Mitel GPS Orion-S--HD Receiver User Manual

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Space Flight Technology, German Space Operations Center (GSOC)

Deutsches Zentrum für Luftund Raumfahrt (DLR) e.V.

User’s Manual for the GPS Orion-S/-HD Receiver

O. Montenbruck, M. Markgraf

Doc. No.

:

GTN-MAN-0110

Version

:

1.0

Date

:

June 22, 2003

Document Title:

ii

User’s Manual for the GPS Orion-S/-HD Receiver

 

Document Change Record

Issue

Date

Pages

Description of Change

1.0

June 22, 2003

all

First release

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Disclaimer

Information in this manual has been compiled with adequate care and represents the best knowledge of the authors. Any errors remaining after its release will be fixed upon notification. In no way shall DLR or the authors be held liable for direct or indirect damage resulting from missing or erroneous information. Furthermore, DLR reserves the right to change interfaces and system specifications in future releases.

Document No.

Issue 1.0

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June 22, 2003

© DLR/GSOCNo part of this document shall be reproduced in any form or disclosed to third parties without prior authorization.

Document Title:

iii

User’s Manual for the GPS Orion-S/-HD Receiver

 

Table of Contents

Document Change Record......................................................................................................

ii

Table of Contents....................................................................................................................

iii

Scope and Applicability ..........................................................................................................

1

Acronyms and Abbreviations.................................................................................................

2

1.

Introduction.........................................................................................................................

 

3

 

1.1

GPS Orion Receiver ....................................................................................................

3

 

1.2

Functional Overview ....................................................................................................

3

 

1.3

Receiver Versions ........................................................................................................

4

2.

Receiver Hardware .............................................................................................................

5

 

2.1

Main Board ...................................................................................................................

5

 

2.2

Interface Board.............................................................................................................

7

 

2.3

Antenna ........................................................................................................................

 

7

3.

Operations Guide ...............................................................................................................

8

 

3.1

Basic Receiver Handling..............................................................................................

8

 

 

3.1.1

Hardware Setup ...............................................................................................

8

 

 

3.1.2

Precautions.......................................................................................................

8

 

 

3.1.3

Serial Communication......................................................................................

8

 

 

3.1.4

Start-Up and Initialization .................................................................................

9

 

 

3.1.5

Output Selection.............................................................................................

10

 

 

3.1.6

Pulse-per-Second Signal ...............................................................................

11

 

 

3.1.7

Troubleshooting..............................................................................................

11

 

3.2

Special Applications ...................................................................................................

12

 

 

3.2.1 Aiding for Ballistic Trajectories.......................................................................

12

 

 

3.2.2

Lift-off Signal ..................................................................................................

13

 

 

3.2.3

IIP Prediction ..................................................................................................

13

 

 

3.2.4 Aiding for LEO Satellites ................................................................................

15

 

 

3.2.5

Relative Navigation ........................................................................................

16

 

 

3.2.6 External LNA Power Supply...........................................................................

17

4. Command and Output Message Reference ..................................................................

18

 

4.1

Overview ....................................................................................................................

 

18

 

4.2

Protocol Description...................................................................................................

20

 

 

4.2.1

WinMon Format..............................................................................................

20

 

 

4.2.2

NMEA Format.................................................................................................

21

 

4.3

Commands.................................................................................................................

22

 

 

4.3.1

Basic Receiver Configuration.........................................................................

23

 

 

4.3.1.1 UR – Update Rate ..........................................................................................................................

23

 

 

4.3.1.2 DR – Data Rate...............................................................................................................................

24

 

 

4.3.1.3

SM – Sentence Mode.....................................................................................................................

24

 

 

4.3.1.4

MC – Media Correction..................................................................................................................

25

 

 

4.3.2

Status Queries................................................................................................

26

 

 

4.3.2.1

TA – Transmit Almanac.................................................................................................................

26

 

 

4.3.2.2

TE – Transmit Ephemeris..............................................................................................................

26

 

 

4.3.3

Initialization.....................................................................................................

27

 

 

4.3.3.1

PV – Position-Velocity....................................................................................................................

27

 

 

4.3.3.2

DW – Doppler Window...................................................................................................................

28

 

 

4.3.4

Reference Trajectory Aiding ..........................................................................

29

 

 

4.3.4.1

AM – Aiding Mode ..........................................................................................................................

29

 

 

4.3.4.2

RM – Run Mode ..............................................................................................................................

29

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Document Title:

 

 

 

iv

User’s Manual for the GPS Orion-S/-HD Receiver

 

4.3.4.3

LO – Load Orbit...............................................................................................................................

30

4.3.4.4

TO – Transmit Orbit........................................................................................................................

30

4.3.4.5

LE – Load Epoch ............................................................................................................................

31

4.3.4.6

LT – Load Trajectory......................................................................................................................

31

4.3.4.7

ET – End Trajectory .......................................................................................................................

31

4.3.4.8

TT – Transmit Trajectory...............................................................................................................

31

4.4 Output Messages (WinMon Format) .........................................................................

32

4.4.1

Periodic Receiver Data ..................................................................................

32

4.4.1.1

F00

– Geographic Navigation Data (Mitel).................................................................................

32

4.4.1.2

F03

– Channel Status Data (Mitel) ..............................................................................................

32

4.4.1.3

F04

– Satellite Summary (Mitel)...................................................................................................

32

4.4.1.4

F05

– Processing Status (Mitel)...................................................................................................

32

4.4.1.5

F08

– Operating Parameters (Mitel)............................................................................................

32

4.4.1.6

F40

– Cartesian Navigation Data.................................................................................................

33

4.4.1.7

F41

– Pseudorange and Range Rate (Smoothed)....................................................................

34

4.4.1.8

F42

– Pseudorange, Carrier Phase and Range Rate (Raw)...................................................

35

4.4.1.9

F43

– Channel Status ....................................................................................................................

36

4.4.1.10

F44

– Clock Data............................................................................................................................

38

4.4.1.11

F45– Relative Navigation Data (WGS-84 System)...................................................................

39

4.4.1.12

F46

– Relative Navigation Data (RTN Frame)...........................................................................

39

4.4.1.13

F47

– IIP Prediction........................................................................................................................

40

4.4.1.14

F48

– Configuration and Status Parameters..............................................................................

40

4.4.2

Working Parameters ......................................................................................

41

4.4.2.1

F50

– Reference Epoch for Trajectory Polynomials .................................................................

41

4.4.2.2

F51

– Trajectory Polynomials .......................................................................................................

41

4.4.2.3

F52

– User Spacecraft Mean Elements ......................................................................................

42

4.4.3

Diagnosis Messages ......................................................................................

43

4.4.3.1

F98

– Command Response..........................................................................................................

43

4.5 Output Messages (NMEA Format) ............................................................................

44

4.5.1

$PASHR,POS Navigation Data .....................................................................

44

4.5.2

$PDLRM,IIP Instantaneous Im pact Point Data .............................................

46

4.5.3 $PDLRM,XSD Extended Status Data............................................................

48

4.5.4 $PDLRM,RAW Raw Measurement Data.......................................................

49

References..............................................................................................................................

 

 

 

50

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Document Title:

1

User’s Manual for the GPS Orion-S/-HD Receiver

 

Scope and Applicability

This manual provides a user’s guide for the DLR’s GPS Orion receivers for space and high dynamics applications. It describes the hard and software interfaces required for operating the receiver in standalone and embedded applications. Information in this document supplements and supercedes related sections of the GPS Orion Product Brief [1] and the GP2000 Series Demonstrator Board User’s Guide [2]. It is applicable for s/w versions D06H (OrionHD) and D07N (Orion-S).

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Acronyms and Abbreviations

A

Ampere

AGC

Automatic Gain Control

ASCII

American Standard Code for Information Interchange

C/N0

Carrier-to-Noise Ratio

COM

Communication

dB

Decibel

DC

Direct current

DLR

Deutsches Zentrum für Luftund Raumfahrt

EPROM

Erasable Programmable Read Only Memory

FLL

Frequency-Locked Loop

GPS

Global Positioning System

GSOC

German Space Operations Center

I/F

Intermediate Frequency

IIP

Instantaneous Impact Point

IQ

In-phase and Quadrature (correlator output)

L1

GPS frequency (1575.42 MHz)

LEO

Low Earth Orbit

LNA

Low noise amplifier

MITEL

Company name

NMEA

Nautical Marine Electronics Association

NVM

Non-Volatile Memory

ORION

Product name

PC

Personal Computer

PLL

Phase-Locked Loop

PPS

Pulse-per-second

PRN

Pseudorandom Noise

R/F

Radio Frequency

RAM

Random Access Memory

RX

Receiver

SAW

Surface Acoustic Wave

SMA

Sub Miniature Assembly

SNR

Signal-to-Noise Ratio

SV

Space Vehicle

TC

Telecommand

TCXO

Temperature Controlled Oscillator

TM

Telemetry

TTL

Transistor-Transistor-Logic

TX

Transmitter

UART

Universal Asynchronous Receive and Transmit

V

Volt

W

Watt

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1. Introduction

1.1GPS Orion Receiver

The GPS Orion receiver represents a prototype design of a terrestrial GPS receiver for 12 channel single frequency tracking built around the Mitel (now Zarlink) GP2000 chipset ([3], [4]). The receiver main board comprises a GP2015 frontend and DW9255 saw filter, a GP2021 correlator as well as an ARM60B 32-bit microprocessor. It can be supplemented by an optional interface board featuring a switching regulator, serial line drivers (RS 232) and a backup battery.

Fig. 1.1 GPS Orion main board

A basic software for the GPS Orion receiver has earlier been made available by Mitel Semiconductor as part of the GPS Architect Development Kit. It is restricted to purely terrestrial applications and has received numerous extensions and modifications to provide accurate navigation under the rapidly varying signal conditions encountered in typical space missions. Key upgrades include enhanced tracking loops, a synchronization of measurements to integer GPS seconds, the provision of precise carrier phase measurements, a revised navigation algorithm, as well as a software based aiding of the signal acquisition using reference trajectory data. In addition to the above software changes, the original hardware design has been amended by a supplementary pin for output of the pulse-per-second signal.

1.2Functional Overview

DLR’s family of GPS Orion receivers comprises various firmware versions for space and high dynamics applications. Available software configurations are:

Orion-S for low Earth satellites and formation flying

Orion-HD for high dynamics platform like sounding rockets and reentry vehicles

Features common to all receiver models are summarized below.

12 fully independent tracking channels 2-bit sampling

3rd order PLL with FLL assist

Low noise code, carrier and Doppler measurements

Acquisition aiding using reference trajectory information Navigation update rate of up to 2 Hz

Configurable ASCII output messages in WinMon and NMEA format Pulse-per-second signal

Low power consumption (2 W at 5 Volts)

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Small form factor (50 x 95 mm) and weight (50 g)

Sufficient radiation tolerance LEO usage

Battery buffered non-volatile memory and real-time clock

Two serial ports

Discrete input pin

5V supply for active antenna (16-28dB)

OrionMonitor control software for Windows PCs

A hardware description of the Orion-S/HD receiver is provided in Chap. 2 of this manual. Chap.3 addresses the receiver operation and the command and log functionality is described in full detail in Chap. 4.

1.3Receiver Versions

The Orion receiver is available in various versions, which basically differ by the employed receiver software. Aside from the standard receiver (Mitel reference design [3], [4]), which is restricted to terrestrial applications, a space (-S) version and a high dynamics (-HD) version are available. These employ specific trajectory models to enable a safe and rapid signal acquisition under rapid motion of the host vehicle. For satellites in low Earth orbit, aiding is provided by an analytical orbit model using twoline elements, whereas a set of piecewise polynomials is employed to approximate the trajectory of ballistic vehicles (sounding rockets, reentry capsules) in the HD version. Various commands specific to each of these versions are provided to load, dump and use the respective aiding information.

The two versions also differ by their choice of FLL/PLL loop settings that are adapted to the specific application needs. A narrow bandwidth of the carrier tracking loop is chosen in the Orion-S receivers to achieve the most accurate carrier phase measurements under typical line-of-sight accelerations of 1 G. Wide bandwidth settings, in contrast are chosen for in the HD receivers to accommodate the extreme dynamics of a powered flight and the re-entry shock.

Finally, a relative navigation mode is offered by the Orion-S receiver to support its use in basic formation flying and rendezvous & docking applications.

A detailed account of the prototype software for the GPS Orion receiver is given in the GPS Architect Software Design Manual [5]. Subsequent modifications for the S and HD version are described in [6].

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2. Receiver Hardware

2.1Main Board

A block diagram of the GPS Orion receiver main board is shown in Fig. 2.1 ([4]). The receiver is designed to work with an active antenna and +5 V power supply for the preamplifier is provided on the central antenna feed.

Fig 2.1 Block diagram of the GPS Orion receiver main board (from [4])

After passing an R/F ceramic filter, the L1 signal (1575.42 MHz) is down-converted and digitized in the GP2015 front-end chip [7]. An external discrete filter and a DW9255 SAW filter [8] are used to filter the first (175.42 MHz) and second (35.42 MHz) intermediate frequencies, while an on-chip filter is used for the third analog IF (4.31 MHz). Finally, the signal is digitized and sampled to create a digital IF of 1.405 MHz with 2-bit quantization. The fundamental reference frequency for the mixing process is provided by a 10.0 MHz TCXO with a specified stability of 2.5 ppm. It also used to derive a 40 MHz clock frequency for the correlator.

The subsequent signal processing is performed in the GP2021 correlator chip [9], which provides 12 fully independent C/A code correlator channels. It also offers two UART ports for external I/O as well basic memory management capabilities that can be used when working with the ARM micro-processor. The GP2021 chip furthermore maintains a low accuracy realtime clock fed by a 32.568 kHz crystal. It also derives a 20 MHz clock frequency for the ARM processor.

All software tasks operate in the 32-bit P60ARM-B micro-processor [10] that provides a peak performance of 20 MIPS and has a typical spare capacity of 35% at 1 Hz navigation rate and 25% at 2 Hz. Upon start-up (or a reset) of the receiver, a boot loader (stored in EPROM) is activated that copies the executable code and initialisation data from the EPROM into the

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RAM memory. The EPROM is arranged into two 16 bit wide chips (256 kB total), while RAM is partitioned into four 8 bit wide memory chips with a total size 512 kB. The RAM memory contents can be maintained by a dedicated backup power supply line with a current of approximately 0.1 mA.

The main board offers an SMA (or MCX) connector for the GPS antenna. It is connected to the interface board via a 9-pin header that provides two bi-directional serial lines, the main and backup power supply, an input discrete and a reset line. Optionally, a tenth pin is made available for the pulse-per-second signal. A summary of the pin assignment is provided in Table 2.1.

Table 2.1 Pin assignment for GPS Orion interface connector

Pin Function

1Ground

2Vdr (memory backup positive supply)

3RX B serial input

4RX A serial input

5TX B serial output

6TX A serial output

7Discrete input line (used as a “lift-off” signal)

8Vdd level sense circuit output (used as a “reset” if connected to GND)

9Vdd (+5V prime power supply input)

10PPS output (optional)

General physical and electrical parameters of the Orion main board are summarized in Table 2.2. The GPS Orion receiver and its components have not been validated for space applications. Nevertheless, limited information on the radiation hardness of the core chipset suggests it’s suitability up to a total dose of about 15 krad [11]. However, no latch-up protection is presently provided to safeguard against destruction of CMOS circuits under the action of heavy ions. Other than the standard Orion receiver, the main boards of the Orion-S and -HD receivers are not equipped with a “supercap” capacitor, since this is not considered vacuumproof. This means that the non-volatile memory and real-time clock is lost whenever the main board is disconnected from the backup power supply (pin 2).

Table 2.2 Physical and electrical parameters of GPS Orion main board

Parameter

Value

Dimension

95mm x 50mm x ~10m m

Weight

ca. 50g

Operations Temperature

-40°C to +85°C (as per [1])

Storage Temperature

-50°C to +110°C (as per[1])

Main power supply

+5V DC (+/- 10%), 400 mA (2W)

Backup power supply

>+2.2V DC, ca. 100 μA

Data I/O levels

CMOS TTL (0V, +5V)

RF input

 

Connector

SMA (or MCX)

Active antenna power supply

+5V DC, 50 mA

Impedance

50Ω

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2.2Interface Board

The interface board provides auxiliary devices that are required for standalone operation of the Orion receivers. It comprises

a switching regulator allowing operation from unregulated power supplies,

a rechargeable battery to maintain the non-volatile memory and real-time clock during power down times and

two RS232 serial line drivers for communication with standard peripheral devices.

Key parameters of the interface board are summarized in Table 2.2.

Table 2.2 Physical and electrical parameters of GPS Orion interface board

Parameter

Value

Dimension

95mm x 50mm x 20mm

Weight

70g

Operating voltage

8–30V

Efficiency of switching regulator

85%

Total power consumption (I/F and main board)

2.4 W

Battery

+3.6V NiCad, 110 mAh ([1])

I/O ports

2 x RS232 (±10V)

 

Sub-D9 connector (male)

The two serial ports support the ground, receive and transmit line using the standard pin assignment for Sub-D9 connectors (Table 2.2). Pins 7 and 8 are cross-connected since the Orion receiver does not support a hardware handshake. Likewise the three pins 1, 4, and 6 are connected among each other.

Table 2.2 Pin assignment for RS232 Sub-D9 connectors (Port A and B)

Pin

Description

Remarks

Schematic

1

DCD (Data Channel Received

Connected with DTR and DSR (pins 4 , 6)

 

 

Line Signal Detector)

 

 

2RxD (Receive Data)

3TxD (Transmit Data)

4

DTR (Data Terminal Ready)

Connected with DCD and DSR (pins 1 , 6)

5

GND (Signal Ground)

 

6

DSR (Data Set Ready)

Connected with DCD and DTR (pins 1 , 4)

7

RTS (Request to Send )

Connected with CTS (pin 8)

8

CTS (Clear to Send)

Connected with RTS (pin 7)

9

RI (Ring Indicator)

Not connected

2.3Antenna

The GPS Orion receiver is operated with an active antenna (or a passive antenna and external preamplifier) having a minimum gain of 16 dB and a noise-figure of less than 4 dB More specifically, the ANPC-131 antenna of M/A COM is recommended (cf. [4]), for terrestrial applications. It offers an LNA gain of +26 dB and a 1.5 dB noise-figure at the L1 frequency (1575.42 MHz).

For space applications dedicated antenna designs with heat and vacuum resistant radomes are generally required. For sounding rockets wrap around antennas, helix tip antennas or blade antennas with separate preamplifiers are available on request. GPS antennas for satellite applications are offered by e.g. Sensor Systems Inc.

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3. Operations Guide

3.1Basic Receiver Handling

3.1.1Hardware Setup

For operating the GPS Orion receiver in a ground based test environment, the following hardware items are typically required:

Orion main board

Orion interface board with power cable

Power supply or battery (typically +12 V, 250 mA)

Active GPS antenna (ca. 26 dB gain) with cable and SMA (or MCX) connector (male)

PC with Windows operating system

Serial interface cable (cross-link with female-female sub-D9 connectors)

Upon first operation, mount the main board on top of the interface board and connect both board via the 9-pin connector. Since the standard interface board provides no PPS interface, pin 10 of the main board (optional) will remain unused in this configuration. Next,

connect the active antenna to the antenna plug on the main board

connect port A (left) of the interface board to the PC’s COM1 port

connect blue cable to ground pin of power supply (minus pole of battery)

The receiver will start to operate once the red cable is connected to the plus pole of the power supply.

3.1.2Precautions

To avoid an undesirable behavior or even destruction of the receiver, the following handling instructions shall be considered:

The center pin of the antenna connector provides a +5V power supply for the low noise amplifier of an active GPS antenna. To avoid short cuts it is strongly advisable to disconnect the receiver from the power supply prior to (dis-)connecting the antenna or pre-amplifier.

R/F attenuators between the receiver and the pre-amplifier must be equipped with a DC by-pass to avoid heating of the attenuator or an overload of the receiver’s DC power feed.

Always connect the plus pin of the power supply last and disconnect it first. Otherwise spurious ground connections via the serial cable or the antenna line may keep the receiver unintentionally powered up.

3.1.3Serial Communication

The Orion-S and -HD receivers use port A (left connector) as the prime port for command input and message output. By default, this port employs the following RS232 communication parameters:

19200 baud

no parity

8 data bits

1 stop bit

no handshaking

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For proper communication, these values must match the settings of the PC communication port.

While the Orion receiver is most conveniently used via a dedicated monitoring and control program (e.g. OrionMonitor), elementary operations may likewise be carried out via a standard terminal program. As an example, the HyperTerminal program provided with the Windows operating systems can be used to monitor receiver output messages in real-time and to record the data stream to a file. Vice-versa, commands can be loaded to the receiver from pre-configured files or entered via the keyboard. In the latter case, the STX (0x02) and ETX (0x03) characters marking the command start and end can be generated by pressing the CNTL-B and CNTL-C keys, respectively. If desired, consecutive commands may be separated by white space like blanks or line feeds. Please note, that the correct checksum must be provided for each command to allow proper execution.

3.1.4Start-Up and Initialization

At power-up the receiver performs the following initialization steps:

The boot loader is executed and the program code is loaded from EPROM to RAM memory.

If non-volatile memory has been retained since the previous activation, the receiver restores the latest almanac, broadcast ephemerides, ionospheric and UTC parameters, trajectory aiding parameters, as well as the current time.

If the receiver was temporarily disconnected from the backup power supply or the respective NVM data are corrupted, the time, almanac and trajectory aiding parameters are initialized with hard-coded default data (Note: The actual values used for the default initialization depend on the particular software release and may vary between receivers). The ephemeris data are marked as unavailable.

A boot message identifying the current software version is issued.

Subsequently, the signal tracking is started and the receiver starts outputting a predefined sequence of messages at a 1 Hz rate. The same steps are performed when the reset button on the interface board is pressed.

Depending on its previous usage the receiver should start tracking and deliver navigation fixes between a minimum of 30 s (hot start with known time, position and ephemerides) and a maximum of 15 min (cold start). To speed-up the signal acquisition various commands can be employed to provide the receiver with a priori information. A comprehensive initialization sequence is listed below. Some steps are optional and may be skipped as desired.

To discard all existing receiver settings issue the CS (cold start) command followed by a reset (or reboot) of the receiver. This will return the receiver into a native state with time, almanac, and trajectory aiding parameters determined by the firmware defaults.

Set the current date and time (using the SD and ST commands). For static receiver operation an accuracy of 10 min is generally sufficient. For LEO operations and initializations in the free-flight phase of ballistic vehicles a maximum error of 10 s is tolerable.

For unaided operation, set the geographic coordinates (using the IP command) or the initial state vector (using the PV command). For static receiver operation an accuracy of 1° is generally sufficient and the altitude can be assumed as zero (sea level).

For aided operation set the trajectory parameters (using the LO command for LEO operations or the LT and ET commands for ballistic trajectories).

Load a set of current almanac parameters based on e.g. a YUMA almanac (using the LA and F13 commands). If desired, the almanac may be complemented by ionospheric correction data and UTC leap second information (F15 command).

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If the above steps have taken more than two minutes, the receiver may have started to scan through the permitted range of frequency bins. Reset or reboot the receiver to start the signal search in the central frequency bin.

Select the desired aiding mode (using the AM command).

Set other operations parameters (e.g. output rates, elevation mask, etc.) as desired.

The receiver should now be indicating proper tracking and a valid 3D navigation fix as part of the periodic navigation and status messages.

3.1.5Output Selection

The output of the GPS Orion-S/HD receiver can, to a limited degree, be configured according to the user needs. All relevant commands and the available output messages are described in full detail in Chap. 4 of this User’s Guide.

In start-up configuration1 the receiver outputs an F00 (geodetic) and F40 (Cartesian) navigation message with time, position and velocity as well as the number of tracked satellites once per second. Channel status information is available as part of the F03 or F43 message that is likewise issued at the 1 Hz update rate. Navigation and status data belong to a class of periodic receiver messages that can be controlled using the DR (Data Rate) command. It sets the output interval of a specified message number in multiples of the navigation interval. Furthermore messages can be polled once or disabled completely. The data rate selection is available for the F00/03/04/05/08 WinMon messages (i.e. the standard Mitel message set of the original Orion receiver firmware), the F40/41/42/43/45/46/47/48 WinMon messages (specific for the Orion-S and/or –HD receiver) as well as a limited set of standard and proprietary NMEA type navigation and status messages.

Dedicated commands are available for polling specific configuration and operations parameter on demand. These comprise the SA command (Send Almanac, ephemerides and iono/UTC data), the TA command (Transmit Almanac), the TE command (Transmit Ephemeris), the TO command (transmit orbit) and the TT (Transmit Trajectory) command.

Aside from the periodic and polled outputs, the receiver autonomously issues various messages on the occasion of special events:

At start-up, a boot message (F99 format) is transmitted that dentifies the current software version.

Upon reception and processing of most commands a response message (F98 format) is issued.

Broadcast ephemeris parameters (F14 message) are transmitted in the Orion-S at start-up and whenever new values become available as part of the GPS navigation message.

These messages are cannot be deactivated and may result in temporary output buffer overflows, when the communication channel does not provide a sufficient bandwidth for all periodic and non-periodic data.

1 On customer request, other default configurations may be implemented in the firmware of project specific software releases.

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3.1.6Pulse-per-Second Signal

Supporting receiver versions provide a one-pulse-per-second signal (CMOS TTL level) at pin 10 of the interface connector. The PPS signal is available in case of valid navigation. It has a one millisecond duration and its starting edge is aligned to the occurrence of an integer GPS second with an accuracy of better than 1 μs. The typical error amounts t ca. 0.2 μs and is determined by the limited resolution of the correlator timing (175 ns) and the accuracy with which the modelled GPS time of the receiver matches the true GPS system time (<0.1 μs with S/A off). When using long antenna cables in ground based tests, the PPS will experience a systematic shift in accord with the added signal time.

Irrespective of the availability of an output pin for the PPS hardware signal, the measurements and navigation solution of the receiver are aligned to the integer GPS second whenever a continuous 3D navigation solution has been achieved.

3.1.7Troubleshooting

If deemed necessary, various electrical and functional checks may be performed at any time to validate the proper receiver operation:

The product of the supply voltage and current consumption shall match the nominal power consumption of 2.4±0.1W. A lower value may indicate errors in the boot process caused by e.g. twisted EPROMs or a broken address/data line on the main board.

When connected to a terminal program, the receiver shall output a continuous stream of (mostly numeric) ASCII characters. Failures to do so may indicate problems with the physical connection (e.g. twisted RX/TX lines of the serial cable) or a wrong configuration (baud rate, etc.) of the PCs COM port.

The receiver shall respond to commands (for a simple test, try the <STX>DR0010A<ETX> and <STX>DR000117<ETX> commands to toggle the F00 message output). Failures may again indicate problems with the physical connection or the communication software.

With adequate open sky visibility the receiver shall achieve code lock (“C”) with an SNR value of better than 10 dB on (at least) one channel within a maximum of 5 min irrespective of its initialization state. Otherwise, problems in the antenna system (passive versus active antenna, inappropriate or erroneously connected pre-amplifier, broken antenna cable, etc.) may be suspected.

If other problems in the antenna system can be ruled out, one may further verify that the center pin of the antenna connector has a DC level of +5.0±0.1V with respect to ground.

In case of persistent failures inspection by the manufacturer may be required.

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3.2Special Applications

3.2.1Aiding for Ballistic Trajectories

To allow a rapid acquisition and an optimal channel allocation in case of high vehicle dynamics the Orion-HD receiver can be aided by a priori trajectory information. For sounding rockets or other ballistic missions the nominal flight path is represented by a piecewise, low order polynomial approximation stored within the receiver (Fig. 3.1, [12]). Using this information the GPS satellites in view and the expected Doppler shift can be computed at any time after launch.

z

 

Sounding Rocket

 

 

 

Trajectory

 

 

 

 

 

 

 

 

w0,t0

 

 

 

 

 

ax,bx,c x

 

 

 

 

 

ay,by,c y

 

 

 

 

 

az,bz,c z

 

 

 

 

 

 

w0,t 0

 

 

 

 

 

 

 

 

 

 

 

 

 

ax ,bx,c x

 

w0,t0

 

 

ay ,by,c y

 

 

 

az ,bz,c z

 

ax,bx ,cx

 

 

 

ay,by ,cy

 

 

 

 

az,bz ,cz

 

 

 

 

y

Time

x

1st segment

2nd segment

3rd segment

Fig. 3.1 Piecewise polynomial approximation of the reference trajectory of a sounding rocket. Each time interval is represented by its start epoch (GPS week and seconds) and three coefficients per axis.

To minimize the computational workload in each step, a simple 2nd-order polynomial

 

æ x ö

 

æa

ö

 

æb ö

 

 

æc

ö

 

 

 

 

r (t) =

ç ÷

=

ç

a

x ÷

+

ç

x ÷

 

) +

ç

c

x ÷

(t - t

 

)2

(3.1)

ç

y

÷

ç

y ÷

ç

b

(t - t

0

ç

y ÷

0

 

 

 

 

 

y ÷

 

 

 

 

 

 

ç z

÷

 

ç a ÷

 

çb

÷

 

 

çc

÷

 

 

 

 

 

è ø

 

è

 

z ø

 

è

 

z ø

 

 

è

 

z ø

 

 

 

 

is used to approximate the trajectory over discrete time intervals in the WGS84 reference frame. Upon differentiation, one obtains an associated approximation of the instantaneous Earth-fixed velocity vector

æ x ö

æb

ö

æc

ö

 

 

ç

&

÷

ç

x ÷

ç

x ÷

 

 

&

0 ) ,

(3.2)

v(t ) = ç y ÷

= çby ÷

+ 2çcy ÷(t -t

ç

&

÷

ç

÷

ç

÷

 

 

è z ø

èbz ø

è cz ø

 

 

which is linear in time. Accordingly, the individual time intervals should be chosen in such a way as to exhibit a near constant acceleration. Up to 15 polynomials can be configured and stored which is sufficient to provide a position accuracy of about 2 km and a velocity accuracy of roughly 100 m/s in representative missions.

Based on the polynomial approximation of the nominal trajectory, the reference position and velocity of the host vehicle are computed once per second. The result is then used to obtain the line-of-sight velocity and Doppler frequency shift for each visible satellite, which in turn serve as initial values for the steering of the delay and frequency locked loops. The positionvelocity aiding thus assists the receiver in a fast acquisition or re-acquisition of the GPS sig-

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nals and ensures near-continuous tracking throughout the boost and free-flight phase of the ballistic trajectory.

The command interface of the Orion-HD receiver supports a total of six different instructions to support the handling of ballistic trajectory information:

The LT (Load Trajectory) command initiates the upload of a set of trajectory polynomials.

Each trajectory polynomial is then loaded in the form a single F51 command message.

The sequence is terminated by the ET (End Trajectory) command.

The reference epoch for the trajectory polynomials can be configured using the LE (Load Epoch) command, unless it is automatically detected through a hardware lift-off signal (see below).

Using the TT (Transmit Trajectory) command, the currently loaded trajectory information can be dumped. When issued, the receiver outputs an F50 message providing the reference epoch and sequence of F51 messages containing the individual trajectory polynomials.

Finally, the aiding can be activated (or deactivated) through the AM (Aiding Mode) command.

Both the reference epoch and the trajectory polynomials are stored in non-volatile memory and made available upon a reboot of the receiver.

The aiding is designed to support a rapid acquisition and re-acquisition after temporary signal losses. It controls the initial configuration of a previously void tracking channel but has no impact on those channels that have already achieved a continuous code and carrier lock and follow the signal dynamics with their respective tracking loops. When aiding is activated, the Doppler and visibility prediction depends only on the a priori trajectory polynomials, and the time since the reference epoch. As such, a faulty or outdated navigation solution has no impact on the initialization of new channels and safe acquisition can even be achieved if during boosted flights that do not allow a linear prediction of the latest state vector. On the other hand, erroneous values may be predicted in case of a major deviation from the nominal flight profile. The choice of aided versus unaided operation must therefore be based on a careful risk assessment. Aiding is clearly advisable, if continued tracking cannot be assured due to e.g. a changing field-of-view or switching between antennas. Unaided operation, on the other hand, may be preferable, if a stable initial acquisition and continued GPS visibility can be assured but the actual flight profile is not know with good confidence before the mission.

3.2.2Lift-off Signal

The discrete input pin of the GPS Orion-HD main board can be employed to automatically sense the lift-off time of a sounding rocket and set the reference epoch for the trajectory aiding. The lift-off signal is defined to remain low while the rocket is grounded and switch to high level at lift-off. While set to low, the receiver continuously overwrites the reference time for the trajectory polynomials by the current time. This update is performed at each TIC and is thus accurate to about 0.1 s. For proper function, the lift-off signal must remain high throughout the entire flight.

3.2.3IIP Prediction

The instantaneous impact point (IIP) describes the touch-down point of a sounding rocket under the assumption of an immediate end of the propelled flight. It is representative of a situation in which the rocket motor is instantaneously switched off by the mission control center following e.g. a guidance error during the boost phase. As part of the range safety operations during a sounding rocket launch, a real-time prediction of the IIP is performed to monitor the expected touch down point in case of a boost termination. The computation and dis-

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