Mitel GPS Orion-S-HD Receiver User Manual
Space Flight Technology, German Space Operations Center (GSOC)
Deutsches Zentrum für Luft- und 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
Disclaimer
mation in this manual has been compiled with adequate care and represents the best
a-
t-
ing or erroneous information. Furthermore, DLR reserves the right to change
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
Infor
knowledge of the authors. Any errors remaining after its release will be fixed upon notific
tion. In no way shall DLR or the authors be held liable for direct or indirect damage resul
ing from mis s
interfaces and system specifications in future releases.
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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 A pplications...................................................................................................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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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 Nav igation 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 Impact 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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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 Luft- und 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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User’s Manual for the GPS Orion-S/-HD Receiver
1. Introduction
1.1 GPS 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 Sem iconductor 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.
1.2 Functional 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.
1.3 Receiver 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
2.1 Main 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. I t 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
1 Ground
2 Vdr (memory backup positive supply)
3 RX B serial input
4 RX A serial input
5 TX B serial out put
6 TX A serial output
7 Discrete input line (used as a “lift-off” signal)
8 Vdd level sense circuit output (used as a “reset” if connected to GND)
9 Vdd (+5V prime power supply input)
10 PPS 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 vacuum proof. This means that the non-volatile memory and real-time clock is lost whenever the main
board is disconnected from the backup power su pply (pin 2).
Table 2.2 Physical and electrical parameters of GPS Orion main board
Parameter Value
Dimension 95mm x 50mm x ~10mm
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
Data I/O levels CMOS TTL (0V, +5V)
RF input
Connector SMA (or MCX)
Active antenna power supply +5V DC, 50 mA
Impedance
>+2.2V DC, ca. 100 µA
50Ω
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2.2 Interface 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)
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 cros s-connected since the
Orion receiver does not support a hardware handshake. Likewise the three pins 1, 4, and 6
are connected among each other.
Sub-D9 connector (male)
Table 2.2 Pin assignment for RS232 Sub-D9 connectors (Port A and B)
Pin Description Remarks Schematic
1 DCD (Data Channel Received
Line Signal Detector)
2 RxD (Receive Data)
3 Tx D (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
Connected with DTR and DSR (pins 4, 6)
2.3 Antenna
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
3.1 Basic Receiver Handling
3.1.1 Hardware 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.
3.1.2 Precautions
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.
3.1.3 Serial 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.
3.1.4 Start-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 cu rrent 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 de-
fault 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.
3.1.5 Output 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 des cribed
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 param eter 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 identifies 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 sof t-
ware releases.
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3.1.6 Pulse-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 si gnal 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.
3.1.7 Troubleshooting
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 su spected.
• 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.
Document No. Issue 1.0
GTN-MAN-0110 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: 12
User’s Manual for the GPS Orion-S/-HD Receiver
3.2 Special Applications
3.2.1 Aiding 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 roc kets or other ballistic mis sions 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
w0,t
0
ax,bx,c
x
ay,by,c
y
az,bz,c
z
w0,t
0
ax,bx,c
x
ay,by,c
y
az,bz,c
z
x
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.
1st segment 2nd segment
Sounding Rocket
Trajectory
w0,t
0
ax,bx,c
ay,by,c
az,bz,c
3rd segment
x
y
z
y
Time
To minimize the computational workload in each step, a simple 2nd-order polynomial
t
=r (3.1)
a
y
=
a
z
a
x
b
x
y
z
x
b
+
y
b
z
c
x
c
tt
+−
y
c
z
2
)()()( tt
−
00
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
&
b
x
&
y
t
=
v , (3.2)
&
z
x
b
=
y
b
z
c
x
c
+
y
c
z
)(2)(
tt
−
0
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 mi ssions.
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-
Document No. Issue 1.0
GTN-MAN-0110 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: 13
User’s Manual for the GPS Orion-S/-HD Receiver
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 temp orary 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 polynom ials, 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 m ajor deviation from the nominal flight
profile. The choic e 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.
3.2.2 Lift-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.
3.2.3 IIP 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 ce nter 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-
Document No. Issue 1.0
GTN-MAN-0110 June 22, 2003
DLR/GSOCNo part of this document shall be reproduced in any form or disclosed to third parties without prior authorization.
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