Brandywine GPS8 Plus User Manual

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Page 1

User Guide

GPS Time and Frequency System

Model GPS8 Plus

P/N 071000003

Revision 2.0

October 2004

Brandywine Communications

2230 South Fairview Street

Santa Ana, CA 92704

(714) 755 1050 (714) 755 0175

http://www.brandywinecomm.com

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Revision History

REVISION DATE COMMENTS

1.9 09-16-04 Preliminary release of GPS8 Plus user guide.

2.0 10-11-04 Revision of entire user guide.

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Safety Warnings

WARNING: This unit contains lethal AC voltages. Disconnect the unit from the AC supply before removing the cover.

WARNING: The lightning flash with an arrowhead inside of an equilateral triangle is intended to alert the user to the presence of un-insulated “dangerous voltage” withi n the product’s enclosure. The “dangerous volt age” may be of sufficient magnitude to constitute as a risk of electrical shock to people.

CAUTION: The exclamation point inside of an equilateral triangle is intended to alert the user to the presence of important operation and maintenance instructions in the user guide.

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Table of Contents

1 Introduction ............................................................................................................................. 6

1.1 Scope of Section............................................................................................................ 6

1.2 Purpose of Equipment.................................................................................................... 6

1.3 Specification...................................................................................................................7

1.4 Controls, Indicators, and Connectors........................................................................... 11

2 Installation............................................................................................................................. 13

2.1 Scope of Section.......................................................................................................... 13

2.2 Unpacking and Inspection............................................................................................ 13

2.3 Installation and Testing ................................................................................................ 13

3 Operating Instructions........................................................................................................... 14

3.1 Scope of Section.......................................................................................................... 14

3.2 Operation...................................................................................................................... 14

3.3 Serial Communications ................................................................................................ 17

3.4 TxD1/RxD1 (Rear Panel J13) Baud Rate and Character Format................................ 18

3.5 TxD1/RxD1 (Rear Panel J13) User Commands and GPS8 Plus Responses ............. 19

3.6 Message Formats......................................................................................................... 22

3.6.1 Time and Date with Status, Position Averager Status, and Output Status ......... 22

3.6.2 Sounder Control .................................................................................................. 23

3.6.3 1

3.6.4 Dilution of Precision Values and Satellites Used................................................. 26

3.6.5 Time Code and Serial Data Output Formats ....................................................... 27

3.6.6 Frequency and Phase Controller Data ................................................................ 30

3.6.7 Health Status of Satellites ................................................................................... 36

3.6.8 Leap Second Information (Firmware 13+)........................................................... 37

3.6.9 2

3.6.10 Location and Signal Noise Ratio of Satellites...................................................... 39

3.6.11 Firmware Version Number................................................................................... 40

3.6.12 Offset of Local Time Data and Command........................................................... 41

3.6.13 Position of GPS Antenna..................................................................................... 42

3.6.14 Additional Information (Including Magnetic Declination) ..................................... 43

3.6.15 Request Precision Time ...................................................................................... 43

3.6.16 Software Reset Command .................................................................................. 44

3.7 TxD2/RxD2 Messages (Rear Panel J12)..................................................................... 45

3.7.1 Type 1 Format ..................................................................................................... 45

3.7.2 Type 2 Format ..................................................................................................... 45

3.7.3 Type 3 Format (GPS4 Format)............................................................................ 47

3.8 IRIG B and AFNOR NF 2 87-500 Time Code Output.................................................. 48

3.8.1 IRIG B and AFNOR NF 2 87-500 Time Code Bit Allocation ............................... 49

3.9 NASA36 Time Code Output......................................................................................... 51

3.10 2137 Time Code Output............................................................................................... 53

3.11 VELA Time Code Output (Slow Code)......................................................................... 54

3.12 Alarm Contacts (J14) ................................................................................................... 55

4 GPS8 Plus Keypad and Display Operation .......................................................................... 56

4.1 Scope of Section.......................................................................................................... 56

4.2 General Principles of Operation of Keypad and Display.............................................. 56

4.3 Messages after Power On............................................................................................ 57

4.3.1 Operating Mode Selection................................................................................... 58

4.3.2 GPS Locked Mode .............................................................................................. 58

4.3.3 Free Run Mode.................................................................................................... 59

4.3.4 1 PPS Lock Mode................................................................................................ 59

4.3.5 Manual Time and Date Entry............................................................................... 59

4.4 Position Screen ............................................................................................................ 60

4.5 Satellites Tracked Screen ............................................................................................ 60

4.6 1 PPS Status Screen ................................................................................................... 61

4.7 Mean Phase Screen..................................................................................................... 62

4.8 Estimated Mean Frequency ......................................................................................... 63

Pulse Output Data and Command (J10) ........................................................ 23

Pulse Output Data and Command (PCB CON6 Pin 3).................................. 38

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4.9 Frequency Control........................................................................................................ 63

4.9.1 Test Configuration of Frequency Controller ........................................................ 64

4.10 Position Mode Configuration........................................................................................ 64

4.10.1 Position Mode Selection ...................................................................................... 65

4.11 Local Time Configuration ............................................................................................. 65

4.11.1 Local Time Offset Adjustment ............................................................................. 65

4.12 Time Code Configuration ............................................................................................. 66

4.12.1 Time Code Selection ........................................................................................... 66

4.12.2 J13 TXD1 Configuration ...................................................................................... 67

4.12.3 J12 TXD2 Configuration ...................................................................................... 67

4.13 J10 Pulse Output Configuration ................................................................................... 68

4.13.1 J10 Timed Pulse Configuration ........................................................................... 68

4.13.2 J10 Pulse Period Configuration........................................................................... 69

4.13.3 J10 Pulse Time Code Configuration.................................................................... 70

4.14 Status Screen and Alarm Configuration 1.................................................................... 71

4.14.1 Selection of Alarm Masks 1 ................................................................................. 71

4.15 Status Screen and Alarm Configuration 2.................................................................... 72

4.15.1 Selection of Alarm Masks 2 ................................................................................. 72

4.16 Status Screen 3............................................................................................................ 73

4.17 Status Screen 4............................................................................................................ 73

5 Principles of Operation.......................................................................................................... 74

5.1 Scope of Section.......................................................................................................... 74

5.2 Theory of Operation ..................................................................................................... 74

6 Maintenance and Calibration ................................................................................................ 77

6.1 Scope of Section.......................................................................................................... 77

6.2 Routine Maintenance ................................................................................................... 77

6.3 Fault Finding................................................................................................................. 78

6.4 Oscillator Module.......................................................................................................... 78

7 Diagrams............................................................................................................................... 79

8 Appendix I Output Alarm Masks ........................................................................................... 86

9 Appendix II Phase Noise....................................................................................................... 90

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1 Introduction

1.1 Scope of Section

Section 1 provides a general description of the GPS8 Plus. The introduction is divided into three parts; purpose of the equipment, physical and electrical specification, and an identification of the external controls, indicators, and connectors.

1.2 Purpose of Equipment

The GPS8 Plus is a multi-output precision time and frequency standard that uses the Global Positioning System (GPS) to steer and hold an internal oscillator and clock system precisely on time. It is designed for use in telecommunications, power utility, and military communication applications. Time and frequency information is maintained to high accuracy by the internal oscillator even if no satellites can be tracked. Each space vehicle in the GPS system transmits time, frequency, and navigation information in two formats.

• P code: a precision code restricted to authorized users.

• C/A code: an unrestricted code available for general use.

In both cases the time information is maintained by the USA Department of Defense and is traceable to the USA Naval Observatory. The standard GPS8 Plus uses the C/A code.

The GPS8 Plus provides a wide range of output options to form a complete time and frequency management system. In addition, satellite data and navigational information is outputted in the form of longitude, latitude, and altitude for applications where precise positions must be known. For fixed position operation, precise averaging of the position is carried out by the GPS8 Plus over 24 hours.

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1.3 Specification

SPECIFICATION DESCRIPTION OPTION

Satellite Signal GPS L1 1.57542 GHz Satellite Code C/A 1.023 MHz Receiver Type Parallel eight channels (8 satellites tracked

continuously and simultaneously) Receiver Sensitivity -133 dBm Input Impedance 50 ohm Standard Antenna Active omni-directional 30 dB gain Cold Start Requirement Automatic (no time or position input needed)

Typically 8 minutes to time synchronization Satellite Acquisition Time Typically < 20 seconds (warm start) Data Update Rate 1 per second Timing Accuracy Tracking Satellites (XTAL_1) Timing Stability Tracking No Satellites

-Frequency Accuracy

-Tracking At Least One Satellite

Frequency Stability Tracking No Satellites

Within ±150 ns of GPS time

Standard deviation of 34 ns

Rubidium (Rb_2) oscillator

Better than 200 ns per hour

High stability OCXO (XTAL_1) oscillator

Better than 1 µs per hour

Standard OCXO (XTAL_2) oscillator

Better than 10 µs per hour

Temperature Compensated Crystal Oscillator (TCXO)

Better than 120 µs per hour

Rubidium (Rb_2) disciplined oscillator

1 s 10 s 100 s 1000 s 10000 s 100000 s

3E-11 1E-11 3E-12 3E-12 2E-12 8E-13

Precision OCXO (XTAL_1) disciplined oscillator

1 s 10 s 100 s 1000 s 10000 s 100000 s

2E-12 3E-12 1E-11 1E-11 3E-12 1E-12

Standard OCXO (XTAL_2) disciplined oscillator

1 s 10 s 100 s 1000 s 10000 s 100000 s

1E-9 2E-10 3E-10 3E-10 3E-11 1E-12

Temperature Compensated (TCXO) disciplined

oscillator

1 s 10 s 100 s 1000 s 10000 s 100000 s

1.5E-9 5E-10 4E-10 3E-10 3E-11 1E-12

Rubidium oscillator (Rb_2)

-11

5 x 10

High Stability OCXO (XTAL_1)

1 x 10

Standard OCXO (XTAL_2)

5 x 10

Temperature compensated (TCXO) oscillator

5 x 10

per month

-10

per day after 30 days of operation

-9

per day after 30 days of operation

-9

per day after 30 days of operation

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SPECIFICATION DESCRIPTION OPTION

Static Operation Standard Deviation Position Average (measured in northern hemisphere over sixteen 24 hours averaged positions) Mobile Operation Maximum Operational Velocity Disciplined Oscillator Control Resolution (Rb_2)

2.3 meters N/S 1.9 meters E/W 5.0 meters Vertical

460 meters per second

-14

3 x 10

SPECIFICATION DESCRIPTION OPTION

Monitoring/Status Indicators

Output Monitors 9 output monitor LEDs at each output socket for

Sounder Selectable audible marker of GPS receiver 1 PPS,

Built In Test

Manual Controls Primary power switch Data Entry Option

Operating Temperature Range

Humidity 95% non-condensing Power Supply DC Supply Options

Power Consumption 10 W typically Case 19” 1U rack mounting 12.01 inches deep Weight 12 lbs typically

Power LED (green)

Fault LED (red)

Time Valid LED (green)

Tracking Satellites LED (green)

Reject Data LED (yellow)

present signal (and status available via TxD1/RxD1)

Internal 1 PPS, or none

Antenna

Receiver

Initialization

Oscillator control

Oscillator frequency

Configuration parameters

Synthesizer phase-locked loop

RAM

FLASH/EPROM

4 way keypad

2 line LCD display

Instrument: -10 to +50°C

Antenna: -40 to +75°C

115/230 VAC ± 10% 45 - 65 Hz

48 VDC

24 VDC

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INPUTS AND OUTPUTS REF DESCRIPTION OPTION

Telecom Frequency Options Alarm J14

Option J10

1st Programmable Pulse Output

2nd Programmable Pulse Output

1 PPS J9

Code J8

See

Below

5-PIN din 180 DEG

9 way D

socket

9 way D

socket

50R BNC

PCB ONLY CON6

PIN 3

50R BNC

75R BNC

E1 2.048 MHz T1 1.544 MHz Free contacts from status monitor changeover relay and center contact can be linked to + 5 V or 0 V internally TxD1/RxD1 RS232 (OR RS422 user selectable inside) User commands select and control output types, request time data, frequency controller data, GPS data, and instrument output status TxD2/RxD2 RS232 (OR RS422 user selectable inside) Automatic 1 per second time and status output OR output on receipt of user time request Carries 5 VDC at center pin for powered antennas 4 V output at 40 mA load One digital (DCLS 0 to 5 V from 50R) time code same as selected modulated time code or is 1 User selectable 1st pulse rate in place of the time code Selected time code or pulse level 0 to 5 V from 50R source

-1 pulse per 2 ms to 1 pulse per day, with 1 ms to 9999 ms pulse duration active high or active low

-1 pulse per day, active high 100 ms is the default value

-Pulse time accuracy (on and off) is UTC

±150 ns

-1 pulse per 2 ms to 1 pulse per day, with 1 ms to 9999 ms pulse duration active high or active low

-1 pulse per minute, active high 100 ms is the default value

-Pulse time is 350 µs early 1 PPS, 0 to 5 V square wave from 50R

One user selectable modulated time code IRIG B AFNOR NASA36 VELA 2137 Level 2.17 Vpp carrier high level from 10R Maximum 1.7 Vpp into 50R (internally adjustable) IRIG B is the default selection 8 KHz square wave (Telecom frame rate) 8 KHz J7 Square wave 0 to 5 V from 75R

pulsed output

Port 1 J13

Port 2 J12

GPS Ant J11

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INPUTS AND OUTPUTS REF DESCRIPTION OPTION

Telco (two) J6 & J5

75R BNC

Freq 2 (two) J4 & J3

75R BNC

Freq 1 (two) J2 & J1

50R BNC

Outputs are link selectable (internally) with the following options: Telecom (2.048 MHz or 1.544 MHz as selected above) 1 MHz (internal jumper selection) 5 MHz (internal jumper selection) 10 MHz (internal jumper selection) Output level 0 to 5 V from 75R Output level square wave 2.37 Vpp into 75R (G703 ¶ 6) Outputs are from Bandpass filter module 2, specified at time of purchase, with the following options: Telecom (2.048 MHz or 1.544 MHz as selected above) 1 MHz 5 MHz 10 MHz Each output level 1 Vrms from 50R, 75R, or 120R by internal jumper selection (50R is the default) Sine wave 3 Vpp from 75R meets G703 ¶ 10 Transformer isolation (75R connector grounded to case) Frequency accuracy and holdover similar to tables above Meets primary reference clock requirements G811 Harmonic distortion -30 dBc or better 5 MHz or 10 MHz phase noise, see Appendix II Outputs are from Bandpass filter module 2, specified at time of purchase, with the following options: Telecom (2.048 MHz or 1.544 MHz as selected above) 1 MHz 5 MHz 10 MHz Output level 1 Vrms from 50R Frequency accuracy and holdover as per tables above Transformer isolation (50R isolated connectors) Harmonic distortion -30 dBc or better 5 MHz or 10 MHz phase noise, see Appendix II

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1.4 Controls, Indicators, and Connectors

Figure 1 and Figure 2 show the GPS8 Plus front panel, rear panel, indicators, connectors, and controls. The following two tables below describe these indicators, connectors, and controls.

INDICATOR DESCRIPTION

Power Green LED indicates that the primary power is

available.

Fault Red LED indicates one of the following:

1. Phase lock loop control voltage within 10% of its limit.

2. Synthesizer lost lock.

3. One or more output failures.

4. Processor malfunction.

5. No satellite reception for 8 hours.

Time Valid -Green LED indicates the time and position data

are synchronized since turned on.

-Turns off after no satellite reception for 8 hours.

Tracking Satellites Green LED indicates that at least one satellite is

being tracked.

Reject Data Yellow LED indicates that the last time data

frame was rejected (error bypass active and frequency in holdover).

Display 2 lines, each with 24 characters low noise back-

lit liquid crystal display.

Keypad 4 way keypad for programming and display

control.

Table 1 GPS8 Plus Front Panel

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CONNECTOR/CONTROL DESCRIPTION

Primary Power 3 pin IEC socket for 115 VAC power Fuse Main HT fuse for primary power supply Alarm 5 pin 180° latching DIN socket Port 1 9 way D type socket for serial RS232 or RS422

data communications

Port 2 9 way D type socket for serial RS232 or RS422

data communications

GPS Ant BNC socket (grounded) 50R for antenna signal

and power Option Option Status 1 PPS

1 PPS Status Analog Code (IRIG B, AFNOR, NASA36, VELA,

and 2137) Analog Code Status 8 KHz 8 KHz Status Telco Telco Status 2

Telco Telco Status 1 Frequency 2 Frequency 2 Status 2

Frequency 2 Frequency 2 Status 1

Frequency 1 Frequency 1 Status 2

Frequency 1 Frequency 1 Status 1

-BNC socket (grounded) 50R DC pulse output

-Green LED showing pulse output is active

-BNC socket (grounded) 50R 1 PPS output

referenced to UTC

-Green LED showing 1 PPS output is active

-BNC socket (grounded) 50R (with optional

galvanic isolation)

-Green LED showing analog code output is active

-BNC socket (grounded) 75R

-Green LED showing 8 KHz output is active

-BNC socket (grounded) 75R

-Green LED showing Telco output 2 is active

-BNC socket (grounded) 75R

-Green LED showing Telco output 1 is active

-BNC socket (grounded) 75R

-Green LED showing frequency 2 output 2 is

active

-BNC socket (grounded) 75R

-Green LED showing frequency 2 output 1 is

active

-BNC socket (isolated) 50R

-Green LED showing frequency 1 output 2 is

active

-BNC socket (isolated) 50R

-Green LED showing frequency 1 output 1 is

active

Table 2 GPS8 Pl us Rear Panel

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2 Installation

2.1 Scope of Section

Section 2 describes the procedure prior to and during the GPS8 Plus installation.

2.2 Unpacking and Inspection

Carefully remove the GPS8 Plus from the shipping carton. The following items should be included in the shipment:

• 1 GPS8 Plus

• 1 power cord

• 1 user guide

2.3 Installation and Testing

The standard GPS8 Plus is housed in a 19” 1U rack mounting case, which can be installed in a rack using the front panel mounting slots.

The active high gain antenna is a sealed omni-directional device, which must be mounted with the cable exit at the bottom. For optimum performance the unit should be located outside on a roof, away from objects which may impair satellite visibility or generate high frequency interference.

Before connecting power to the receiver check that the power supply is in accordance with the specification and/or details on the rear panel and check that the power cord has a secure earth connection. An initial self test should be carried out before final installation or connection to other units. The power indicator will be illuminated. After about nine seconds the unit will test all the software controlled indicators in the following order:

1. Fault

2. Time Valid

3. Tracking Satellites

4. Reject Data

If all LEDs are turned on simultaneously, an internal hardware fault has been detected by the processor. The diagnostic messages from TxD1 (SK2/J13) during power on should be examined for a fault report.

After the indicators have been tested they will be set to reflect the current status.

At this point the unit can be switched off and finally installed or configured as described in section 3.

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3 Operating Instructions

3.1 Scope of Section

Section 3 covers the operation, initialization, and configuration of the GPS8 Plus. It is assumed that the unit has been installed in accordance with section 2.3 and that power has been applied to the unit.

3.2 Operation

During normal operation the GPS8 Plus automatically receives, analyses, and processes data from up to eight GPS satellites simultaneously. This data is used to provide time, frequency, and position information. The information is made available to the user through a range of output interfaces.

In applications where the unit is to be used for general test and measurement purposes, it can be switched on and off as necessary. However, if the unit is to be used as a time and/or frequency standard it should be running continuously to allow long term software processing to take place, which is essential if the unit is to achieve its optimum performance. With the internal ovened oscillator XTAL_1 option, this process takes a minimum of 24 hours. The unit carries out automatic position averaging and then automatically shifts into the known position to obtain the highest possible time and frequency precision. The position averaging process also takes place during the first 24 hours after the unit is switched on.

As described in section 2.3 when the GPS8 Plus is switched on it will carry out a self-test routine before entering the operational program, which sets the correct initial conditions for the receiver system. Satellite tracking is automatic, provided that at least one is visible and the GPS8 Plus already has a valid position in its battery backed receiver. Now time and frequency control can start within approximately 1 minute. If three or more satellites are visible, the 3-D position information will also be processed. The GPS8 Plus internal frequency standard will warm up in 5 minutes (OCXO) or 10 minutes (Rubidium). At the end of the warm up period the frequency controller is reinitialized.

Reception of at least one satellite is confirmed by the green Tracking Satellites indicator being illuminated. Subsequent synchronization of the time to UTC (up to 15 minutes from satellite reception) is confirmed by the green Time Valid indicator being illuminated. The voltage control of the internal oscillator maintains synchronization of all outputs with GPS. If the control voltage of the internal oscillator is within 10% of the end-of-range, the red Fault indicator will illuminate. This indicator also illuminates if any of the 10 outputs that should be present are not detected by the internal output signal detector.

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The final phase and frequency locking process may take several hours depending on the oscillator type fitted and satellite reception. Because the GPS8 Plus uses a patented "Intelligent Phase Locked Loop" system that incorporates a specially developed dynamic time constant, the control parameters are continuously modified to achieve lock in the shortest possible time to optimize its performance for the best time and frequency accuracy.

Note that after a position has been successfully navigated, the number of tracked satellites drops down to 4 or less, the satellite receiver automatically selects 2-D positioning mode, and continues to recover time. If the number of tracked satellites then drops down to 3, 2, or 1, the positioning mode is suspended but time is recovered. If no satellites are tracked, the GPS8 Plus runs in ‘holdover’ continuing to maintain all the outputs. After 8 hours of lost reception the red Fault indicator is turned on.

If the unit has a current satellite ephemeris at switch on, the start up time (after system initialization has been completed) is approximately:

• Initial acquisition : 1 – 15 seconds

• Satellite re-acquisition : 25 seconds

• Tracking: 20 seconds

• Navigating: 1 minute from start up

• Position averaging complete: 24 hours from start up

Synchronization of the receiver time will normally be completed within 1½ minutes after the unit is switched on. If the receiver does not have an almanac already backed up in memory after it has been switched on, it will take typically 8 or 9 minutes to acquire the necessary parameters to enable precise time acquisition, as indicated by the Time Valid indicator on the GPS8 Plus front panel or by the assertion of the Time Valid status bit in the time message.

For fixed position operation, precise averaging of the position is carried out by the GPS8 Plus over 24 hours. The reason for this averaging is depicted in the illustration below. The illustration shows the raw time data before processing is carried out by the GPS8 Plus is improved by a factor of 3 when the GPS8 Plus has averaged its position. The vertical scale on the graphs is the 1 PPS deviations in nanoseconds.

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RAW 1PPS DATA FROM G PS RECEIVER - POSITION FINDIN G

500

250

-2 5 0

TIME DEVIATION IN ns

-5 0 0 0 4 8 12

Elapsed Tim e in Hours

RAW 1PPS DATA FROM GPS RECEIVER - POSITION AVERAGED

500

250

-2 5 0

TIME DEVIATION IN ns

-5 0 0 0 4 8 12

E lap s e d tim e in H o urs

Illustration 1 GPS8 Plus Averaging Position Result

FUR3.LOG

FUR4.LOG

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3.3 Serial Communications

The GPS8 Plus has two asynchronous serial ports that communicate with the user. They are suitable for connection to other units such as printers, computers, and terminals. They can be configured as RS232 or RS422 ports by turning around a simple header inside the unit. The options fitted to this unit are identified in the specification (section 1.3). This section describes the options in more detail.

Each serial port is accessible via its own 9 way D socket, J13 and J12 respectively. An RS232 and RS422 selection header is mounted inside of the unit just behind each of the 9 way D sockets on the main PCB. The header has five straps traversing a standard 16 pin dual-in-line footprint at one end. If inserted with the straps at the right-hand end viewed from the front of the main PCB, the configuration of the associated connector is RS422 as defined in the table below:

PIN GPS8 PLUS RS422

FUNCTION

1 Ground 0 V 2 Not used 3 Tx data- Output 4 Rx data+ Input 5 Ground 0 V 6 Rx data- Input 7 Not used 8 Tx data+ Output 9 Not used -

Table 3 RS422 Connections J13 and J12 (Factory Default)

If the header is inserted with the straps at the left-hand end viewed from the front of the main PCB, the configuration of the associated connector is RS232 as defined in the table below:

PIN GPS8 PLUS RS232

FUNCTION

1 Ground 0 V 2 Tx data Output 3 Rx data Input 4 Not used Input 5 Ground 0 V 6 Not used (RS422 Rx data-)

7 Not used 8 Not used (RS422 Tx data+) 9 Not used -

Table 4 RS232 Connections J13 and J1 2 ( Factory D ef ault)

Note that the unused pins (6 and 8) have RS422 signal connections, if a 5-way jumper is used to select RS232. This can be eliminated by using a 3-way jumper for RS232 selection. The 3 jumpers are at the left-hand end of the header viewed from the front of the main PCB for RS232 selection.

MANUAL P/N 900000007 REV 2.0

DIRECTION

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3.4 TxD1/RxD1 (Rear Panel J13) Baud Rate and Character Format

TxD1 is designed for communication with the user because it gives the user access to the unit status and GPS reception information. RxD1 receives

commands and data requests from the user.

SW1 (a red 8-way switch pack) located on the main PCB controls TxD1/RxD1 baud rate and character format at J13. Prior to the alteration of the switch, disconnect the power connection to the GPS8 Plus and practice all normal safety precautions when opening the unit to access the 8-way DIL switch SW1.

SWITCH 1 2 3 4 5 6 7 8

ON For baud rate selection see

the table below

OFF No parity Even

Table 5a TxD1/RxD1 Character Format

SW1-1 SW1-2 SW1-3 SW1-4 BAUD

OFF OFF OFF OFF 75

ON OFF OFF OFF 110

OFF ON OFF OFF 134.5

ON ON OFF OFF 150

OFF OFF ON OFF 300

ON OFF ON OFF 600

OFF ON ON OFF 1200

ON ON ON OFF 1800

OFF OFF OFF ON 2400

ON OFF OFF ON 4800

OFF ON OFF ON 4800

ON ON OFF ON 9600

OFF OFF ON ON 9600

ON OFF ON ON 19200

OFF ON ON ON 19200

ON ON ON ON 19200

Table 5b TxD1/RxD1 Baud Rate

Parity Odd

parity

8 data

bits

7 data

bits

2 stop

bits

1 stop bit

The example below is of the SW1 setting for 4800 baud, 8 data, no parity, and 1 stop:

!!!!

!!!! !!!!

!!!!

OFF ON

!!!!

12345678

Example 1 SW1 Factory Default Setting

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3.5 TxD1/RxD1 (Rear Panel J13) User Commands and GPS8 Plus Responses

The GPS8 Plus asynchronous serial port RxD1/TxD1 provides time, position, and status data on request from the user. Furthermore, it can accept user commands. For example, to set the output time to UTC or local, the user must set the local time offset from the UTC, set the pulse length on-time or period, and set the duration of an output pulse in 1 millisecond units.

The user can obtain information from the GPS8 Plus by sending single ASCII character requests terminated by <CR><LF>. The GPS8 Plus response message structure is based on the NMEA-0183 standard for interfacing Marine Electronics Navigation Devices. The same structure is used for commands from the user that sets the GPS8 Plus operating parameters. User characters are sampled at 1 millisecond intervals and will be missed if more than 1 character is

received per millisecond. If there is no time delay between the characters sent to

the GPS8 Plus, the maximum baud rate for the user commands is 4 800.

The requested record will be directed to the GPS8 Plus serial port TxD1 output. All characters transmitted by the GPS8 Plus are consistent with ASCII character or control codes. When hexadecimal numbers are transmitted to communicate status bit values they are transmitted as ASCII characters 0 through 9 and A through F (A through F characters are always upper case characters).

Reserved characters are used to indicate the beginning and end of records in the data stream and to delimit data fields within a record. As an exception the time request may be a single character and optionally without termination to minimize the overhead of obtaining a time stamp response from the GPS8 Plus.

Example 2 ASCII Character Request

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The list of reserved characters is given in the table below:

CHARACTER H EX VALUE USAGE

$ 24 Start of record identifier

<CR><LF> 0D 0A End of record identifier

, 2C Record field delimiter * 2A Checksum field delimiter

Table 6 Rese rved Characters

The notation ‘CK’ indicates the optional checksum value of the message that is computed by an exclusive-OR of all bytes between the ‘$’ and ‘*’ characters. The ‘$’, ‘*’, and checksum are not included in the checksum computation. For commands sent to the GPS8 Plus, the checksum may be omitted in which case the associated ‘*’ must also be omitted. The GPS8 Plus will recognize the <CR> as the end of the message and will not attempt to verify the missing checksum. If the checksum is attached by the user, it will be checked by the GPS8 Plus and the message will be rejected if an error is detected. Errors in the format of the message will also cause a rejection.

The list of available single character requests is given in the table below:

REQUEST RESPONSE

a<CR><LF> Version number, time, position averager, and output status information b x<CR><LF> Sounder control, where x = 0, 1, or 2 c<CR><LF> 1st pulse output (local/UTC) time or pulse period, polarity, and pulse length d<CR><LF> Dilution of precision values and satellites used e<CR><LF> Modulated time code, pulsed output, TxD1 format, and TxD2 format f<CR><LF> Frequency and phase controller data h<CR><LF> Health status of satellites j<CR><LF> Leap second date and UTC-GPS seconds offset value k<CR><LF> 2nd pulse output (local/UTC) time or pulse period, polarity, and pulse length l<CR><LF> Location and signal strength of satellites (up to three records) n<CR><LF> Version number of firmware and GPS engine o<CR><LF> Run UTC/local time and local time hours offset p<CR><LF> Position of GPS receiver antenna S<CR><LF> Additional information (including magnetic deviation) t Precision time request *

Z See software reset command 3.6.16

Table 7 Single Character Request s

* The precision time request is handled as a priority. The ‘t’ request does not need to be followed by a <CR><LF>. If a <CR><LF> is appended, it will be ignored.

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The GPS8 Plus continuously polls the GPS receiver for satellite status and associated information. If the user request requires information from the GPS engine, the GPS8 Plus immediately forwards the information it has already acquired in its internal buffer. The associated response is returned immediately. If several requests have been sent prior to completion of the responses, the order of responses may be different from the order of requests.

The precision time request is given high priority. The time at which the request character was received is logged at the next whole millisecond and the GPS8 Plus transmits as soon as the serial port is free. This special message is not in the NMEA format, but in the industry standard Type 2 Serial Data Format that is described below in section 3.6.15.

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3.6 Message Formats

3.6.1 Time and Date with Status, Position Averager Status, and Output Status

Requested by a<CR><LF>

Field 1 2 3 4 5 6 7 8 $PRCCG, A, 007, 19:53:19, 07/06/00, 9, 3, 1, 0000, 00*09

FIELD CONTENTS OF FIELD

1 Fixed text “A” 2 Version number of GPS8 Plus firmware 000 to 999 3

4 The status is the ASCII representation of a hexadecimal character between 0

5 Number of position average samples accumulated (target is 100) 6 Position average mode:

7 The output status (16 bits in 4 digits) as detected by the output detectors at

8 The control status (8 bits in 2 digits) as reported by the frequency controller.

BIT 15 BIT 14 BI T 13 BIT 12 BI T 11 BIT 10 BIT 9 BIT 8

Reserved Reserved Reserved Reserved Reserved Option fail 1 PPS fail Modulated

J10 (SK4) J9 (CON8) J8 (SK3)

BIT 7 BIT 6 BI T 5 BIT 4 BI T 3 BIT 2 BIT 1 BIT 0

Synthesizer

PLL

unlocked

J7 (SK12) J6 (SK10) J5 (SK8) J4 (SK9) J3 (SK11) J2 (SK7) J1 (SK6)

BIT 7 BIT 6 BI T 5 BIT 4 BI T 3 BIT 2 BIT 1 BIT 0

One or

more OP

failures

detected

Hours minutes seconds Day of month, month, and year

and F. Each bit of the hexadecimal character represents the following status condition when set to 1 (or the opposite at 0): Bit 3 (MSB) Bit 2 Bit 1 Bit 0 (LSB) Valid time Not GPS locked Local time Leap year

“0” = not averaging “1” = averaging “2” = known averaged position in use

each of the 10 sockets. Each hexadecimal character represents 4 bits and each bit that is set to 1 corresponds to the status described in the following tables below when true. When no failures are detected, four zeros are transmitted.

Each bit that is set to 1 corresponds to the status described in the following table below when true.

code fail

Table 8a Field 7 Bits

(LSB)

8 KHz fail Telco fail Telco fail Freq 2 fail Freq 2 fail Freq 1 fail Freq 1 fail

Table 8b Field 7 Bits

(LSB)

satellites for

8 hours

Reserved Oscillator

frequency

control near

its limit

Table 9 Field 8 Bits

MANUAL P/N 900000007 REV 2.0

Frequency

error

oscillator cannot be controlled

Reserved Rubidium

oscillator

status shows

rubidium cold

Reserved

Page 23

3.6.1.1 Example of Output

$PRCCG, A, 007, 13:56:48, 09/06/00, 9, 100, 2, 0000, 00*0B

The meaning of the response is i.e. software version 007, requested at 13 hours 56 minutes 48 seconds, on the 9

of June 2000, time status shows time valid, synchronized, and UTC in leap year. The 100 position samples have been accumulated so the position averaging is complete, the GPS receiver is reporting that it is in a known position, and there are no output or control failures.

3.6.2 Sounder Control

Requested by b< >0 <CR><LF>

The GPS8 Plus sounder can be turned on and off by this command. The sound output can be triggered by the GPS receiver 1 PPS (provided that the satellites are being received and the position has been found) or it can be initiated by the 1 PPS derived from the GPS8 Plus precision time base. Note that there is a space between the command letter ‘b’ and the control number 0, 1, or 2. The three commands are as follows:

• b 0<CR><LF>: satellite receiver 1 PPS initiates the sound

• b 1<CR><LF>: GPS8 Plus internal 1 PPS initiates the sound

• b 2<CR><LF>: the sounder is turned off

3.6.3 1

Pulse Output Data and Command (J10)

Requested by c<CR><LF>

Sending the single letter ‘c’ followed by <CR><LF> requests a message from the GPS8 Plus that describes its 1 format transmitted to the GPS8 Plus sets the pulse output parameters, which are stored in non-volatile RAM. The 1

pulse output from J10. The same message

pulse output (logic levels 0 V and 5 V from 50R) can be commanded to be normally low going high for the pulse duration or normally high going low for the pulse duration. There are two ways to specify the pulse occurrence and duration. Also, note that the output at J10 can be commanded to be a time code, see the ‘e’ command.

1. The pulse output can be specified to occur at the UTC or local time. In this case, the pulse outputs always start at the second’s edge (UTC to within ±150 ns) and terminate at a specified number of milliseconds later. The ‘don’t care’ characters in the pulse time definition allow hour, minute, or second values to be ignored. The default pulse output setting is UTC midnight and positive pulse 100 ms long. The minimum and maximum pulse duration is 1 ms and 9999 ms respectively.

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2. Alternatively, multiple pulses per time period can be specified. In this case, the period of the pulse stream is specified together with the pulse active duration. The minimum period is 2 ms and the maximum period is 9999 ms. Immediately after receipt of the command, the current pulse (if active) is terminated and the first new pulse commences at the start of the next second’s edge. If the pulse period specified in milliseconds is a submultiple or a multiple of seconds, the pulses remain synchronized with 1 PPS. If the contrary is true, the pulses may require multiple seconds before the start of a pulse coincides with 1 PPS. However, they are always strictly related to real-time milliseconds as long as the unit is locked to the GPS.

Remember that if the checksum is omitted in the user command to the GPS8

Plus, the ‘*’ must be omitted from the end of the message so that the GPS8 Plus

recognizes that no checksum is available. Two forms exist as seen below:

• Form 1 (pulse output at specified time, local or UTC):

Field 1 2 3 $PRCCG, C, UHHMMSS, +9999*CK<CR><LF>

• Form 2 (pulse output with specified period):

Field 1 2 3 $PRCCG, C, P9999, +9998*50<CR><LF>

FIELD CONTENTS OF FIELD

1 Fixed text letter “C” 2

3 ‘+’ = pulse output when active is +5 V from 50R, when inactive is 0 V from 50R.

9999 From 1 to 4 digits (leading zeros NOT required) defines the pulse length in

EITHER ‘U’ = UTC of the pulse output time or ‘L’ = local time of the pulse output time. Followed by HHMMSS = hour minute second of the pulse output time, where leading zeros MUST be present in the time field (e.g. 5 hours is 05 hours). If HH or MM or SS are to be ignored, XX is transmitted (where X = ASCII code 58 hexadecimal). OR ‘P’ = specified period in milliseconds, which is the following number 2 to 9999.

‘–’ = pulse output when active is 0 V from 50R, when inactive is +5 V from 50R.

milliseconds.

Note that the GPS8 Plus responds to the pulse setting command by echoing the message that is recomputed from the values originally transmitted by the user after they have been decoded for use by the GPS8 Plus and its own checksum has been added to the message output. Illegal commands are ignored, except when the pulse length is commanded to be greater than the pulse period. In this case, the pulse length is reset to 1 ms.

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3.6.3.1 Examples of 1

Note that the examples of the 1

Pulse Commands and Responses

pulse commands and responses are when the

UTC or local time is specified.

1. $PRCCG, C, L202500, +100*79<CR><LF> The pulse output commences at 20 hours 25 minutes 00 seconds local time and is active at +5 V for 100 ms.

2. $PRCCG, C, UXXXXXX, +1*65<CR><LF> The pulse output commences once per second and is active at +5 V for 1 ms.

3. $PRCCG, C, UXXXX10, -500*66<CR><LF> The pulse output commences once per minute at 10 seconds past the minute and is active at 0 V for 500 ms.

4. $PRCCG, C, UXX0000, +500*61<CR><LF> The pulse output commences once per hour at 00 seconds past the minute and is active at +5 V for 500 ms.

5. $PRCCG, C, UXX00XX, +500*61<CR><LF> The pulse output commences once per second at 00 minutes past the hour for one minute (until the minute changes to 01) and is active at +5 V for 500 ms.

3.6.3.2 Examples of Pulse Periods

Note that the examples of the pulse periods are when the pulse periods are specified instead of the UTC or local time. The pulse period may be a minimum of 2 ms to a maximum of 9999 ms. The pulse width may be a minimum of 1 ms to a maximum of 9998 ms. If the pulse length command asks for a length greater than the period, it is automatically reset to 1 ms.

Illustration 2 Pulse Period

1. $PRCCG, C, P1000, +500*65<CR><LF> The pulse output has a period of 1000 ms and is active high for 500 ms. This is a square wave at 1 Hz. The positive edge commences at the second’s edge. It will remain synchronized.

2. $PRCCG, C, P2, +1*52<CR><LF> The pulse output has a period of 2 ms and is active high for 1 ms. This is a square wave at 500 Hz. The positive edges will remain synchronized with the seconds’ edges because the period of 2 ms is a sub-multiple of 1 second.

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3. $PRCCG, C, P2, -1*54<CR><LF> The pulse output has a period of 2 ms and is active high for 1 ms. This is a square wave at 500 Hz. The negative edges will remain synchronized with the seconds’ edges because the period of 2 ms is a sub-multiple of 1 second and a negative pulse is specified.

4. $PRCCG, C, P9999, +9998*50<CR><LF> The pulse period is 9999 ms and the pulse output is high for 9998 ms. The output is high except for 1 ms at the end of the pulse. The period is not a multiple of 1 Hz. The output goes high for the first time at the 1 Hz edge following the receipt of the command. The next pulse starts 1 ms before 10 seconds has elapsed. 9999 seconds must pass before the pulse commences again at a second’s edge.

3.6.4 Dilution of Precision Values and Satellites Used

Requested by d<CR><LF>

Field 1 2 3 4 5 6 7 8 9 10 11...15 16 17 $GPGSA, A, 1, NN, NN, NN, NN, NN, NN, NN, NN,….,PP.PP, HH.HH, VV.VV*CK<CR><LF>

FIELD CONTENTS OF FIELD

1 “A” or “M” Operational mode:

“M” = 2-D mode only “A” = 2-D/3-D auto-switching mode

2 “1” – “3” Positioning status:

“1” = positioning interrupted “2” = 2-D positioning “3” = 3-D positioning

3 – 14 NN Satellite numbers 01 – 32 used for positioning.

Note a null field is outputted unless a satellite is available.

15 PP.PP PDOP (combined DOP)

Note “00.00” is outputted unless 3-D positioning is performed.

16 HH.HH HDOP (horizontal DOP)

Note “00.00” is outputted while positioning is interrupted.

17 VV.VV VDOP (vertical DOP)

Note “00.00” is outputted unless 3-D positioning is performed.

3.6.4.1 Example Message

$GPGSA, A, 3, 03, 15, 17, 19, 21, 22, 23, 27,…,01.96, 01.05, 01.66*37<CR><LF>

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3.6.5 Time Code and Serial Dat a Out put Form a ts

Requested by e<CR><LF>

Sending the single letter ‘e’ followed by <CR><LF> requests a message from the GPS8 Plus that describes its time code outputs and serial data formats. The same message format transmitted to the GPS8 Plus can be used to set the time

code and serial data formats. The pulsed out put is report ed on ly. Use the ‘c’

command to define the pulsed output requirement in terms of UTC/local time pulses or period defined pulses. In each case the pulse length is required.

Field 4 is reserved to allow future definition of different formats from TxD1. TxD1 is fixed as a serial type 2 requested time message.

Field 1 2 3 4 5 $PRCCG, E, X, X, X, X*CK<CR><LF>

FIELD CONTENTS OF FIELD

1 E Fixed letter “E” identifies the command 2

4 1 – 3 or X Reserved for TxD1 serial data format selection (not in use) 5

0 – 5 or X Time code 1 type

0 None – 1 KHz sine wave carrier at maximum level only 1 IRIG B – 1 KHz modulated carrier 2 XR3/2137 – 1 KHz modulated carrier 3 VELA – 1 KHz modulated carrier 4 NASA36 – 1 KHz modulated carrier 5 AFNOR – 1 KHz modulated carrier X Do not change current output selection

0 – 3 or X Pulsed output type

0 None 1 Digital time code (0 to 5 V from 50R) as time code 1 above 2 UTC/local time defined pulse output – set by ‘c’ command only 3 Period defined multi-pulse output – set by ‘c’ command only X Do not change current output selection

1 – 3 or X TxD2 automatic or requested serial data output format

selection (J12) see section 3.7 1 Type 1 format: 20:34:45 31/05/01 151 4 2 Type 2 format: 20:34:45.678 31/05/01 151 4 3 GPS4 format:

2001, 151:20:34:45, 3, 1

At power on, TxD2 automatically transmits the selected time message once per second. On receipt of a lower case ‘t’ TxD1 outputs the time at which ‘t’ was received in the currently selected format above and becomes a request port thus only transmitting the time message on receipt of the ‘t’ from the user. The user must send ‘r’ to the switch to turn on the automatic output of the time message once per second.

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3.6.5.1 Example Messages

MESSAGE DESCRIPTION

$PRCCG, E, 1, 1, 1, 1*2C<CR><LF> -TC1 at J8 modulated time code output is IRIG B (1 KHz

carrier).

-TC2 at J10 pulse output is IRIG B (0 and 5 V from 50R).

-TxD1 at J12 is type 2 serial data request only (this output is not currently changed by the command).

-TxD2 at J12 is type 1 format data, automatically output 1 per second until ‘t’ request when it becomes requested only until ‘r’ is received. ‘t’ and ‘r’ do not require <CR><LF> termination.

$PRCCG, E, 2, 1, 1, 1*2F<CR><LF> -TC1 at J8 modulated time code output is XR3/2137 (1

KHz carrier).

-TC2 at J10 pulse output is XR3/2137 (0 and 5 V from 50R).

-TxD1 at J12 is type 2 serial data request only (this output is not currently changed by the command).

$PRCCG, E, 3, 1, 1, 1*2E<CR><LF> -TC1 at J8 modulated time code output is VELA (1 KHz

carrier).

-TC2 at J10 pulse output is VELA (0 and 5 V from 50R).

-TxD1 at J12 is type 2 serial data request only (this output is not currently changed by the command).

$PRCCG, E, 4, 1, 1, 1*29<CR><LF -TC1 at J8 modulated time code output is NASA36 (1

KHz carrier).

-TC2 at J10 pulse output is NASA36 (0 and 5 V from 50R).

-TxD1 at J12 is type 2 serial data request only (this output is not currently changed by the command).

$PRCCG, E, 1, 1, 1, 2*29<CR><LF> -TC1 at J8 modulated time code output is IRIG B (1 KHz

carrier).

-TC2 at J10 pulse output is IRIG B (0 and 5 V from 50R).

-TxD1 at J12 is type 2 serial data request only (this output is not currently changed by the command).

-TxD2 at J12 is type 2 format data, automatically output 1 per second until ‘t’ request when it becomes requested only until ‘r’ is received. ‘t’ and ‘r’ do not require <CR><LF> termination.

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To change TC2 to a timed pulse output, the ‘c’ command must be used because the specification of the pulse time or pulse period and pulse duration is required. This example message $PRCCG, C, P2, +1*52<CR><LF> makes TC2 output a 1 ms positive pulse with a 2 ms period (500 Hz and +ve edge on time).

Send e<CR><LF> to query the output setup as seen in the example message below:

MESSAGE DESCRIPTION

$PRCCG, E, 1, 3, 1, 2*2D -TC1 at J8 modulated time code output is IRIG B (1 KHz

carrier).

-TC2 at J10 pulse output is a pulse with period specification.

-TxD1 at J12 is type 2 serial data request only (this output is not currently changed by the command).

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3.6.6 Frequency and Phase Controller Data

Requested by f<CR><LF>

Field 1 2 3 4 5 6 7 8 9 10 11 12 $PRCCG, F, +XXX, +XXX, +XXX, +XXXXX, +X.XE-XX, +X.XXEXX, DDDHHMMSS, +XX.X, XX, XX, XX*CK<CR><LF>

The field lengths shown in the table below are typical, but not fixed (the leading zeros and/or spaces are not necessarily transmitted).

FIELD CONTENTS OF

FIELD

1 F Fixed text “F” identifies the

message type

2 +XXX

3 +XXX Phase controller value (ns)

4 +XXX

5 +XXXXX

6 +X.XE-XX

7 +X.XXE-XX

8 DDD:HH:MM:SS

9 +XX.X Reserved for temperature in

10 XX

11 XX

12 XX Oscillator type (00 to FF) Fixed 2 ASCII hex chars

Instantaneous phase (ns) (signed decimal integer)

(signed decimal integer)

Average phase (ns) (signed decimal integer)

Frequency controller value (signed decimal integer +0 = nominal center)

Last frequency controller correction

(decimal scientific notation)

Frequency trend

(decimal scientific notation) UTC at last correc tio n (day of

year, hour, minute, and second)

degrees Celsius (not used in GPS8 Plus controller, which outputs +69.4)

Status of phase and frequency controller (00 to

FF) PLL constraint (TCSW) and status

DESCRIPTION RANGE

ASCII ‘F’

–499999999 +500000000 Typical +0 2 chars to 10 chars Will always be +0 in GPS8 Plus controller output 2 chars –32768 to +32767 Minimum of 2 chars Limited to 5 chars Sign replaced by % if out of range –32768 to +32767

2 chars to 6 chars

1.0E-8 to 0.00E-14

7 chars to 9 chars

±1.0E-8 to ±0.00E-14

7 chars to 9 chars Fixed 12 chars

+0.0 is 4 chars

+69.9 is 5 chars

Fixed 2 ASCII hex chars

3.6.6.1 Example Message

$PRCCG, F, -450, -382, +0, -2186, +3.0E-13, +1.14E-10, 144:09:54:00, +69.4, 60, 03, 03*3B<CR><LF>

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3.6.6.2 Instantaneous Phase

The instantaneous phase measurement between the 1 PPS recovered from the satellite receiver and the 1 PPS generated by the internal precision oscillator is reported in nanoseconds. The resolution of the measurement is 50 ns and the maximum value is limited to ½ seconds (beyond this, the value is converted by the calculation of the 1 second phase and the sign is reversed). A positive sign always means the phase of the 1 PPS derived from the local oscillator and the output to the user is ahead of the satellite receiver 1 PPS. Similarly a negative sign means that the unit output 1 PPS is behind the satellite receiver 1 PPS.

The instantaneous phase value is outputted even when the satellite receiver 1 PPS signal is known to be invalid.

3.6.6.3 Average Phase

The average phase is the mean value of the instantaneous phase readings that have been accepted. The resolution of this output is 1 ns. Occasionally the instantaneous phase readings can be rejected by the GPS8 Plus (these readings are not included in the average) for several reasons, such as the ones listed below.

• Corrupt satellite data.

• Out of limit phase measurement.

• Excessive inaccuracy due to the re-introduction of selective availability

(SA).

3.6.6.4 Phase Controller Value

The phase controller value is the current value of the phase controller used to adjust the local 1 PPS to UTC. In the GPS8 Plus this phase control is always set to zero because following initialization the 1 PPS is maintained in phase with the satellite 1 PPS by frequency control. If a large 1 PPS phase offset is detected for any reason (e.g. after a long holdover period), values from 6 µs to 20 ms are removed by phase stepping the output 1 PPS in 400 ns steps. An error larger than 20 ms is corrected by resynchronization.

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3.6.6.5 Frequency Controller Value

The frequency controller value is the decimal value for the 16 bit DAC used for controlling the frequency of the internal oscillator. The oscillator is initially factory calibrated with a center control value, which is stored in EEPROM. This value is transmitted to the DAC after switch on. As the oscillator gradually ages, this value increases or decreases to maintain the correct output frequency. The

maximum range of the DAC is ±32767 counts, where a positive increase

indicates an increase in oscillator frequency.

At switch on the GPS8 Plus frequency control system starts with a fast time constant to achieve frequency correction of the oscillator in the shortest possible time. This process causes some rapid frequency changes while the oscillator is coarsely adjusted via the DAC. When a sufficient measurement and control history has been accumulated in memory and processed, a medium time constant is implemented, which results in smaller and less frequent correction to the local oscillator. Finally, after more extensive measurement and control data has been stored and processed, the slow time constant is implemented, which results in optimum frequency control strategy and hence accuracy of the internal oscillator.

The time taken to change from FAST to MEDIUM to SLOW depends on several parameters such as oscillator stability, satellite reception, and temperature changes. Typical values are ½ hour to MEDIUM and 1.9 hours to SLOW for a high stability crystal ovened oscillator. Typical values are 2.2 hours to MEDIUM and 9 hours to SLOW for a standard rubidium oscillator.

OSCILLATOR

TYPE

OCXO 5 minutes 10 minutes 52 minutes

High Stability

OCXO

Rb_2 10 minutes 2.2 hours 9 hours

WARM UP

TIME

5 minutes 30 minutes 1.9 hours

Table 10 O scillator Changing Time

TIME IN

FAST

TIME IN

MEDIUM

3.6.6.6 Last Frequency Controller Correcti on

The last frequency controller correction is the value of the last frequency correction applied by the 16 bit DAC. When the unit has found its average position and the oscillator has overcome the high value of aging experienced in the first few days after switch on, this value will usually be either zero or the lowest possible frequency increment (e.g. 3E-13 for XTAL_1). The GPS8 Plus controller algorithm works to keep each correction step magnitude below the short term noise level of the oscillator type fitted, such that the action of correction does not degrade the short term stability of the oscillator, which is better than what can be recovered from the GPS.

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3.6.6.7 Frequency Trend

The frequency trend is the movement in the average phase over a specified measurement period. The measurement period is dependent on the oscillator option fitted and the time elapsed since start up. For example, a general purpose ovened oscillator period starts at 1 second, after ten samples of 1 second the period increases to 10 seconds, and after ten samples the period increases to

100 seconds, at which point the measurement resolution is 1 x 10

. The sample intervals and measurement periods for four oscillator options are shown below.

OSCILLATOR TYPE SAMPLE

INTERVAL

TCXO 1 second 1 second

OCXO (XTAL_2)

High Stability OCXO (XTAL_1)

Rubidium (Rb_2 & Rb_1)

Table 11 Oscillator Sample Intervals and Measurement Periods

1 second 1 second

10 seconds 10 seconds

100 seconds 100 seconds

MEASUREMENT

10 seconds 100 seconds

100 seconds 1000 seconds

1000 seconds 10000 seconds

PERIOD

FREQUENCY

MEASUREMENT

RESOLUTION

1E-9 1E-10 1E-11 1E-9 1E-10 1E-11 1E-10 1E-11 1E-12 1E-11 1E-12 1E-13

Phase readings are stored at every sample interval and the value transmitted is updated at this point even when the measurement period is 1000 seconds. This output gives the user a continuous and uninterrupted general indication of the frequency accuracy of the internal disciplined oscillator.

3.6.6.8 UTC at Last Correction

The UTC at last correction is the time at which the last frequency controller correction was applied.

3.6.6.9 Status of the Phase and Frequency Controller

Two ASCII hexadecimal characters indicate the status of the phase and frequency controller, where bit 1 represents the described status below.

msb lsb msb lsb

PLL limit

Reject

PLL

data

not time mark

Table 12 Phase and Frequency Controller Status

/TMV

valid

MANUAL P/N 900000007 REV 2.0

Reject phase

(HW limit)

Reject phase

(noise)

Rubidium

status

cold

Medium Slow

Page 34

3.6.6.10 PLL Constraint

The PLL constraint consists of two ASCII hexadecimal characters that represent 4 bits of the status, where bit 1 represents the described status below.

msb lsb msb lsb

PLL

control

‘TCSW’

Bit 3

PLL

control

‘TCSW’

Bit 2

PLL

control

‘TCSW’

Bit 1

Table 13 PLL Constraint Status

PLL

control

‘TCSW’

Bit 0

Valid

time

code

Output or

synthesizer

fault

Frequency

control

fault

PLL control represents the user’s test command of a constraint called ‘TCSW’, which has been applied to the frequency disciplining section. The values are shown in the table below.

PLL Control

‘TCSW’

‘0’ No constraint and standard 3 time constant loop is running ‘1’ PLL is commanded to ‘FAST’ control only ‘2’ PLL loop is open with the EFC set for the center frequency ‘3’ PLL loop is open with the EFC frozen at its last setting ‘4’ PLL loop is open with the EFC set to maximum ‘5’ PLL loop is open with the EFC set to minimum

Table 14 TCSW Values

PLL Constraint

STATUS DESCRIPTION

Valid Time Set to 1 when the GPS8 Plus has synchronized to the satellite receiver. It is

reset to 0 if 8 hours elapsed without satellite reception or after the user reenters time from the keypad until the GPS 8000 has re-synchronized to the GPS time. It is reset to 0 if 15 consecutive 1 PPS recovered time pulses are outside the allocated phase limit. It is reset to 0 if a frequency error is detected.

Reject Code Set to 1 at power-on and when satellite receiver time recovery data is marked

not valid, late, or inconsistent. During this condition the GPS8 Plus is

outputting back-up time not verified by comparison with the GPS data. Output or Synthesizer Fault

Frequency Control Fault

Set to 1 if any or up to 10 output failures have been detected or if the E1/T1

synthesizer is not locked. Each of the 10 outputs is monitored for amplitude

and a failure results in this bit being set. The associated indicator LED for the

output will be turned OFF. If the E1/T1 synthesizer is faulty, a red LED

indicator is turned ON in the main PCB.

Set to 1 if the frequency controller is unable to set the oscillator frequency to

track the reference frequency from the GPS receiver.

The oscillator type consists of two ASCII hexadecimal characters that represent 4 bits of the status, where bit 1 represents the described status below.

msb lsb msb lsb

Reserved Reserved Reserved Reserved Oscillator

type Bit 3

Table 15 PLL Constraint Status

Oscillator

type

Bit 2

Oscillator

type Bit 1

Oscillator

type

Bit 0

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The oscillator type is determined by a jumper setting on the oscillator module and additional option parameters stored in the EEPROM.

LK1 Link Setting: ‘1’ = jumper is fitted and ‘0’ link is open. In addition, the ‘5-6’ link is the nearest link to CON4 and the ‘1-2’ link is the nearest link to IC2.

5-6 3-4 1-2 NO OSCILLATOR

TYPE

0 0 0 00 Get oscillator

from EEPROM*

0 0 1 01 Standard

rubidium

0 1 0 02 High precision

OCXO (SC)

0 1 1 03 Low precision

OCXO (AT)

1 0 0 04 Standard

precision

OCXO (AT)

1 0 1 05 TCXO Positive OSA8410

1 1 0 06 TCXO with

poor 1 PPS

reference

1 1 1 07 MTI230-0501

with poor 1

PPS reference

Table 16 Oscillator Jumper Settings

CONTROL

SLOPE

Positive LPFRS 24 hours

Negative MTI 250-0502 12 hours

Positive HCD81 2 hours

Negative MTI 230-0501 2 hours

- - -

Negative MTI 230-0501 12 hours

EXAMPLE

OSCILLATOR

AV5H

MINIMUM TOTAL

DISCIPLINING

TIME BEFORE

CENTER

FREQUENCY IS

AUTOMATICALLY

SAVED IN THE

BLANK EEPROM

45 minutes

EEPROM CODE

00 High grade rubidium/CS Positive LPFRS opt A 48 hours

01 – 07 AS PER TABLE ABOVE FOR LINKS 001 – 111

08 Standard grade plain XTAL Positive 09 Brandywine High Stability

Positive 9 hours

OCXO (SC)

Table 17 EEPROM Code

MANUAL P/N 900000007 REV 2.0

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3.6.7 Health Status of Satellites

Requested by h<CR><LF>

Field 1 2 3 $PFEC, GPanc, YYMMDDhhmmss, XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX*CK<CR><LF>

FIELD CONTENTS OF FIELD

1 GPanc

2 YYMMDDhhmmss

“0” “1” Unhealthy (not used for positioning or timing) “2” Healthy (used for positioning and timing)

Fixed text identifier

Almanac date/time Where YY = year, MM = month, DD = day, hh = hour, mm = minute, and ss = seconds 32 columns being the status of satellite PRN 1 – 32, where the status is as follows: Almanac not collected yet or satellite is not launched yet

3.6.7.1 Example Response

Field 1 2 3 $PFEC, GPanc, 990524095412, 22222222220022222220222222202220*4D<CR><LF>

MANUAL P/N 900000007 REV 2.0

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3.6.8 Leap Second Information (Firmware 13+)

Requested by j<CR><LF>

Field 1 2 3 4 $PRCCG, J, YYMMDDhhmmss, +1, dd*CK<CR><LF>

FIELD CONTENTS OF FIELD

1 J Fixed text identifier “J”

YYMMDDhhmmss Leap second date

Where YY = year, MM = month, DD = day, hh = hour, mm = minute, and ss = second

000000000000

+1 Leap second is to be inserted at the leap second date. –1 Leap second is to be deleted at the leap second date. 00 Leap second date has passed or the parameter is not yet

SS GPS-UTC seconds offset. 4

00 The GPS-UTC value has not yet been acquired. This field

The leap second date information has not been acquired. Usually a GPS satellite announces the date, time, and value for a leap second adjustment repeatedly for two to six months before the adjustment is actually executed. After the adjustment has been made the announcement is continued for some period of time. In this period of post-execution announcement, if the pre-execution announcement was not received, the leap second field 3 below is filled with 00 because it is no longer available from the satellites.

available.

accumulates leap seconds since the GPS system started operation on January 6, 1980. As of April 2001 this value is

13.

3.6.8.1 Example Response

Field 1 2 3 4 $PRCCG, J, 9901010000, +1, 13*CK<CR><LF>

MANUAL P/N 900000007 REV 2.0

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3.6.9 2

Pulse Output Data and Command (PCB CON6 Pin 3)

Requested by k<CR><LF>

Note that units using this pulse output are specified by specific part numbers.

A second pulse output is available internally at CON6 pin 3 and may be optionally routed to the user as a second pulse output. Sending the single letter ‘k’ followed by <CR><LF> requests a message from the GPS8 Plus that describes its 2

pulse output from CON6 pin 3. The same message format transmitted to the GPS8 Plus sets the pulse output parameters, which are stored in non-volatile RAM. The pulse output (CMOS logic levels at 0 V and 5 V internally) can be commanded to be normally low going high for the pulse duration or normally high going low for the pulse duration. There are two ways to specify the pulse occurrence and duration. Unlike the 1

pulse output, this

output cannot be normally commanded to be a time code.

1. The 2

pulse output can be specified to occur at the UTC or local time. In

this case, the 2

pulse outputs always start at the second’s edge (UTC to within ±150 ns) and terminate at a specified number of milliseconds later. The ‘don’t care’ characters in the pulse time definition allow hour, minute, or second values to be ignored. The default 2

pulse output setting is 1 PPM, a positive pulse, and 100 ms long. The minimum pulse duration is 1 ms and the maximum pulse duration is 9999 ms.

2. Alternatively, the 2

pulse output can be defined in terms of the period of the pulse stream together with the pulse active duration. The minimum period is 2 ms and the maximum period is 9999 ms. The minimum pulse length is 1 ms and the maximum pulse length is 1 ms less than the period. Immediately after receipt of the command the current pulse (if active) is terminated and the first new pulse commences at the start of the next second’s edge. If the pulse period (specified in milliseconds) is a submultiple or a multiple of the seconds, the pulses remain synchronized with 1 PPS. If the contrary is true, the pulses may require multiple seconds before the start of a pulse coincides with 1 PPS. However, they are always strictly related to real-time milliseconds as long as the unit is locked to the GPS.

Remember that if the checksum is omitted in the user command to the GPS8

Plus, the ‘*’ must be omitted from the end of the message so that the GPS8 Plus

recognizes that no checksum is available. Two forms exist as seen below:

• Form 1 (2

pulse output at specified time, UTC or local): Field 1 2 3 $PRCCG, K, UHHMMSS, +9999*CK<CR><LF>

• Form 2 (2

pulse output with specified period): Field 1 2 3 $PRCCG, K, P9999, +9998*50<CR><LF>

MANUAL P/N 900000007 REV 2.0

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Refer to the ‘c’ command description above for more pulse command examples. The ‘c’ command is almost identical to the ‘k’ command except the ‘c’ command specifies the 1 ‘c’ for the 2

pulse output instead of the 2nd pulse output. Since ‘k’ replaces

pulse output, any checksum examples given for the ‘c’ command is

different for the ‘k’ command.

3.6.10 Location and Signal Noise Ratio of Satellites

Requested by l<CR><LF>

The response to the ‘l’ request contains a maximum of three records.

Field 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 $GPGSV, N, n, NN, SS, XX, XXX, XX, SS, XX, XXX, XX, SS, XX, XXX, XX, SS, XX, XXX, XX*CK<CR><LF>

FIELD CONTENTS OF FIELD

1 N Total number of messages (1 – 3) 2 n Number of this message 3 NN Number of satellites in line-of-site (with elevation > 5 degrees

only) (00 – 12) 4 SS 1st satellite PRN 5 XX 1st satellite elevation (degrees 05 – 90) 6 XXX 1st satellite azimuth (degrees 000 – 359) 7 XX 1st satellite signal-noise-ratio (dB Hz 00 – 99)

8 – 11 2nd satellite details 12 – 15 3rd satellite details 16 – 19 4th satellite details

3.6.10.1 Example Messages

$GPGSV, 2, 1, 07, 03, 41, 270, 51, 06, 18, 083, 33, 17, 64, 083, 49, 19, 09, 329, 45*74 $GPGSV, 2, 2, 07, 22, 74, 208, 48, 23, 21, 126, 45, 25, 06, 195, 39*4B

ABBREVIATION DESCRIPTION

PRN Shows the unique identification number of the satellite from 1 – 32. ELV Shows the elevation of each satellite in degrees. AZ Shows the azimuth of each satellite in degrees. S/N Shows the signal to noise ratio of each satellite in dB. Below 40 is poor, 40 –

46 is average, and above 46 is good. The signal to noise ratio is a relative value for a single receiver and may not be the same for any two receivers operating in identical conditions.

MANUAL P/N 900000007 REV 2.0

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3.6.11 Firmware Version Number

Requested by n<CR><LF>

The software version number is only outputted.

Field 1 2 3 4 $PRCCG, N, nnn, ppppppp-vvv, t*CK<CR><LF>

FIELD CONTENTS OF FIELD

1 Fixed text letter “N” 2 3 digit decimal firmware serial number with leading zeros included 3 7 digit engine program and 3 digit version number 4 Test result

If the firmware version is EGPS13 and the engine is 4850102 version 009, the example message will be $PRCCG, N, 013, 4850102-009, 1*CK<CR><LF>.

The firmware resides in both flash memory and EPROM. If the flash memory has been reprogrammed, the current version may be a higher number than the number written by the label on EPROM. It is possible to revert to EPROM

firmware by fitting the jumper at LK1. If flash me mory is to be repr o gra mmed,

LK1 must be open.

MANUAL P/N 900000007 REV 2.0

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3.6.12 Offset of Local Time Data and Command

Requested by o<CR><LF>

The local time parameters listed below are reported by the GPS8 Plus in response to the single character o<CR><LF> request. The same format is used in a user command transmitted to the GPS8 Plus to set the local time parameters, which are stored in non-volatile RAM. The message format must be entered exactly as shown below, including the colon and dummy data for the

minute offset (not used). Remember that if the checksum is omitted, the ‘* ’ must be omitted from the end of the message so that the GPS8 Plus recognizes that

no checksum is available.

Note that the pulse output time (refer to the ‘c’ response and associated command) has its own local time/UTC switch, which is independent of the ‘o’ command local time/UTC switch for time and time code outputs. However, the sign and number of the hour offset of the local time is programmed by the ‘o’ command alone. The user can set the GPS8 Plus time code output to UTC, but it generates pulse outputs which follow local time.

Field 1 2 3 $PRCCG, O, U, +HH:00*CK<CR><LF>

FIELD CONTENTS OF FIELD

1 Fixed text letter “O” 2 ‘U’ = time and time code outputs are UTC.

‘L’ = time and time code outputs are local time.

3 ‘+’ = adds the following hour offset to UTC to obtain the local time.

‘–’ = subtracts the following hour offset from UTC to obtain the local time. HH = number of hour offset (must have leading zero if < 10 hours or two zeros if the value is zero). The maximum offset is + or – 23 hours. :00 = hour and minute separator and data reserved for minute offset.

3.6.12.1 Example Commands

1. $PRCCG, O, L, +01:00*7A<CR><LF> The time code output and the time returned in response to the ‘t’ time request is local time. The local time is the UTC time added with an hour. The count offset time rolls over through days to years if required.

2. $PRCCG, O, L, -23:00*7C<CR><LF> The time code output and the time returned in response to the ‘t’ time request is local time. The local time is the UTC time subtracted with 23 hours. The count offset time borrows from days through to years if required.

When a command has been accepted by the GPS8 Plus the message is echoed, but the contents of the response are recalculated from the data used by the GPS8 Plus to implement the user request. A checksum is always returned.

MANUAL P/N 900000007 REV 2.0

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3.6.13 Position of GPS Antenna

Requested by p<CR><LF>

Field 1 2 3 4 5 6 7 8 9 10 11 12 13 14 $GPGGA, hhmmss, DDMM.MMMM, N, DDDMM.DDDD, E, s, NN, 00.00, AAAAAA.A, M, GGGG.G, M,.,*CK<CR><LF>

FIELD CONTENTS OF FIELD

1 hhmmss Time (UTC) of position update 2 DDMM.MMMM Latitude (degrees, minutes, and decimal fraction of minutes) 3 N “N” = north and “S” = south 4 DDDMM.MMMM Longitude 5 W “W” = west and “E” = east 6 Status “0” = positioning not operational (or known position)

“1” = positioning operational

“2” = differential positioning 7 NN Number of satellites used for positioning 8 DOP DOP (2D: HDOP, 3D: PDOP) 9 Altitude AAAAAA.A (– 00999.9 to 017999.9)

10 M Unit for altitude (M = meters) 11 GGGG.G Geoid altitude (– 999.9 to 9999.9) 12 M Unit for Geoid altitude (M = meters) 13 Null field Reserved for DGPS data (time elapsed since last RTCM-

SC104 data updating)

14 Null field Reserved for DGPS station identification

3.6.13.1 Example Response

$GPGGA, 095427, 5147.1358, N, 00049.8371, E, 1, 06, 01.71, 000026.0, M, 0046.3, M,.,*73

MANUAL P/N 900000007 REV 2.0

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3.6.14 Additional Information (Including Magnetic Declination)

Requested by s<CR><LF>

Field 1 2 3 4 5 6 7 8 9 10 11 $GPRMC, hhmmss, A, DDMM.MMMM, N, DDDMM.DDDD, E ,000.0, 000.0, DDMMYY, MMM.M, W*CK<CR><LF>

FIELD CONTENTS OF FIELD

1 hhmmss Time (UTC) of position update 2 A or V A = positioning

V = positioning interrupted or known position 3 DDMM.MMMM Latitude (degrees, minutes, and decimal fraction of minutes) 4 N “N” = north and “S” = south 5 DDDMM.MMMM Longitude (degrees, minutes, and decimal fraction of

minutes) 6 W “W” = west and “E” = east 7 Speed Speed in knots (000.0 to 999.9) 8 True course True course in degrees (000.0 to 359.9) 9 DDMMYY UTC date as day, month, and year (1994 to 2040)

10 MMM.M Magnetic deviation in degrees (000.0 to 180.0) 11 W “W” (MAG = TRUE – DEV) or “E” (MAG = TRUE + DEV)

3.6.14.1 Example Response

$GPRMC, 095431, A, 5147.1362, N, 00049.8371, E, 000.3, 005.7, 240599, 003.1, W*6F

3.6.15 Request Precision Time

Requested by t<CR><LF>

The purpose of this request is to capture time for data logging, time tagging, and etcetera. This message takes priority over other messages which will be inhibited while ‘t’ is active. If a message is already being transmitted, use of the ‘t’ request may cause a disjointed message when the high priority output is immediately transmitted. The time in the message reflects the instance the stop bit of ‘t’ was received and advanced by 1 ms to compensate for the typical character delay. If the serial port is busy when a request is received, it will be ignored. Maximum request rate will depend on the baud rate. For example, at 4800 baud the serial port can transmit about ten messages per second. The format of the message is type 2 (refer to section 3.7).

MANUAL P/N 900000007 REV 2.0

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3.6.16 Software Reset Command

The unit can be reset by an NMEA style command such as the one seen below:

$PRCCG, Z, 0*2F<CR><LF>

It is not necessary (but highly recommended) that the checksum be included at the end of this message. The value is always ‘2F’ since the content of the command is fixed. The message must contain the correct header, the correct number of characters, the ‘Z’, the ‘0’, and the correct checksum. The ‘0’ refers to the number zero (ASCII code 30 hexadecimal) and describes the type of reset to use. If the message corresponds correctly to the reset command and the checksum is valid, the GPS8 Plus returns the message as an acknowledgment and waits 160 ms and then restarts the initialization as though the unit had just been switched on. The initialization messages are outputted as each stage is completed. Note that there is a pause of about 5 seconds after the reset (a decimal point is outputted as each second passes) in which the download of new firmware to the GPS8 Plus flash memory could be initiated.

MANUAL P/N 900000007 REV 2.0

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3.7 TxD2/RxD2 Messages (Rear Panel J12)

The GPS8 Plus second serial output is configured to output serial data in the user’s chosen format automatically after initialization. The format selection can be changed by entering a simple command into the TxD1/RxD1 command port at J13. Three formats are discussed below. Other customer specific outputs and

protocols are available. The baud rate and character format are fixed as described above (4800, 8, N, and 1). To request data from the port, the user

sends the character ‘t’ to request time. The port sends the message, reporting the time at which ‘t’ was received and the automatic 1 per second output stops. No further output is transmitted until another request ‘t’ is received. To restore the automatic 1 per second output, the user must send ‘r’ to the port.

3.7.1 Type 1 Format

Field 1 2 3 4 HH:MM:SS DD/MN/YY NNN W<CR><LF>

FIELD CONTENTS OF FIELD

1 Hours, minutes, and seconds (UTC) 2 Day of month, month, and year 3 Day of year (January 1 = 001) 4 Day of week (Monday = 1 and Sunday = 7)

3.7.1.1 Example Message

The example message has a fixed format of 25 characters, including spaces and termination.

20:34:03 08/11/91 312 5<CR><LF>

3.7.2 Type 2 Format

Field 1 2 3 4 5 HH:MM:SS.SSS DD/MN/YY NNN W S<CR><LF>

FIELD CONTENTS OF FIELD

1 Hours, minutes, and seconds (UTC) 2 Day of month, month, and year 3 Day of year (January 1 = 001) 4 Day of week (Monday = 1 and Sunday = 7) 5 Status

MANUAL P/N 900000007 REV 2.0

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The status character is an ASCII representation of the hexadecimal figure from 0 to F (refer to section 3.3) where the bits are allocated as follows:

BIT 3 (MSB) BIT 2 BIT 1 BIT 0 (LSB)

Valid time Reject code Local time Leap year

Table 18 Status Bit Allocation

• Valid time is set to 1 when the GPS8 Plus has synchronized to the satellite

receiver. It is reset to 0 if 8 hours elapsed without satellite reception or after the user re-enters time from the keypad until the GPS 8000 has resynchronized to the GPS time. It is reset to 0 if 15 consecutive 1 PPS recovered time pulses are outside the allocated phase limit. It is reset to 0 if a frequency error is detected.

• Reject code is set to 1 at power on and when the satellite receiver time

recovery data is marked not valid, is late, or inconsistent. During this condition the GPS8 Plus is outputting back up time not verified by comparison with the GPS data. It is quite normal for the occasional ‘Reject Data’ indicator to illuminate.

• Local time is set to 1 for local time and reset to 0 for UTC time.

• Leap year is a year divisible by 4, including the century if it is also divisible

by 4 (e.g. 2000 but not 2100).

3.7.2.1 Example Message

The example message has a fixed format of 31 characters, including spaces.

The example below means a request was made at thirty four minutes, three seconds, and 123 milliseconds past eight in the evening on Tuesday the 15

of March 1988 with the GPS synchronized master clock reporting valid time, GPS locked, and UTC in a leap year.

20:34:03.123 17/03/88 077 2 9<CR><LF>

The character ‘9’ status is described in the table below:

BIT 3 (MSB) BIT 2 BIT 1 BIT 0 (L SB)

1 0 0 1

Valid time Synchronized

last second

(no reject

code)

Table 19 Character ‘9’ Status

UTC Year is a leap

year

The second port responds identically to TxD1/RxD1. Both ports can be polled rapidly and asynchronously to report the time to independent users.

MANUAL P/N 900000007 REV 2.0

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3.7.3 Type 3 Format (GPS4 Format)

Field 1 2 3 4 YYYY, DDD:HH:MM:SS, T, S<CR><LF>

FIELD CONTENTS OF FIELD

1 Year (UTC) 2 Day of year (January 1 = 001), hours, minutes, and seconds 3 TFOM 4 Status of time

Please note that TFOM refers to Time Figur e of Merit and S r efers to Status.

TFOM DESCRIPTION

9 10 ms < PPS 8 1 ms < PPS ≤10 ms 7 100 µs < PPS ≤ 1 ms 6 10 µs < PPS ≤ 100 µs 5 1 µs < PPS ≤ 100 ns 4 100 ns < PPS ≤ 1 µs 3 PPS ≤ 100 ns

Table 20 TFOM Values

Status S DESCRIPTION

0 Coasting 1 Time locked 2 Holdover 3 Recovery 4 Power-up

Table 21 Status Values

3.7.3.1 Example Message

The example message has a fixed format of 23 characters, including commas and termination.

2001, 156:15:17:43, 3, 1<CR><LF>

MANUAL P/N 900000007 REV 2.0

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3.8 IRIG B and AFNOR NF 2 87-500 Time Code Output

IRIG B and AFNOR NF 2 87-500 time code is the generated standard by the GPS8 Plus. This time code consists of a 1 KHz carrier, which is modulated in the pattern described below. The carrier and time code is synchronized precisely to the GPS time by the GPS8 Plus. To select the desired time code, use the NMEA style ‘e’ command described in section 3.6.5. The type of code in operation is queried by sending e<CR><LF> to the command port.

FORMAT DESCRIPTION

Time Universal Time (UTC)

Time frame 1.0 second

Code digit weighting BCD and SB as follows:

a Binary Coded Decimal time-of-year code word: 30 binary digits.

(1) Seconds, minutes, hours, and days. (2) Recycles yearly.

b Straight Binary time-of-day code word: 17 binary digits.

(1) Seconds only. (2) Recycles every 24 hours.

a BCD: word begins at index count 1. Binary coded elements occur

between the position identifier elements (7 for seconds, 7 for minutes, 6 for hours, 8 and 2 for days) until the code word is complete. An index marker occurs between the decimal digits in each group to provide separation for visual resolution.

b SB: word begins at index count 80. Five decimal digits (17 binary

coded elements) occur with a position identifier between the 9 binary coded elements.

Bit order Least significant digit occurs first.

Element rate 100 per second

Element identification

a On time reference point for all elements is the leading edge. b Index marker: 2 ms

(Binary zero or un-coded element) c Code digit : 5 ms d Position identifier - 10 per second: 8 ms

(Refers to the leading edge of the succeeding element) e Reference marker - 1 per second: Two consecutive position identifiers.

Table 22 IRIG B Format Specificatio n

The ‘on time’ point to which the code word refers to is the leading edge of the second position identifier.

• Resolution: 10 ms (un-modulated) or 1 ms (modulated)

• Carrier frequency: 1 KHz (modulated)

and 10th

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3.8.1 IRIG B and AFNOR NF 2 87-500 Time Code Bit Allocation

Each time code bit has 10 ms of duration therefore it contains 10 cycles of the 1 KHz carrier. In the table below ‘0’ is 2 ms higher level duration, ‘1’ is 5 ms higher level duration, and ‘Marker’ is 8 ms higher level duration. The standard output level for the higher level carrier (AFNOR specification) is 0 dB into 600R (2.17 V

± 10%). The lower level is ⅓ of the higher level or the higher level - 10 dB.

There are 100 bits total in one second with bit 0 (an 8 ms marker) starting at the second’s edge. The GPS8 Plus allows the selection of IRIG B time code without the AFNOR extensions (TC1 = 1 in the ‘e’ command) or with AFNOR (TC1 = 5 in the ‘e’ command). If IRIG B is selected, the control bits are set to ‘0’.

BIT WEIGHT CODE BIT WEIGHT CODE

00 Marker Pr IRIG B 50 Year 1 AFNOR 01 Seconds 1 IRIG B 51 Year 2 AFNOR 02 Seconds 2 IRIG B 52 Year 3 AFNOR 03 Seconds 4 IRIG B 53 Year 4 AFNOR 04 Seconds 8 IRIG B 54 Always 0 IRIG B 05 Always 0 IRIG B 55 Year 10 AFNOR 06 Seconds 10 IRIG B 56 Year 20 AFNOR 07 Seconds 20 IRIG B 57 Year 40 AFNOR 08 Seconds 40 IRIG B 58 Year 80 AFNOR 09 Marker P1 IRIG B 59 Marker P6 IRIG B 10 Minutes 1 IRIG B 60 Month 1 AFNOR 11 Minutes 2 IRIG B 61 Month 2 AFNOR 12 Minutes 4 IRIG B 62 Month 4 AFNOR 13 Minutes 8 IRIG B 63 Month 8 AFNOR 14 Always 0 IRIG B 64 Always 0 IRIG B 15 Minutes 10 IRIG B 65 Month 10 AFNOR 16 Minutes 20 IRIG B 66 Always 0 AFNOR 17 Minutes 40 IRIG B 67 Always 0 AFNOR 18 Always 0 IRIG B 68 Always 0 IRIG B 19 Marker P2 IRIG B 69 Marker P7 IRIG B 20 Hours 1 IRIG B 70 Day of month 1 AFNOR 21 Hours 2 IRIG B 71 Day of month 2 AFNOR 22 Hours 4 IRIG B 72 Day of month 4 AFNOR 23 Hours 8 IRIG B 73 Day of month 8 AFNOR 24 Always 0 IRIG B 74 Always 0 AFNOR 25 Hours 10 IRIG B 75 Day of month 10 AFNOR 26 Hours 20 IRIG B 76 Day of month 20 AFNOR 27 Always 0 IRIG B 77 Always 0 IRIG B 28 Always 0 IRIG B 78 Always 0 IRIG B 29 Marker P3 IRIG B 79 Marker P8 IRIG B 30 Day of year 1 IRIG B 80 Counter clock 2^0 IRIG B 31 Day of year 2 IRIG B 81 Counter clock 2^1 IRIG B 32 Day of year 4 IRIG B 82 Counter clock 2^2 IRIG B 33 Day of year 8 IRIG B 83 Counter clock 2^3 IRIG B 34 Always 0 IRIG B 84 Counter clock 2^4 IRIG B 35 Day of year 10 IRIG B 85 Counter clock 2^5 IRIG B 36 Day of year 20 IRIG B 86 Counter clock 2^6 IRIG B 37 Day of year 40 IRIG B 87 Counter clock 2^7 IRIG B 38 Day of year 80 IRIG B 88 Counter clock 2^8 IRIG B 39 Marker P4 IRIG B 89 Marker P9 IRIG B 40 Day of year 100 IRIG B 90 Counter clock 2^9 IRIG B 41 Day of year 200 IRIG B 91 Counter clock 2^10 IRIG B 42 Always 0 IRIG B 92 Counter clock 2^11 IRIG B 43 Always 0 IRIG B 93 Counter clock 2^12 IRIG B 44 Day of week 1 AFNOR 94 Counter clock 2^13 IRIG B 45 Day of week 2 AFNOR 95 Counter clock 2^14 IRIG B 46 Day of week 4 AFNOR 96 Counter clock 2^15 IRIG B 47 Always 0 IRIG B 97 Counter clock 2^16 IRIG B 48 Always 0 IRIG B 98 Counter clock 2^17 IRIG B 49 Marker P5 IRIG B 99 Marker P0 IRIG B

MANUAL P/N 900000007 REV 2.0

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MANUAL P/N 900000007 REV 2.0

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3.9 NASA36 Time Code Output

NASA36 time code can be selected by the user using the ‘e’ command. NASA36 time code operates in a similar manner to IRIG B with the bit allocations described below. 1 KHz is the standard carrier for NASA36 time code. To select the desired time code, use the NMEA style ‘e’ command described in section 3.6.5. The type of code in operation is queried by sending e<CR><LF> to the command port.

FORMAT DESCRIPTION

Time Universal Time (UTC) or local time

Time frame 1.0 second

Code digit weighting BCD as follows:

a Binary Coded Decimal time-of-year code word: 30 binary digits.

(1) Seconds, minutes, hours, and days.

(2) Recycles yearly.

Word begins at index count 1. Binary coded elements occur between

the position identifier elements 600 ms duration every 100 ms from 100

ms to 900 ms inclusive.

Bit order Least significant digit occurs first.

Element rate 100 per second

Element identification

a On time reference point for all elements is the leading edge. b Index marker: 2 ms

(Binary zero or un-coded element) c Code digit : 6 ms

(Binary one) d Position identifier - 9 per second: 6 ms

(Refers to the leading edge of the succeeding element) e Reference marker - 1 per second: Five consecutive position identifiers,

followed by a zero

Resolution 10 ms (un-modulated)

1 ms (modulated)

Carrier frequency 1 KHz when modulated

Table 23 NASA36 Format Specification

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BIT WEIGHT BIT WEIGHT

00 Reference always 0 50 Position identifier 01 Seconds 1 51 Hours 10 02 Seconds 2 52 Hours 20 03 Seconds 4 53 Always 0 04 Seconds 8 54 Always 0 05 Always 0 55 Always 0 06 Always 0 56 Always 0 07 Always 0 57 Always 0 08 Always 0 58 Always 0 09 Always 0 59 Always 0 10 Marker 60 Position identifier 11 Seconds 10 61 Day of year 1 12 Seconds 20 62 Day of year 2 13 Seconds 40 63 Day of year 4 14 Always 0 64 Day of year 8 15 Always 0 65 Always 0 16 Always 0 66 Always 0 17 Always 0 67 Always 0 18 Always 0 68 Always 0 19 Always 0 69 Always 0 20 Position identifier 70 Position identifier 21 Minutes 1 71 Day of year 10 22 Minutes 2 72 Day of year 20 23 Minutes 4 73 Day of year 40 24 Minutes 8 74 Day of year 80 25 Always 0 75 Always 0 26 Always 0 76 Always 0 27 Always 0 77 Always 0 28 Always 0 78 Always 0 29 Always 0 79 Always 0 30 Position identifier 80 Position identifier 31 Minutes 10 81 Day of year 100 32 Minutes 20 82 Day of year 200 33 Minutes 40 83 Day of year 400 34 Always 0 84 Day of year 800 35 Always 0 85 Always 0 36 Always 0 86 Always 0 37 Always 0 87 Always 0 38 Always 0 88 Always 0 39 Always 0 89 Always 0 40 Position identifier 90 Position identifier 41 Hours 1 91 Control functions 42 Hours 2 92 Control functions 43 Hours 4 93 Control functions 44 Hours 8 94 Control functions 45 Always 0 95 Position identifier 46 Always 0 96 Position identifier 47 Always 0 97 Position identifier 48 Always 0 98 Position identifier 49 Always 0 99 Position identifier

Table 24 NASA36 Bit Allocation

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3.10 2137 Time Code Output

The 2137 time code can be selected by the user. 1 KHz is the standard carrier frequency for 2137, which otherwise resembles the code known as XR3. To select the desired time code, use the NMEA style ‘e’ command described in section 3.6.5. The type of code in operation is queried by sending e<CR><LF> to the command port.

FORMAT DESCRIPTION

Time Universal Time (UTC)

Time frame 1.0 second

Code digit weighting BCD

Binary Coded Decimal time-of-day code word: 20 binary digits.

Hours, minutes, and seconds. Recycles every 24 hours.

Code word structure Word begins during the first 40 ms index count of the current time

frame. Binary coded elements occur every 40 ms during the current

time frame (6 for hours, 7 for minutes, and 7 for seconds) until the code

word is complete. A reference marker occurs during the last 40 ms

index count of the current time frame.

Bit order Most significant digit occurs first.

Element rate 25 per second

Element identification

a On time reference point for all elements is the leading edge. b Index marker: 12 ms

(Binary zero) c Code digit: 24 ms

(Binary one) d Reference marker - 1 per second: 36 ms

Resolution 1 ms (1 KHz modulated)

4 ms (250 Hz modulated)

200 ms (un-modulated)

Carrier frequency 1 KHz or 250 Hz when modulated

Table 25 2137 Format Speci f ication

The ‘on time’ point which the code word refers to is the trailing edge of the reference marker.

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3.11 VELA Time Code Output (Slow Code)

VELA time code can be selected by the user. To select the desired time code, use the NMEA style ‘e’ command described in section 3.6.5. The type of code in operation is queried by sending e<CR><LF> to the command port.

The VELA logic level time code output is a positive pulse commencing at the second's edge with a duration of 200, 500, or 800 ms according to the details below.

SECOND FUNCTION DURATION SECOND FUNCTION DURATION

0 P0 800 ms 30 P3 800 ms 1 Year 8 31 200 ms 2 Year 4 32 Minutes 40 3 Year 2 33 Minutes 20 4 Year 1 34 Minutes 10 5 200 ms 35 Minutes 8 6 200 ms 36 Minutes 4 7 DOY 200 37 Minutes 2 8 DOY 100 38 Minutes 1

9 200 ms 39 200 ms 10 P1 800 ms 40 P4 800 ms 11 DOY 80 41 MSS 8000 12 DOY 40 42 MSS 4000 13 DOY 20 43 MSS 2000 14 DOY 10 44 MSS 1000 15 DOY 8 45 MSS 800 16 DOY 4 46 MSS 400 17 DOY 2 47 MSS 200 18 DOY 1 48 MSS 100 19 200 ms 49 200 ms 20 P2 800 ms 50 P5 800 ms 21 200 ms 51 MSS 80 22 200 ms 52 MSS 40 23 Hours 20 53 MSS 20 24 Hours 10 54 MSS 10 25 Hours 8 55 MSS 8 26 Hours 4 56 MSS 4 27 Hours 2 57 MSS 2 28 Hours 1 58 MSS 1 29 200 ms 59 R 800 ms

P0 to P5 and R are 800 ms markers. The durations not shown are 200 ms for a zero weighting or 500 ms for the weighting shown. The count MSS is the count of minutes elapsed since the clock has been synchronized. It is set to 9999 at switch on, reset at synchronization, and incremented once per minute if the clock is not synchronized. If the counter reaches the maximum, it ceases to count any further. The maximum count of 9999 minutes represents 6.9 days.

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3.12 Alarm Contacts (J14)

The status output is provided by relay RL1, which has a normally open and a normally closed contact with changeover to a common terminal. LK5 inside of the unit allows jumpers to be placed which provides optional connection to 0 V and 5 V for Alarm_Open and Alarm_Closed respectively as shown below. Signal NALRM turns on TR1 to hold the relay in a non-alarm state. If signal NALRM goes ‘low’, the relay contact changes to the alarmed condition. If power is available, the red STATUS indicator will be illuminated on the front panel of the unit.

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4 GPS8 Plus Keypad and Display Operation

4.1 Scope of Section

Section 4 provides a description of the GPS8 PLUS keypad and display operation.

4.2 General Principles of Operation of Keypad and Display

The display has two lines of 24 characters and its function is organized as a series of two line screens displaying information to the user. The screens are easily selected after initialization by use of the arrow keys, pressing the Up Arrow key for the next screen or the Down Arrow key for the previous screen.

The keypad offers simple yet rapid operation through 4 keys, the ‘Up Arrow’, ‘Down Arrow’, ‘ENTER’, and ‘ESCAPE’.

Some screens allow the user to enter data for GPS8 Plus configuration. Pressing ENTER will allow the user to enter data, which will freeze the current screen and turn on a cursor next to the item to be modified. After modification, the user may exit the display without saving the entry by pressing ESCAPE or confirm the entry by pressing ENTER. If a screen does not allow data entry and ENTER is pressed in an attempt to enter data, the brief message seen below is displayed.

*NO DATA ENTRY POSSIBLE*

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4.3 Messages after Power On

At power on an update of the screen will begin. A blank screen such as the one seen below is displayed. During this time the user can initiate the download of new firmware to the flash memory of the GPS8 Plus.

▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒

The series of messages below are displayed during the initialization process while the initialization and test are carried out. Afterwards, the main default screen with the time of day and status is finally reached.

RAM...PASS NV_RAM...PASS

ROM...PASS

RAM...PASS NV_RAM...PASS

Test Alarm & LEDs....

POWERUP 00:00:07 001 UTC

THU JAN1 2004

At power down the GPS8 Plus does not retain the time and date. At power up it increments the time from switch on. The screen with the time indicates the status of the time as seen in the POWERUP screen above. Other screens are selected by pressing the Up Arrow and Down Arrow keys, which are described below. When a different screen is selected from the main default screen and data entry is not in progress, pressing ESCAPE will return the user to the main default screen.

At the main default screen when data entry is not in progress, pressing ESCAPE will select and display the internal sounder state. The three possible internal sounder states are shown below. The message describing the sounder state is displayed briefly on the top line of the screen.

SOUND IS GPS ENGINE 1PPS

WED MAR 24 2004

SOUND IS INT OSC 1PPS

WED MAR 24 2004

SOUND IS OFF

WED MAR 24 2004

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4.3.1 Op eratin g M od e Sele ction

GPSLOCK 21:30:25 084 UTC

WED MAR 24 2004

The main default screen normally looks like the screen seen above after a few minutes of operation. The first field in the top line allows the user to select one of three operating modes. The mode is selected by pressing ENTER, which displays an arrow with a blinking cursor. Pressing the Up Arrow and Down Arrow keys allow the user to review the three modes of operation listed in the table below. A mode is selected by pressing ENTER. This mode is remembered after power down therefore this mode continues at the next power up.

MODE DESCRIPTION

GPSLOCK Normal operation of the GPS8 Plus as a disciplined GPS time and frequency

standard.

FREERUN Operation of the GPS8 Plus as a free running time and frequency source. In this

mode the user can enter the time and date and press ENTER to run from that time. No corrections are applied from the GPS. If the selection of FREERUN is confirmed by pressing ENTER followed by ESCAPE to quit the time and date entry, the GPS8 Plus will continue to run from the current time with the frequency control frozen at the last setting. The subsequent time difference over 1 second against the GPS can be seen on the Instantaneous Phase report. Mean Phase and Frequency Difference are estimated until the phase error exceeds 15 microseconds.

PPSLOCK Operation of the GPS8 Plus as a disciplined time and frequency standard using only

the 1 PPS derived from the GPS receiver. In this mode the user can enter the time and date and press ENTER to run from that time. The entered time is corrected to run in synchronism with the real time 1 PPS from the GPS receiver with no use of the time or date information from the GPS.

4.3.2 GP S Lo cked Mode

This is the default mode during normal operation as a disciplined time and frequency standard. The normal status of GPSLOCK may be replaced by one of the statuses described in the table below.

STATUS DESCRIPTION

POWERUP GPSLOCK is selected, but the GPS8 Plus has not yet acquired time and is running

from the initialized time and date at switch on with frequency outputs set using corrections remembered from the previous operation.

HLDOVER GPSLOCK is selected, but data from the GPS receiver cannot be used or is not

available. The time and date continue to count from the last time locked to the GPS. (The last frequency controller update time, day number, and elapsed holdover time are available on other displays.)

SLEWING GPSLOCK is selected, the time and date have been acquired, and the

synchronization process is slewing the GPS8 Plus time and date into synchronism with the GPS.

LOCKING GPSLOCK is selected, the time and date have been acquired, but the continuous

locking process has not been completed and the time and date are not confirmed as valid.

GPSLOCK Normal presentation when the time displayed from the GPS8 Plus clock/calendar

was updated from the GPS data in the previous second and the frequency of the time-base is being corrected by the 1 PPS from the GPS.

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4.3.3 Free Run Mode

Only the description FREERUN is displayed in the status field for this mode. The status is described below.

STATUS DESCRIPTION

FREERUN The time clock is running from the GPS8 Plus internal TCXO, OCXO, or Rubidium

frequency standard and is never updated by data from the GPS. Likewise, the frequency of the time-base is fixed at the last correction value applied by the controller. If this mode was selected after GPSLOCK and the user pressed ESCAPE to quit the time and date entry, then the time is continuing to count from the last GPS time received. Otherwise it counts from a keypad entered time (see section 4.3.5) or from the initialized time at switch-on.

4.3.4 1 PPS Lock Mode

When PPSLOCK mode has been selected the three possible status indications shown in the table below replace PPSLOCK until the locking process is complete or during moments when the locking process is interrupted. The statuses are described below.

STATUS DESCRIPTION

PPSLOCK The GPS8 Plus is running as a disciplined time and frequency standard using only

the 1 PPS derived from the GPS receiver as a reference. If this mode was selected after GPSLOCK and the user pressed ESCAPE to quit the time and date entry, then the time is continuing to count from the last GPS time received. Otherwise it counts from a keypad entered time (see section 4.3.5) or from the initialized time at switchon.

PPSLKNG The locking process has not been completed. The time displayed is the time

continuing from the moment the PPSLOCK mode was selected or from manual keypad entry, which is being synchronized by the GPS8 Plus to the reference 1 PPS update.

NO 1PPS The GPS8 Plus is in PPSLOCK mode, but no 1 PPS is being detected. Time and

date continue to be maintained from the free-running GPS8 Plus internal frequency standard.

4.3.5 Manual Time and Date Entry

The time and date can only be entered in FREERUN or PPSLOCK mode and only UTC may be entered. Pressing ENTER will freeze the displayed time and date (while all outputs continue with the current advancing time and date). When ENTER is pressed again this confirms the mode. The displayed up arrow with the blinking cursor moves to the colon position allowing the user to enter the hours. At any point prior to the last date entry the user may quit the time entry process by pressing ESCAPE and the GPS8 Plus time and date will be restored. The Up Arrow and Down Arrow keys are used to adjust each field and the selected value is confirmed by pressing ENTER. After the selected value is confirmed the blinking cursor moves to the next field. This way the user proceeds in the order of hours, minutes, seconds, year, month, and day of month number. The year number is selected or confirmed prior to the day of month entry to allow the correct number of days to be displayed for February.

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For FREERUN mode, pressing ENTER after the day of month number updates the time-base values with the entered values and the count will continue from this moment. The Time Valid indicator is illuminated and the outputs that use the status show time as being valid to permit the testing of systems that require it. The day of week is calculated by the GPS8 Plus and does not require data entry.

For PPSLOCK mode, pressing ENTER starts a synchronization process that synchronizes the entered time to the 1 PPS reference. Synchronization is a combination of jam-sync, time-slewing, and fine frequency adjustments depending on the magnitude of the time interval between the user’s final press of the ENTER key and the 1 PPS update. When lock has been acquired, PPSLOCK is displayed and the Time Valid indicator is illuminated.

4.4 Position Screen

From the time and date screen, pressing the Up Arrow key once selects the position screen.

N51°47.1378’E000°49.8364

ALT:00006.5m MODE: KNWN

No data entry is possible for this screen. The top line shows the latitude in degrees, minutes, and decimal fractions of minutes followed by the longitude in the same units. The second line shows the altitude in meters. The following four possible modes are described in the table below.

MODE DESCRIPTION

NONE Initialized value of the field while the status is being established from the receiver.

ACQ The receiver is acquiring satellite and almanac data, if required. The GPS8 Plus

requires a recent UTC parameter from the almanac and will not indicate valid time prior to this acquisition. The position display is not yet valid.

NAV The receiver is navigating – a valid position is displayed.

KNWN The GPS8 Plus has averaged the position data over 24 hours of operation,

downloaded the precise position to the receiver, and commanded it into KNOWN position mode. The ‘known’ position is the one displayed. This is the standard mode of operation giving the highest time and frequency accuracy for static users of the GPS8 Plus. A dynamic position finding mode is available for other applications. Selection is described in section 4.11.

4.5 Satellites Tracked Screen

From the position screen, pressing the Up Arrow key once selects the satellites tracked screen.

SATELLITES TRACKED: 8

03 15 16 18 21 22 27 31

No data entry is possible for this screen. The top line shows the number of satellites tracked and the bottom line shows their PRN numbers.

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4.6 1 PPS Status Screen

From the satellites tracked screen, pressing the Up Arrow key once selects the 1 PPS status screen.

1PPS STATUS: SAMPLE USED

INST PHASE:+50ns

No data entry is possible for this screen. The top line gives some information about the status of the reference 1 PPS and the bottom line indicates the latest instantaneous time interval from the GPS8 Plus 1 PPS to the reference 1 PPS. A positive sign indicates that for this sample, the unit 1 PPS was ahead of the reference 1 PPS from the receiver. The measurement is normally displayed in nanoseconds, but for large time differences microseconds or milliseconds are used. For example, a GPS8 Plus fitted with TCXO (the lowest grade of internal oscillator) that has been allowed to free run for several days may have a screen similar to the one seen below.

1PPS STATUS: FREE RUN

INST PHASE:+4.03240ms

The status indications that are possible are explained in the table below. The first message in the list is the one that is displayed if that status is detected, but other conditions further down the list may also apply. The aim is to give the user the best possible guide to the 1 PPS measurement and the GPS8 Plus subsequent action. The magnitude of the instantaneous measurement is always reported even when it cannot be used.

STATUS DESCRIPTION

FREE RUN 1 PPS samples are not being used because the user has selected the FREE

RUN mode of operation. The 1 PPS instantaneous phase continues to be reported.

DAC AT LIM The 16 bit DAC controlling the GPS8 Plus reference oscillator frequency has

less than 12% control range remaining. The most probable explanation is that the user has commanded the DAC to its maximum or minimum setting for test purposes and the message is a reminder of why 1 PPS samples cannot be used.

TIMREC NVAL The 1 PPS reference could not be used because the 1 PPS is marked as

invalid by the GPS receiver.

REJECT TIME The time data from the receiver for this second was rejected by the GPS8

Plus because of apparent data errors.

PHASE LIMIT The 1 PPS instantaneous phase measurement is rejected because the value

exceeds the limit accepted for the GPS8 Plus frequency reference.

LOOP FROZEN The user has initiated a GPS8 Plus test (such as centering the DAC) so the

frequency controller is disabled.

SAMPLE USED The instantaneous phase measurement has been used in the phase averager

and for frequency estimation.

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4.7 Mean Phase Screen

From the 1 PPS status screen, pressing the Up Arrow key once selects the mean phase screen.

MEAN PHASE: 29ns

HOLDOVER: 00:00:00

No data entry is possible for this screen. Calculation of the mean phase of the GPS8 Plus 1 PPS from the received 1 PPS is carried out for the 1 PPS samples that have been accepted as valid. The samples are placed in a buffer to allow a moving average calculation of the phase. At each new second, if the sample is accepted, it is placed in the buffer while the oldest accepted sample is removed and the mean of the buffer contents is calculated. The first line of the screen shows the result. The second line of the screen has a count of hours, minutes, and seconds with a maximum of 99 hours, 59 minutes, and 59 seconds. The count is incremented during any period in which the GPS8 Plus is unable to obtain or use correction data for its time and frequency controller and is reset when the correction data is again available.

The mean phase calculation is carried out even when in FREE RUN mode. However, the magnitude of the phase measurements accepted depend on the quality of the oscillator fitted to the GPS8 Plus and when the range is exceeded the screen will typically show the message seen below.

MEAN PHASE: OUT OF RANGE

HOLDOVER: 84:12:48

In this example the GPS8 Plus has been commanded into the FREE RUN mode and the phase readings are out of range for the averager. The instantaneous phase which is always reported over a range of ±0.5 seconds will be available on the previous screen. The HOLDOVER counter shows that the GPS8 Plus has run for over 84 hours in this mode.

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4.8 Estimated Mean Frequency

From the mean phase screen, pressing the Up Arrow key once selects the estimated mean frequency screen.

ESTIMATED MEAN FREQUENCY

-2.4E-11 over 1000s

FREQUENCY DESCRIPTION

ESTIMATED

MEAN

FREQUENCY

The frequency calculation from the movement in the MEAN PHASE over the measurement interval indicated. The mean phase is saved at regular sample intervals in a 100 sample buffer. The measurement interval is increased from 1 to 10 sample intervals. When 101 samples have been collected, the calculation is carried out over the full 100 samples on a rolling basis. In this stage at each sample interval the latest phase sample is entered into the buffer, the earliest is discarded, and a new frequency result is displayed. The time between samples is varied according to the oscillator type to increase the resolution of the measurement. The purpose is to give the user a guide to the most recent performance of the unit over the previous measurement interval.

4.9 Frequency Control

From the estimated mean frequency screen, pressing the Up Arrow key once selects the frequency control screen.

FRQ CTRL:-71 AUTO SLO

+0.0E-14 at 012 17:36:00

In this screen the number after FRQ CTRL: is the value of the 16 bit DAC that controls the GPS8 Plus oscillator frequency. The center frequency of a particular oscillator is not necessarily at zero and the calculated DAC position includes an offset for the oscillator calibration, which is updated after the unit has been switched on and allowed to discipline the oscillator for at least several hours. The range of control is -32768 to +32767. After the DAC value the status of the frequency controller is indicated.

STATUS DESCRIPTION

AUTO SLO The status of the controller is indicated as AUTO for normal automatic frequency

control. The word AUTO is followed by FST, MED or SLO. At start-up larger excursions in frequency are applied to help drive the local clock system quickly into phase and frequency synchronization with the GPS reference. As soon as this short term correction history has built an approximate 'integral' value for the controller (typically less than 15 minutes) the correction rate drops from FAST to MEDIUM. Control continues in MEDIUM for at least 36 hours refining the ‘integral’ before switching automatically to SLOW.

FREERUN The screen is not being updated because the FREE RUN mode has been

selected (see 4.3.1)

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4.9.1 Test Configuration of Frequency Controller

User configuration for test purposes is possible for this screen. Pressing ENTER displays the blinking cursor next to the word AUTO. Use the Up Arrow key to select one of the following modes for test purposes. Pressing ENTER confirms the selection, whereas pressing ESCAPE quits the selection and makes no

changes. When the testing has been compl e ted, make sure t hat AU TO is

reselected, otherwise the GPS8 Plus will not reacquire GPS LOCK.

MODE DESCRIPTION

FST ONLY Lock the frequency controller in the FAST update mode.

CENTER Set the electronic frequency control to its center value for oscillator testing or

adjustment.

FREEZE Lock the frequency controller DAC at its current setting.

SET MIN Set the frequency to maximum.

SET MAX Set the frequency to minimum.

Setting the frequency to maximum or minimum will cause an alarm because the DAC value exceeds the alarm threshold. The lower line of this screen shows the magnitude of the latest correction in frequency of the GPS8 Plus’ frequency standard and the time at which it was applied. In the FAST controller setting the oscillator is adjusted at every second if a valid phase comparison with the reference 1 PPS is obtained. In MEDIUM and SLOW controller setting the time between corrections depend on the quality of the installed oscillator.

4.10 Position Mode Configuration

From the frequency control screen, pressing the Up Arrow key once selects the position mode configuration screen.

POSN. MODE CONFIGURATION

MODE: AVERAGE SAMPLE:100

The screen shows the current positioning mode and in the case of the position average mode the number of current position samples. The position averaging is the default mode of operation for the GPS8 Plus. The mode can be changed to suit dynamic applications in which averaging cannot be used.

When averaging is used the target number of samples is 100. The position samples are values of latitude, longitude, and altitude obtained when the GPS8 Plus receiver is navigating with reasonable dilution of precision (DOP). The collection of 100 samples require about 24 hours during which the sample counter on this screen indicates its progress. After sample 100 has been obtained the GPS8 Plus calculates the average position, transmits it to the GPS receiver, and commands the receiver to operate in this KNOWN position. After receipt of the command the position finding mode KNWN is shown on the position screen described in section 4.4 and the position shown is the averaged position.

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4.10.1 Position Mode Selection

To select an alternative mode of operation, the user must press ENTER in this screen which will then display an up arrow with the blinking cursor positioned in front of AVERAGE as seen in the screen above. Pressing the Up Arrow or Down Arrow key allows STATIC or DYNAMIC positioning mode to be selected. The desired mode is confirmed by pressing ENTER, whereas pressing ESCAPE will quit the selection without making any changes. For the current receiver, STATIC or DYNAMIC mode offers the same type of operation. The receiver carries out position estimation continuously on a second by second basis.

4.11 Local Time Configuration

From the position mode configuration screen, pressing the Up Arrow key once selects the local time configuration screen. At this screen the user can select whether the GPS8 Plus outputs are in UTC or local time and the magnitude of the offset of the local time from the UTC.

LOCAL TIME CONFIGURATION

OUTPUT:UTC OFFSET:+01:00

The screen shows the currently selected output mode (UTC or LOC) and the hours and minutes offset that would be applied if local time outputs were selected. A positive sign indicates that the outputs will be advanced in time when the local time is selected.

4.11.1 Local Time Offset Adjustment

Pressing ENTER will position an up arrow with the blinking cursor in front of the UTC. The user can toggle from UTC to LOC by pressing an arrow key. At the desired selection ENTER is pressed, which moves the up arrow to the colon position before the sign, allowing the user to select the sign. Pressing an arrow key will toggle the sign and pressing ENTER moves the cursor to the colon position after the hour offset. The Up Arrow and Down Arrow keys are used to select the hour offset required. At this point pressing ESCAPE restores the original settings with no changes. Alternatively, pressing ENTER confirms all selections and completes the edit process for this screen.

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4.12 Time Code Configuration

From the local time configuration screen, pressing the Up Arrow key once selects the time code configuration screen. On selection its function is displayed in the top line for 2½ seconds.

TIMECODE CONFIGURATION

IRIG B TIMED TYP2 TYP3

The description TIMECODE CONFIGURATION in the top line is then replaced by the names of the output ports as headers to their current settings in line 2 of the screen.

J8 J10(DC) J13 J12

IRIG B TIMED TYP2 TYP3

4.12.1 Time Code Selection

Pressing ENTER will cause an up arrow with the blinking cursor to be displayed in front of the time code name under J8. The Up Arrow and Down Arrow keys are used to scroll through the list of IRIG B, 2137, VELA, NASA36, AFNOR, or NONE. These time codes are amplitude modulated onto a 1 KHz carrier that has positive going zero-crossings ‘on time’. The selection NONE results in an unmodulated 1 KHz sine wave output. Press ENTER to confirm the selection.

The arrow moves the description of the output to J10. J10 is a logic level output at 0 to 5 V from 50R. It can output timed pulses (at times and durations which can be selected at the next screen). Pulses at defined periods (selectable at the next screen) or un-modulated time code that is exactly the same as the time code for J8.

J10 MODE DESCRIPTION

IRIG B The port outputs the same time code as J8 as a DC level, 0 to 5 V from 50R.

IRIG B has been selected as the time code for J8.

TIMED One pulse per day to one pulse per second with 1 ms to 9999 ms duration. The

normal level can be 0 V with pulse level 5 V or the normal level can be 5 V with 0 V for the duration of the pulse. ‘Don’t care’ selections at the entry of hours, minutes, or seconds allow pulses to be outputted more frequently than once per day.

PERIOD In this mode the user defines period of pulses and pulse length. The period range

is 2 ms to 9999 ms and the pulse length is 1 ms to 9998 ms (always less than the period).

NONE No output from J10.

Section 4.14 describes how to set up the TIMED or PERIOD parameters for the pulse output from J10.

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4.12.2 J13 TXD1 Configuration

After confirmation of the J10 mode, pressing ENTER places the cursor next to J13. The description ‘TYP2’ is fixed for this port and cannot be changed by the arrow keys. J13 is the GPS8 Plus asynchronous serial interface RxD1/TxD1 which provides time, position, and status data on request from the user (see section 3.5). As regards to the time code, it has a fixed Type 2 serial data message, which is requested by the user by sending the ‘t’ request to J13 RxD.

4.12.3 J12 TXD2 Configuration

Pressing ENTER again places the cursor next to the output from J12. J12 carries the TxD2/RxD2 asynchronous serial interface and the outputs are in one of three message formats described in section 3.7. The arrow keys make the format selection. Pressing ENTER confirms all selections in this screen. The data is automatically outputted once per second in the selected format unless it is requested by transmission of the ‘t’ request to RxD2 at J12. After the ‘t’ request the unit responds with a single message to each ‘t’ received. The automatic mode is resumed by sending ‘r’ to the port.

TYPE EXAMPLE OF OUTPUT FROM J12

TYP1 00:00:00 00/00/00 000 0<CR><LF> until time is valid

20:34:03 08/11/91 312 5<CR><LF> TYP2 20:34:03.123 17/03/88 077 2 9<CR><LF> TYP3 2001, 156:15:17:43, 3, 1<CR><LF>

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4.13 J10 Pulse Output Configuration

From the time code configuration screen, pressing the Up Arrow key once selects the J10 pulse output configuration screen. On selection the function of the screen is shown in the top line for 2½ seconds.

J10 PULSE CONFIGURATION

CODE: IRIG B (TC1 AS DC)

The function message is replaced by a line describing the output from J10. Pressing ENTER positions the cursor on the up arrow before TIMECODE. Pressing the Up Arrow or Down Arrow key selects other possible modes for the J10 output.

J10 :↑TIMECODE (DC)

CODE: IRIG B (TC1 AS DC)

J10 :↑TIME LENGTH POL

UTC 00:00:00 100ms +

J10 :↑PERIOD LENGTH POL

1000ms 100ms +

J10 :↑ NO OUTPUT

4.13.1 J10 Timed Pulse Configuration

J10 : TIME LENGTH POL

UTC↑00:00:00 100ms +

Select TIME as the mode for the pulse and press ENTER. The up arrow moves to the position after UTC allowing the user to press the Up Arrow or Down Arrow key to toggle UTC to LOC for selection of the pulse time. UTC chooses UTC for the following time of pulse output. LOC chooses local time for the field that follows.

Press ENTER to move to editing the hour field, which can be incremented or decremented from 00 through 23 using the Up Arrow or Down Arrow key. Above 23 or below 00 the “don’t care” selection XX is shown on the screen. For this selection, hours are not used in the time of day comparison. The user can subsequently define minutes and seconds so a pulse will be outputted every hour.

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Press ENTER to edit the minute field and choose the minute count at which the pulse is outputted from 00 to 59. Again an extra count XX is included for selection when the minutes are not used in the time of day comparison. With hours similarly selected the pulse will be outputted once per minute. Press ENTER to accept the minute entry and choose the seconds count for the pulse. Once again 00 to 59 or XX can be selected. For XX:XX:XX seconds are excluded from the comparison so the pulse is outputted every second.

When the time of day for the J10 pulsed output has been entered the cursor moves to the LENGTH field and the screen shows leading zeros to allow the user to edit the 1000 s, 100 s, and 10 s. The cursor blinks on the 1000 ms digit and a value 0 to 9 can be selected using the Up Arrow and Down Arrow keys. Press ENTER at the desired value and the cursor moves to the 100 ms digit. In this way the user enters or confirms existing digit values through to 1 ms for the pulse length definition as a value of 1 ms to 9999 ms.

After the pulse length entry or confirmation the cursor moves to the sign field, allowing a normal positive going pulse to be specified by selecting or confirming the ‘+’, or if desired the ‘-‘ can be selected with the arrow key. For a positive pulse the output of J10 is at 0 V and jumps to 5 V for the output duration of the pulse. For a negative pulse, the reverse is true. The output of J10 is at 5 V and jumps to 0 V for the output duration of the pulse.

Pressing ESCAPE allows the user to return to the previously defined values for the pulse with no change in operation. Only when pressing ENTER for the final time are the values applied.

4.13.2 J10 Pulse Period Configuration

J10 : PERIOD LENGTH POL

1000ms 200ms +

The pulse period may be 2 ms minimum to 9999 ms maximum and the pulse width is 1 ms minimum to 9998 ms maximum, but always less than the period.

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Select PERIOD as the mode for the pulse and press ENTER. The cursor moves to the 1000 ms digit displayed as 0 if it is not used in the current definition. Select the required value using the arrow key and confirm it by pressing ENTER which moves the cursor to the hundredth digit. When each digit has a value press ENTER until all four digits are set. Do not enter a period less than the minimum value of 2 ms. The final ENTER moves the cursor to the pulse length entry. The cursor blinks on the 1000 ms digit of the pulse length and a value 0 to 9 can be selected using the arrow keys. Press ENTER at the desired value and the cursor moves to the 100 ms digit. In this way the user enters or confirms existing digit values through to 1 ms for the pulse length definition as a value of 1 ms to 9998 ms. The pulse length minimum is 1 ms and the pulse length must be less than the period.

After the pulse length entry or confirmation the cursor moves to the sign field, allowing a normal positive going pulse to be specified by selecting or confirming the ‘+’, or if desired the ‘-‘ can be selected with the arrow keys. For a positive pulse the output of J10 is at 0 V and jumps to 5 V for the output duration of the pulse. For a negative pulse, the reverse is true. The output of J10 is at 5 V and it jumps to 0 V for the output duration of the pulse.

Pressing ESCAPE allows the user to return to the previously defined values for the pulse with no change in operation. Only when pressing ENTER for the final time are the values applied. The first pulse to be outputted is synchronized with the GPS8 Plus time.

4.13.3 J10 Pulse Time Code Configuration

J10 : TIMECODE (DC)

CODE: IRIG B (TC1 AS DC)

J10 time code output is a logic level version of the modulated carrier time code that is outputted on J8. Refer to section 4.13 for information on how to select the time code.

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4.14 Status Screen and Alarm Configuration 1

From the J10 pulse output configuration screen, pressing the Up Arrow key once selects the alarm configuration 1 screen. The function of this screen is displayed in the top line of the screen for 2½ seconds followed by a list of output sockets and their status.

ALARM CONFIGURATION 1

J6 J5 J4 J3 J2 J1

NRM NRM NRM NRM NRM NRM

The primary purpose of the screen is to list the status of the monitoring circuits for the signals out of J1 through J6. For outputs which are normal, NRM is displayed underneath the connector number. For outputs which are judged faulty, FLT is displayed. Any output that is FLT will cause the GPS8 Plus status relay to be deactivated signaling an alarm that turns on the red Fault LED on the GPS8 Plus front panel. The fault monitors are not latched and clear automatically when the output signals return to normal levels. There are occasions when the user may not want a particular output to contribute to the overall GPS8 Plus status report by deactivating the status alarm status relay. The alarm configuration screen allows any of the output status reports to be removed from the OR function that signal the unit alarm. This function is useful for outputs which are not used in a particular configuration.

4.14.1 Selection of Alarm Masks 1

Selecting the outputs to be removed from the alarm function is accomplished by pressing ENTER. An up arrow with a blinking cursor is shown on the screen after J6. Continue to press ENTER until the cursor is adjacent to the output that is to be removed from the alarm function. Use the arrow key to toggle the alarm mask ON or OFF. When the mask is applied the status NRM or FLT is displayed as ---. In the example below the output from J5 is not required and any possible alarm state has been masked by the user’s selection.

J6 J5 J4 J3 J2 J1

NRM --- NRM NRM NRM NRM

User mask settings are saved in EEPROM with a check byte. The settings are initialized to the same state at the next power up.

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4.15 Status Screen and Alarm Configuration 2

From the J10 pulse output configuration screen, pressing the Up Arrow key once selects the alarm configuration 2 screen. The function of the screen is displayed in the top line of the screen for 2½ seconds followed by a list of the alarm sources and their statuses.

ALARM CONFIGURATION 2

SYN Rb J10 J9 J8 J7 NRM --- NRM NRM NRM NRM

The first alarm source listed on this screen is SYN, the PLL synthesizer for telecom or other special frequency generation. SYN indicates NRM for a correctly locked synthesizer or FLT is shown for an out-of-lock condition.

The second source listed is Rb under which is indicated the status of an optional Rubidium frequency standard in the unit. As a default this source is already masked. When a Rubidium is fitted, the status condition may be CLD, NRM, or FLT. CLD indicates that the Rubidium has not yet warmed up and its status is likely to indicate that it is not resonance locked. If it remains unlocked when it has warmed up, then FLT is indicated for the Rubidium status.

The remaining alarms are for the signals on the output sockets J10 through J7. They operate in the same way as the alarms on J6 to J1 described in section

4.14. None of the alarm sources on the Alarm Configuration 2 screen is latched and the alarm is cleared when the fault clears.

4.15.1 Selection of Alarm Masks 2

In each case the associated alarm signal can be masked as described in paragraph 4.14.1. User mask settings are saved in EEPROM with a check byte. The settings are initialized to the same state at the next power up.

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4.16 Status Screen 3

From the alarm configuration 2 screen, pressing the Up Arrow key once selects the third status screen. Alternatively, from the main default screen of the time and date, pressing the Down Arrow key twice accesses it.

SYNC FRQ DAC SATS

NRM NRM NRM NRM

The status conditions listed contribute to the overall alarm on the GPS8 Plus and will result in the alarm status being asserted by the alarm relay and the front panel red Fault LED will be turned on.

NAME DESCRIPTION

SYNC Set to FLT shows that the synchronization process has been interrupted. During start

up or if a re-synchronization becomes necessary after a very long holdover period, the synchronization process is launched automatically by the GPS8 Plus when error-free 1 PPS signals are available from the receiver. A further 1 PPS is required to complete the synchronization and might have been disabled at that instant. It will automatically be cleared to NRM when the procedure has been successfully completed.

FRQ Set to FLT if the GPS8 Plus is unable to discipline its oscillator to follow the GPS. It will

automatically be cleared to NRM when the GPS8 Plus succeeds in controlling the oscillator.

DAC Set to LIM when the DAC that controls the frequency of the GPS8 Plus TCXO, OCXO,

or Rubidium is within ±12% of its maximum or minimum setting. It will automatically be cleared to NRM when the DAC is inside the 12% limit of remaining control.

SATS No satellites have been tracked for more than 8 hours.

4.17 Status Screen 4

From the third status screen, pressing the Up Arrow key once selects status screen 4. Alternatively, from the main default screen of the time and date, pressing the Down Arrow key once accesses it.

ISSUE ROM RAM NV_RAM

23P NRM NRM NRM

NAME DESCRIPTION

ISSUE Heading to the firmware issue number.

ROM Set to FLT if the flash memory or EPROM contents give rise to a checksum error

during initialization.

RAM Set to FLT if the RAM fails a comprehensive check during initialization.

NV_RAM Set to FLT if the battery-backed RAM contents fail a checksum test during

initialization. If NV_RAM shows FLT, then the stored parameters are updated by the default values until new values are entered or acquired. After a firmware upgrade, NV_RAM status may be deliberately set to FLT to force its initialization at the first power on of the new firmware.

If the name is set to FLT after power on, then the items do not contribute to the alarm relay or Fault LED activation. If the ROM, RAM, or NV_RAM persistently show status errors consult the manufacturers.

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5 Principles of Operation

5.1 Scope of Section

Section 5 provides a functional description of the GPS8 Plus and a general description of the Global Positioning System.

5.2 Theory of Operation

Operation of the Global Positioning System is based on the principle that the position of an object can be determined by measuring the distance to three or more satellites of known positions. The GPS8 Plus receiver system determines the distance to each satellite by measuring the time taken for the signal to arrive at the antenna (a procedure known as one-way ranging). The signal provides satellite position and time information, which is used by the receiver to calculate the antenna position. Satellite position information must be exact with respect to the earth at the time the signal is transmitted to enable the receiver to compensate for the constant movement and correctly synchronize with the satellite data.

Time information derived from a satellite atomic clock provides the receiver with the exact time of the data transmission. This time is compared with the receiver clock to calculate transmission time. As the speed of electromagnetic radiation is constant (2.998 x 10 gives the satellite distance. The receiver automatically solves simultaneous equations for position and time using the optimum constellation of satellites.

Each satellite also transmits almanac data and status information, which is used by the receiver to determine which satellites are visible and which four will provide the most accurate position and hence time and frequency calculations. Satellite almanac data is particularly important because the coding technique and satellite motion make a random search for a satellite time consuming. For this reason almanac data is stored in non-volatile memory and is retained whenever the receiver is switched off.

The basic GPS8 Plus is comprised of eight main sections as listed below.

1. Receiver/antenna

2. Microcomputer control system

3. Display/keypad

4. Internal oscillator

5. I/O system

6. Power supply

meters/sec), multiplying this figure by the transmission time

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The satellite transmissions are first received by a remote sealed omni-directional antenna assembly, which includes a low noise pre-amplifier and coaxial cable for connection to the main unit. Up to 100 feet of standard RG-58 cable can be used or 300 feet of RG-8.

The GPS receiver inside the unit is a parallel eight channel unit, which simultaneously and continuously tracks up to eight satellites. The received time, position, status, and DOP data is processed and transmitted to the main control board via a bi-directional RS232 port together with a 1 PPS signal recovered from the data while tracking at least one satellite. The 1 PPS signal transmitted by the satellite is aligned to "GPS time", which is offset from UTC by a whole number of seconds. This offset is automatically removed by the receiver.

All functions associated with input and output control are performed by a microcomputer system, which also accepts keypad data and provides screen information for the liquid crystal display if used.

The microcomputer control system has 8 main functions as listed below.

1. Display/keypad control

2. Receiver control

3. Phase control of timing outputs

4. Frequency control of reference oscillator

5. Time code/pulse rate generation

6. Serial/parallel I/O control

7. Built in test/monitoring

8. Expansion selection/control

All control software for the microcomputer system resides in a single 1024 K flash memory supported by a 512 K ‘boot’ ROM, which permits firmware upgrade via the unit serial port. Battery backed RAM and EEPROM store various system parameters during a power down so that constant reprogramming of the unit is avoided. The sine wave frequency (and as an option IRIG B time code) outputs are available via isolating transformers.

The unit can be fitted with a number of different types of oscillator, which provides time and frequency back-up in the event of the loss of satellite reception, as well as filtering to remove short term variations due to the variability of the GPS as a timing reference caused by the possibility of selective availability, which may be introduced to deny the highest positioning accuracy to non-military users. The drift while the GPS8 Plus is operating without satellite synchronization (a period known as “holdover”) will depend on the quality of the oscillator, ambient temperature changes, and the period with no satellite visibility.

However, the high stability grade of the oscillator can achieve better than 10 µs

per day drift in benign environmental conditions.

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The complete unit is normally powered by an AC supply, which uses a high efficiency toroidal transformer to provide two independent low noise supplies regulated to +5 V for the processor and digital support circuitry and +15 V for the analog circuits and output distribution circuits. The +15 V is further regulated to +12 V to maintain low noise and increase isolation.

The analog section of the unit is totally galvanically isolated from the digital section up to eventual common grounding at the output connectors. The signal connections between analog and digital sections are made via opto-isolators and a transformer.

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6 Maintenance and Calibration

6.1 Scope of Section

Section 6 describes the recommended maintenance checks for the GPS8 Plus and outlines a general approach to fault finding and repair.

6.2 Routine Maintenance

During normal operation the GPS8 Plus functions automatically and does not require continuous manual intervention. Preventative maintenance can therefore be restricted to a regular inspection of status indicators and displays. The only component which will degrade with time and may need replacement during the life of the unit is the battery, which powers the GPS receiver and GPS8 Plus' RAM when the main power supply is switched off or disconnected.

INSTRUMENT ON TIME APPROXIM ATE BATTERY LIFE

100% 25 years

66% 10 years

0% 3 years

BT1 is mounted in a socket and can be replaced. Before removing the unit’s cover ensure that the power cord has been detached from the rear of the unit.

Other components which will age and may require adjustment are as follows:

• Liquid crystal display (OPTION ONLY)

• Liquid crystal display

The GPS8 Plus incorporates a variable control RV3, accessible through a hole in the right-hand side of the case viewed from the front. RV3 is factory set to give the optimum contrast ratio on the display. During the life of the unit the contrast ratio may change due to ambient temperature and lighting variations, shock, vibration, and etcetera. RV3 can be re-adjusted as often as required to optimize contrast ratio. This adjustment can be made through an access hole in the side of the unit’s case (the right-hand side of the case viewed from the unit’s front).

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6.3 Fault Finding

CAUTION: In the case of equipment malfunction or failure, it is recommended that the GPS8 Plus is returned to the factory for repair. If this is not practical, fault finding and repair must only be undertaken by a qualified test engineer.

Internal fault finding and repair of the GPS8 Plus requires specialized knowledge. Identification of a malfunction to a module level is straightforward using the self test routines and status indicators described on pages 6, 14, and 15. The following table identifies the most likely module responsible for a fault condition.

BUILT IN TEST SUSPECT MODULE

ANT FLT ANTENNA/RECEIVER INIT FAIL MICROCOMPUTER RCVR FLT RECEIVER OSC I/P FLT INTERNAL OSCILLATOR FRQ ERR FLT INTERNAL OSCILLATOR RAM FAIL MICROCOMPUTER EFC LIM FLT INTERNAL OSCILLATOR ROM FAIL MICROCOMPUTER

A malfunction of the display module, keypad system, or output interfaces will be immediately apparent to the user. A major failure of the power supply will cause a complete loss of functionality. A partial failure can be verified by checking the DC voltages.

It should be noted that the unit contains static sensitive devices and hazardous voltages (115 VAC). If measurements and/or adjust ments m ust be made with the unit switched on it should be powered via an isolation transformer.

6.4 Oscillator Module

The precision oscillator installed in the GPS8 Plus may be one of several different types. The oscillator is factory installed and does not require further adjustment. The type of oscillator fitted is detected automatically and is described in the power on message. The type is specified by 3 jumpers on the oscillator module and for certain types or optionally an entry in EEPROM memory.

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7 Diagrams

FIGURE DESCRIPTION

1 GPS8 Plus Front Panel 2 GPS8 Plus Rear Panel 3 GPS8 Plus Mechanical Outline 4 GPS8 Plus Link Locations 5 Table of Link Settings 0180BN 6 GPS8 Plus Block Diagram

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MANUAL P/N 900000007 REV 2.0

GPS8 Plus Link Locations

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MANUAL P/N 900000007 REV 2.0

Table of Link Settings 0180BN

Link ID Factory

Setting

Result Notes

LK1 OPEN NOT TESTING GPS8 Plus executes main program from flash memory; EPROM is still required

for key I/O management and ‘flash’ programming. Must be open to allow flash

programming. LK1 1-2 TESTING FROM EPROM GPS8 Plus uses EPROM only; Flash need not be programmed. LK2 OPEN NOT RESET Screwdriver reset pins. LK3 1-2 WATCHDOG IS TRIGGERED BY

PROCESSOR

Jumper 2-3 to defeat watchdog during testing with emulator only.

LK4 OPEN NO TERMINATION Jumper fitted for RxD1 100R RS422 termination LK5 OPEN VOLT FREE CONTACTS Jumper 1-2 for 0 V -> alarm open

Jumper 3-4 for +5 V -> alarm closed LK6 OPEN NO TERMINATION Jumper fitted for RxD2 100R RS422 termination LK7 1-2 and 3-4 50R OUTPUT Jumper 1-2 is 120R; Jumper 3-4 is 75R LK8 2-3 10 MHz TCXO Bypasses Freq Doubler Jumper 1-2 for 10 MHz oscillator. See LK15. LK9 OPEN T1 SELECT Jumper for E1 (2.048 MHz)

Fit appropriate VCXO at IC41 LK10 1-2 and 3-4 50R OUTPUT Jumper 1-2 is 120R; Jumper 3-4 is 75R LK11 NOT FITTED NOT IN USE Option for LC/crystal selection if main PCB is fitted with bandpass filter (in place

of plug-in bandpass filter board). LK12 NOT FITTED NOT IN USE As above LK13 NOT FITTED NOT IN USE As above LK14 NOT FITTED NOT IN USE As above LK15 1-2 10 MHz TCXO Selects 10 MHz/2 for BP filter 5 MHz option

Link 2-3 to use the 5 MHz oscillator output directly if a 5 MHz oscillator is fitted. LK16-19 (LK34) NOT FITTED NOT IN USE Option for frequency selection if main PCB is fitted with bandpass filter (in place

of plug-in bandpass filter board). LK20-23 (LK35) NOT FITTED NOT IN USE As above LK24 NOT FITTED NOT IN USE Programming voltage for GPS engine. LK25-28 (LK36) LK25 Select square-wave output from SK8 (J5) as

PLL Synth (usually 2.048 MHz or 1.544 MHz)

The selections are PLL Synth, 1 MHz, 5 MHz, 10 MHz, or DDS Synthesizer.

LK29 OPEN NOT IN USE Can be read by the processor for option selection. LK30-33 (LK37) LK30 Select square-wave output from SK10 (J6) as

PLL Synth (usually 2.048 MHz or 1.544 MHz)

The selections are PLL Synth, 1 MHz, 5 MHz, 10 MHz, or DDS Synthesizer.

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MANUAL P/N 900000007 REV 2.0

GPS8 Plus Block Diagram

T1/E1

BUFFER BUFFER

RAM

POWER

ENAB

TC1

+12V

50R

MODULATED

5MHz

CONTROL

1PPS

TTL ALARM

DIV 5

PROC FAIL

RS232/RS422 OUTPUTS

MONITOR

MICRO

SCLK

50R

FLASH

TXD2/RXD2

SRL

75R

FRQ1 SINE

1kHz

FD[1..7]

INPUT

TXD1/RXD1

AN[7..5]

GPS8 0180BD001

TxD

BSRDO

VCC

CONNECTOR

DIV 6

STATUS

OUTPUT

/DACCS

75R

BOOT

DIN

T1/E1

EXT

SELECT RS232 OR RS422

ANALOGUE GROUND AREA

1kHz

5 WAY DIN

POWER

MONITOR

EPROM

16kHz

FD7

75R

ISOL

1Hz

RCC 4497 BD

RUBIDIUM EFC

HZ1_OUT

TXD1/RXD1 PARAM

BP2 MODULE

OUTPUT

MICROCOMPUTER

FRAME

T1 OR E1

+/-3dB Output Level Trim

50R

OPTO

MONITOR

G703 SECTION 10 (set 3Vpp max sinewave into 75R)

LK? LINK 5

123

DIV 2

6 INPUTS

MISC I/O (PARALLEL OP AND TI OUTPUTS)

FD2

DATA

MONITOR

OSC MODULE

10MHz (Digital)

SCLK

FILTER

FUSE

DIV 3

50R

PASS

16 BIT DAC

OUTPUT

REG

DIV 1

8 OUTPUTS

RxD

+/-3dB Output Level Trim

VALID

1MHz

FILTER

FD4

BUFFER

+12V

MONITOR

SHIFT

OR 1/5/10MHz

SRDO

BUFFER

20 WAY HEADER

3 CTRL

PASS

FILTER

RxD

ALARM

TC1_OUT

+15V

MICRO

5MHz

BENAB

PSU

FD1

50R

BP1 MODULE

OUTPUT

T1 OR E1

50R

FD6

10 WAY HEADER

BAND

OSCILLATOR

10MHz CRYSTAL

TxD

1kHz

COMPARATOR

2 LINE X 24 CHARACTER DISPLAY OPTION

OUTPUT

FROM

10MHz

+15V

DIN

G703 SECTION 6 (2,37Vpp INTO 75R)

LK15 LINK 1

123

RV5

MONITOR

50R

8 WAY DIL SW

BUFFER

CRYSTAL DOUBLER

EPLD

LQ6

50R

PHASE

10MHz CLK

MONITOR

DUART

AREA

ENT

75R

FREQ1 SINE

POWER ON

SELECT RS232 OR RS422

12 34

120R

LK? LINK 5

123

BUFFER

TO BUDGET FREQUENCY

LOGIC

LOOP

COMPARATOR

TYP

OUTPUT

9-WAY D

SO 222/1 DIGITAL AREA

LED DRIVER

AVAILABLE

DIV 4

TIMECODE

FILTER

ISOLATED SUPPLY

120R

SOURCE

600R

CONTROL

OUTPUT

OCXO

FILTER

10MHz

SEL

9-WAY D

TC2_OUT

TC1_OUT

TRACKED

MONITOR

TTL SQ WAVE into 50R

12.352MHz - T1

50R

5MHz

MODULATOR

ADDR

POWER

MONITOR

HIGH PRECISION

VCO

50R

RS232 2)

LOW PHASE-NOISE

MICRO

SAT

TTL

OUTPUT

TTL SQ WAVE into 50R

LK? LINK 5

123

16.384MHz - E1

OR 1/5/10MHz SINE 1Vrms into 50R

REF

CHOOSE

FD5

1Hz

DATA

CTRL

OUTPUT

REF

ALARM

OPTION

RS232 (1)

RUBIDIUM

CODE

DIGITAL

MONITOR

8kHz FRAME

BLOCK DIAGRAM

SYNTHESISER

TIMECODE OR PULSES

MONITOR

DATA

E1/T1

COMPARATOR

POWER

TRANSFORMER

SRL

RV6

RS422 (2)

FREQ2 SINE

OPTION

REJECT

T1/E1

GPS ENGINE

OUTPUT

+12V

LQ5

UNMODULATED

1MHz

MICRO

REF_1PPS

VCO

CONTACTS

FD3

RS422 (1)

BAND

FRQ2 SINE

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8 Appendix I Output Alarm Masks

The GPS8 Plus uses hardware detectors to measure the presence or absence of its outputs. The detectors are monitored by the processor, which turns on the red Fault LED and activates the alarm contacts in the event of an output failure.

The alarm sources can be ‘masked’ so that they no longer appear in the output status record and no longer activate the front panel LED and alarm contacts. This may be necessary if an output is not in use or if a particular output has been customized for an application and the hardware detector indicates a permanent alarm condition.

The output alarms are shown in two status bytes that can be seen in field 7 of the response to the query a<CR>LF>. Any alarm source that is active will set a ‘1’ in its corresponding status bit and four bits are indicated by the hexadecimal value of each character in field 7. For example, the response message is as follows:

Field 7 $PRCCG, A, 018, 10:27:03, 17/12/01, 4, 0, 1, 0003, 80*2C<cr><lf>

Field 7 in the example has BIT 0 and BIT 1 set. The bit descriptions are shown in the tables below.

BIT 15 BIT 14 BIT 13 BIT 12 BIT 11 BIT 10 BIT 9 BIT 8

Reserved Reserved Reserved Reserved Reserved Digital time

J10 (SK4) J9 (CON8) J8 (SK3)

code pulse fail

BIT 7 BIT 6 BIT 5 BIT 4 BIT 3 BI T 2 BIT 1 BIT 0

Synthesiser

PLL

unlocked

J7 (SK12) J6 (SK10) J5 (SK8) J4 (SK9) J3 (SK11) J2 (SK7) J1 (SK6)

8 KHz

frame fail

Frequency square fail

Frequency

sine 2 fail

Frequency

sine 2 fail

The unit has no sine 1 frequency output on J1 and J2 due to the removal of an internal band-pass filter module.

1 PPS fail Modulated

Frequency

sine 1 fail

time code

fail

(LSB)

Frequency

sine 1 fail

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The alarm masks are contained in two bytes corresponding to the 16 bits in the table above. Each byte is stored as its true value, where ‘1’ means the alarm is enabled and its complement (all bits inverted) in EEPROM at address 4 and 5. There are 16 bits of data at each of these addresses and the masks are organized as F1MSK and its complement at address 4 and F2MSK and its complement at address 5. F1MSK contains the bits to mask BIT 0 through BIT 7 of the alarm bytes and F2MSK contains the bits to mask BIT 8 through BIT 15. The complement byte is used for protection. If the value does not agree with the true value as to which bits should be masked, then the default values are loaded (no masks are applied). To change the mask value it is easiest to prepare a table as shown for the example below.

F1MSK CHECK BYTE F1MSK BYTE SHOWING

CORRESPONDING ALARM BIT NO

BIT7 BIT6 BIT5 BIT4 BIT3 BIT2 BIT1 BIT0

0 0 0 0 1 1 1 1 1 1 1 1 0 0 0 0

0 F F 0

Table 26 F1MSK at EEPROM Address 04 with All Four Sine Wave Output Alarms Disabled

F1MSK CHECK BYTE F1MSK BYTE SHOWING

CORRESPONDING ALARM BIT #

BIT

BIT9 BIT8

0 0 0 0 0 0 1 0 1 1 1 1 1 1 0 1

0 2 F D

Table 27 F2MSK at EEPROM Address 05 with 1 PPS Output Alarm Disabled

To enter the new values the <CTRL T> 6 Edit/View EEPROM menu is used.

Enter <CTRL T>

Tests

1. PWM u/U d/D

2. A-D1; D-A1 u/U OR d/D

3. DAC u/U d/D H C L W

4. Delay Adjust

5. Sounder

6. Edit/View EEPROM

7. Reset NVRAM & RE-START

8. PLL Op 0-6, 7 Trace RS, 8 Frq Meas

9. More Tests

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Press 6

6. Edit/View EEPROM

<SPACE> Display All EEPROM F TxD2 Format 1 to 3 O Enter Osc 0 to 9 P Program EEPROM location U Unlock P Program EEPROM location

Press P (must be uppercase P)

Enter Hex Addr (ESC=quit) 00-37

Enter 4<RETURN>

4 Addr=04

Enter Hex Data 0000-FFFF

Enter 0FF0<RETURN>

0FF0 Data=0FF0

GPS8 Plus confirms location has been programmed

Prog time=02ms Value Saved=0FF0

P Program EEPROM location Enter Hex Addr (ESC=quit) 00-37

User can just press <RETURN> for the next address

Addr=05 Enter Hex Data 0000-FFFF

User enters 02FD<RETURN>

02FD Data=02FD Prog time=02ms Value Saved=02FD

P Program EEPROM location Enter Hex Addr (ESC=quit) 00-37

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Press <ESC> to quit programming function of the EEPROM menu

6. Edit/View EEPROM <SPACE> Display All EEPROM F TxD2 Format 1 to 3 O Enter Osc 0 to 9 P Program EEPROM location U Unlock

PRESS <ESC> again to quit test menu

QUIT 6

For the new values to take effect the unit must be powered on again. Note that the initialization messages include a description of the alarm masks showing ‘0’ for alarm outputs that are disabled and ‘1’ for alarm outputs that are enabled.

L=load ESC=RUN.....

Initialize - Clear RAM Initialize DUART TFS Ltd C Fri Dec 14 2001 17:08 [email protected]

Testing IC27 RAM......IC27 RAM PASSED

Test IC27 NV_RAM Contents.IC27 PASSED - Using RAM based Parameters

Testing IC28 ROM......IC28 ROM PASSED Cksum = 5074

Test Alarm ON then OFF

Testing Indicators.......

Osc is XTAL4 OSA 8410 TCXO...Calibration Found Initialize PLL...PLL Initialized Slope Positive TC1=AFNOR TC2=Pulse Pulse 1 msg: $PRCCG,C,U000000,+100*65 Pulse 2 msg: $PRCCG,K,UXXXX00,+100*6D TimCod msg : $PRCCG,E,5,2,2,2*2B Offset msg : $PRCCG,O,U,+00:00*62 Alarm Masks 1234 5678 1234 5678 0000 1111 1011 1111 Set Timers Run GPS8 EGPS18

Enter A<RETURN><CTRL J>

To see the value reported in field 7 of the status message $PRCCG,A,018,13:28:21,17/12/01,8,2,1,0000,00*25<cr><lf>

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9 Appendix II Phase Noise

The standard GPS has been designed for low phase noise. The precision ovened crystal oscillator and the analog distribution is separately powered and opto-isolated from the digital processor section. However, the standard unit with telecom outputs shows some expected cross-talk between the telecom outputs and the standard frequency outputs, primarily visible in the form of a substantial peak at 16 KHz in the phase-noise spectrum. One solution is to disable the telecom frequency synthesizer for applications that do not require the telecom outputs. All the phase noise plots were taken while the units were disciplining.

GPS8 AND GPS8LN PHASE NOISE - 5MHz OUTPUTS

-80

-90

-100

-110

-120

-130

-140

-150

-160

-170

Single Sideband Phase Noise £(f) dBc

GPS8

GPS8LN

Frequency

Single Sideband Phase Noise £(f) dBc

-80

-90

-100

-110

-120

-130

-140

-150

-160

-170 10

GPS8 AND GPS8LN PHASE NOISE - 10MHz OUTPUTS

GPS8

GPS8LN

Frequency

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