Acquisition Test for the Phoenix GPS Receiver

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					Space Flight Technology, German Space Operations Center (GSOC)
      Deutsches Zentrum für Luft- und Raumfahrt (DLR) e.V.




           Acquisition Test for the
           Phoenix GPS Receiver




                             C.Renaudie




                  Doc. No.   :    TN 06-02
                  Version    :    1.1.
                  Date       :    Dec. 31, 2006
Document Title:                                                                                                                 ii
Acquisition Test for the Phoenix GPS Receiver



Document Change Record
 Issue       Date       Pages                                  Description of Change
1.0      Nov. 30, 2006 all                    First release
1.1      Dec. 31, 2006 all                    Editorial changes, revised summary and conclusions




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TN 06-02                                                                                                         Dec. 31, 2006
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Document Title:                                                                                                                            iii
Acquisition Test for the Phoenix GPS Receiver



Table of Contents

Document Change Record ....................................................................................................ii
Table of Contents ..................................................................................................................iii
Scope.......................................................................................................................................1
Acronyms and Abbreviations ...............................................................................................2
1. Introduction.......................................................................................................................3
   1.1 Phoenix GPS Receiver Description............................................................................3
   1.2 Test Concept ..............................................................................................................3
   1.3 Simulation Scenario ...................................................................................................4
   1.4 Hardware and Software Test Configuration ...............................................................5
2. Software Description and Data Analysis Concept ........................................................6
3. Results and Analysis........................................................................................................8
Summary and Conclusions .................................................................................................12
References ............................................................................................................................13




Document No.                                                                                                                  Issue 1.1.
TN 06-02                                                                                                                   Dec. 31, 2006
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Document Title:                                                                                                                 1
Acquisition Test for the Phoenix GPS Receiver



Scope
This report describes the evaluation of the cold start acquisition performance of DLR’s Phoe-
nix-S GPS receiver using different frequency search windows and PDOP masks. The test
setup and configuration for a Spirent STR4500 GPS signal simulator are specified, the data
analysis strategy is explained and the results presented.




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Document Title:                                                                                                                 2
Acquisition Test for the Phoenix GPS Receiver



Acronyms and Abbreviations
A                             Ampere
ARM                           Advanced RISC Machines
C/A                           Coarse Acquisition
COM                           Communication
CS                            Cold Start
dB                            Decibel
DLR                           Deutsches Zentrum für Luft- und Raumfahrt
DW                            Doppler Window
EPROM                         Erasable Programmable Read Only Memory
GPS                           Global Positioning System
GSOC                          German Space Operations Center
L1                            GPS frequency (1575.42 MHz)
LEO                           Low Earth Orbit
LNA                           Low noise amplifier
MITEL                         Company name
NVM                           Non-Volatile Memory
ORION                         Product name
PDOP                          Position Dilution Of Precision
PHOENIX                       Product name
PRISMA                        Mission
R/F                           Radio Frequency
TTFF                          Time-To-First -Fix
V                             Volt
ZARLINK                       Company name




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TN 06-02                                                                                                         Dec. 31, 2006
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Document Title:                                                                                                                 3
Acquisition Test for the Phoenix GPS Receiver


1. Introduction

1.1       Phoenix GPS Receiver Description

The Phoenix receiver is a miniature GPS receiver for high-dynamics and space applications
([1], [2]). It offers single-frequency C/A code and carrier tracking on 12 channels and can be
aided with a priori trajectory information to safely acquire GPS signals even at high altitudes
and velocities.




                                                                   Fig. 1.1 Phoenix GPS receiver main board (Sigtec
                                                                   MG5001 board –[1], [2]- with standard connectors
                                                                   and backup battery)

The receiver is built around the GP4020 baseband processor of Zarlink. The GP4020 chip
combines a 12 channel correlator for L1 C/A code and carrier tracking, a microcontroller core
with 32 bit ARM7TDMI microprocessor and several peripheral functions in a single package.
Phoenix provides a code tracking accuracy of better than 0.5 m and a carrier-phase accuracy
of better than 1 mm at 45 dB-Hz. With a mass of the receiver board of 70 grams and a power
consumption of 0.85 W at begin of life, the receiver is particularly suited for small satellite
missions.

1.2       Test Concept

To assess the time-to-first -fix (TTFF) performance of the spaceborne Phoenix GPS receiver
(#5, Phoenix Extended Navigation System-XNS), artificial GPS signals are generated and
cold start commands are sent to the receiver every fixed interval of time. From the output
measurements obtained by the receiver, TTFF values are computed.
A cold start (CS) of the receiver consists of sending a “reset all” command which causes the
Phoenix to erase all the data in non volatile memory (NVM, where the receiver software is
stored) and to restart the receiver in the default configuration. After a cold start, the receiver
enters a full-sky search mode in which it starts acquisition of all the GPS satellites until it
succeeds in computing a position.
The experiment consisted of the running of a Matlab/Simulink program which enables to per-
form an automatically test by writing regularly CS commands to the communication port to
which the receiver is connected and reading the port to retrieve the receiver output data.
Two output message types were considered and delivered by the Phoenix receiver periodi-
cally:
      •    The F40 message (cf. Table 1.1) which provides the Cartesian state vector (position
           and velocity) of the host vehicle in the Earth-fixed WGS84 system.
      •    The F43 message (cf. Table 1.2) which indicates the current tracking status of each
           channel. It provides information on the predicted and measured Doppler shift, the sig-
           nal-to-noise ratio and the currently achieved lock status.


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Acquisition Test for the Phoenix GPS Receiver

Table 1.1 F40 Cartesian Navigation Data message description providing GPS week, GPS seconds of week,
GPS-UTC, position, velocity in WGS84, navigation status, number of tracked satellites, PDOP

MsgID Chars.         Format              Description
F40        104                           Cartesian navigation data
              1      x                   <STX>
              3      xxx                 Message Id (=F40)
              4      xxxx                GPS week
             12      xxxxxx.xxxxx        GPS seconds of week [s] (of navigation solution)
              2      xx                  GPS-UTC [s]
             12      sxxxxxxxx.xx        x (WGS84) [m]
             12      sxxxxxxxx.xx        y (WGS84) [m]
             12      sxxxxxxxx.xx        z (WGS84) [m]
             12      sxxxxx.xxxxx        vx (WGS84) [m/s]
             12      sxxxxx.xxxxx        vy (WGS84) [m/s]
             12      sxxxxx.xxxxx        vz (WGS84) [m/s]
              1      x                   Navigation status (0=no-Nav,2=3D-Nav)
              2      xx                  Number of tracked satellites
              4      xx.x                PDOP
              2      xx                  Checksum
              1      x                   <ETX>


Table 1.2 F4 Channel Status message

MsgID Chars.         Format              Description
F40        104                           Cartesian navigation data
              1      x                   <STX>
              3      xxx                 Message Id (=F40)
              4      xxxx                GPS week
             12      xxxxxx.xxxxx        GPS seconds of week [s] (of navigation solution)
              2      xx                  GPS-UTC [s]
             12      sxxxxxxxx.xx        x (WGS84) [m]
             12      sxxxxxxxx.xx        y (WGS84) [m]
             12      sxxxxxxxx.xx        z (WGS84) [m]
             12      sxxxxx.xxxxx        vx (WGS84) [m/s]
             12      sxxxxx.xxxxx        vy (WGS84) [m/s]
             12      sxxxxx.xxxxx        vz (WGS84) [m/s]
              1      x                   Navigation status (0=no-Nav,2=3D-Nav)
              2      xx                  Number of tracked satellites
              4      xx.x                PDOP
              2      xx                  Checksum
              1      x                   <ETX>

The output interval was set to 10s (default setting) which is equivalent to a 1Hz output fre-
quency.

1.3   Simulation Scenario

To assess the navigation solution acquisition performance of the GPS receiver, the Spirent
GPS Signal simulator (GSS) generates artificial GPS signals, which closely match the real
signals in orbit received by a spacecraft in low earth orbit. The DLR_AEO scenario is config-
ured for a spacecraft orbiting the Earth in a near polar-orbit of 408 km altitude and 97° incli-
nation and covers a total of 24 h. The epoch is chosen as 1 Oct. 2008, 0:00 GPS Time, i.e.
the beginning of day 3 (GPS seconds of week=259200) of GPS week 1499. During the test,
a simulation including ionosphere and ephemeris errors is used. These sources may affect
the quality of the resulting navigation solution tracking data but are not expected to impact
the TTFF performance.
To assess the cold start performance, the acquisition aiding has been disabled in the Phoe-
nix receiver.


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Acquisition Test for the Phoenix GPS Receiver


1.4    Hardware and Software Test Configuration

The test description is based on the use of a Spirent STR4500 GPS signal simulator [3],
which provides a realistic signal dynamics with one R/F outlet and 12 single frequency (L1)
channels. The Spirent’s graphical SimPLEX software is used to control the simulator.




Fig. 1.2. Spirent STR4500 simulator and its SimPLEX graphical interface

To avoid stray radiation in the laboratory environment and achieve reproducible signal levels,
the receiver is connected to the simulator’s R/F output via shielded cables. The typical signal
level generated by the receiver (L1 C/A code) matches a value of -130 dBm corresponding to
the signal strength sensed by a terrestrial antenna. In addition, a higher than normal signal
level is required to compensate for the higher noise temperature experienced in simulator
testing compared to the usual antenna sky temperature [4]. A software signal amplification of
about 8 dB is therefore used to reproduce the signal strengths observed in open-air receiver
tests.
The Phoenix receiver requires a low noise preamplifier of matching gain between the simula-
tor outlet and the receiver antenna output. Fig. 1.3 describes the set-up used for the tests




Fig. 1.3 Sample hardware setups for signal simulator testing of the Phoenix GPS receiver.

A summary of the employed test hardware and software is given in Table 1.3. All tests were
conducted at DLR/GSOC.

Table 1.3 Hard- and software configuration used in the Phoenix receiver tests
 Item                                Description
 GPS Phoenix receiver                DLR/GSOC board #5
                                     S/W version D09Ao XNSTST (2006/07/21) and (2006/11/21)
 Preamplifier                        DL1A LNA, 27dB nominal amplification
 Signal simulator                    Spirent STR4500 unit S/N 1617, SimPLEX v2-06
                                     12 channels L1 (C/A)
                                     Default signal power setting +8dB




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Document Title:                                                                                                                 6
Acquisition Test for the Phoenix GPS Receiver


2. Software Description and Data Analysis Concept
A Matlab system-function, an S-function using C++ algorithms is created which first opens
the serial port COM1, configures it and controls its opening. Then, the program reads the
receiver output data and outputs the length and the address of the message. Once a naviga-
tion solution is obtained, a cold start command is sent to the port after a fixed amount of time
(1, 3 or 5 minutes). The useful data corresponding to this sequence are saved to the Matlab
workspace:
At any time, the simulation can be stopped and the port closed by activating the manual
switch of the Simulink model; as for the opening of COM1, this action is controlled (error
messages are generated in case of failure).
This program is executed for 24-hours scenarios. For each scenario, the parameters of Dop-
pler window (DW=30, 20 and 11.5 kHz), PDOP and the amount of time between the obten-
tion of a navigation solution and the sending of the cold start command (1, 3 and 5 minutes)
must be defined, which leads to 9 experiments.
Table 3.1 shows the configurations tested, its parameters, as well as a synthesis of the re-
sults.




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Acquisition Test for the Phoenix GPS Receiver



Table 3.1 Automatic tests steps, tests parameters and Phoenix receiver output data




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Acquisition Test for the Phoenix GPS Receiver


3. Results and Analysis
To analyze the signal acquisition and the navigation performance, extensive testing has been
completed using a Spirent STR4500 12-channel L1 C/A code GPS signal simulator. Actual
test results are provided for a Mitel Phoenix receiver adapted for space applications by
DLR/GSOC. The following graphs are plotted directly from the receiver output data saved in
the Matlab workspace.
Figures 4.1 and 4.2 illustrate the case of frequency searches of 30kHz, 20kHz and 11.5kHz,
with a duration of 180 seconds following the successful acquisition of a navigation solution
and before performing a cold start.
Figure 4.1 indicates that, during the 24 hours-scenario and in these conditions, 119 naviga-
tion solutions are computed by the receiver in an average time of 9 minutes 4 seconds when
a Doppler window of 30kHz is taken in account, against 104 and 88 navigation solutions with
an average TTFF of 10 minutes 30 seconds and 13 minutes 4 seconds when DW parame-
ters of 20kHz and 11.5kHz are applied respectively.
Thus, the results for a Doppler window of 30kHz demonstrate better navigation solution ac-
quisition time compared to the other DW cases with an improvement ranging from 1 minute
25 seconds (20kHz DW case) up to 4 minutes (11.5kHz DW case). Less navigation solutions
are then computed with Doppler windows of 20kHz and 11.5kHz when compared to the
30kHz DW case due to a higher TTFF mean.

                           CS navigation solution acquisition test - GSS: Aeolus scenario - PDOP mask=10, waitsecs=3 minutes
                  30


                                                                                                               DW=30kHz
                  25                                                                                           DW=20kHz
                                                                                                               DW=11.5kHz


                  20
 TFTF [MINUTES]




                  15



                  10



                   5



                   0
                       0    2         4         6         8        10       12        14        16        18       20          22      24
                                                                SCENARIO TIME [HOURS]


Fig. 4.1 TTFF with PDOP mask as a function of time for DW=30, 20 and 11.5 kHz; PDOP mask=10, wait-
secs=180s.


Figure 4.2 represents the distribution of navigation solution times in bins of 3 min. It resem-
bles a Maxwell-Boltzmann distribution for the three frequency searches for which TTFF re-
ceiver performance was evaluated.
Overall, 92%, 88% and 65% of the navigation solutions are obtained within 15 minutes for a
Doppler window of 30kHz, 20kHz and 11.5kHz respectively. One can observe a shift of the
pattern as the frequency search window is decreased.


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Acquisition Test for the Phoenix GPS Receiver


                                      CS navigation solution acquisition test - GSS: Aeolus scenario - PDOP mask=10, waitsecs=3 minutes
                         50

                         45

                         40

                         35
 FRACTIONAL COUNTS [%]




                                                                                                              DW=30kHz
                         30                                                                                   DW=20kHz
                                                                                                              DW=11.5kHz
                         25

                         20

                         15

                         10

                          5

                          0
                                  3              6          9          12          15          18        21         24           27       More
                                                                            ACQUISITION TIME [MINUTES]


Fig. 4.2 repartition of TTFF in acquisition time bins for Doppler Window values of 30, 20 and 11.5kHz. PDOP
mask=10, waitsecs=180s.

Fig. 4.3 is generated to demonstrate that the setting of waitsecs which is used to vary the
duration time between a navigation solution acquisition and a cold start command has no
impact on the time-first-to-fix. The graph is obtained for a Doppler window of 30kHz with a
PDOP restriction of 10 and three waitsecs conditions of 300, 180 and 60 seconds.

                                            CS navigation solution acquisition test - GSS: Aeolus scenario - DW=30kHz - PDOP mask=10
                         25
                                                                                                                              waitsecs=5 minutes
                                                                                                                              waitsecs=3 minutes
                                                                                                                              waitsecs=1 minute
                         20
 TFTF [MINUTES]




                         15




                         10




                          5




                          0
                              0         2            4          6      8         10       12        14        16         18       20        22       24
                                                                               SCENARIO TIME [HOURS]


Fig 4.3: TTFF with PDOP mask as a function of time for DW=30kHz for different waiting time intervals; PDOP
mask=10.



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Acquisition Test for the Phoenix GPS Receiver

In order to show that the orbit is fully covered by the results, Fig. 4.4 presents the sampling of
the argument of latitude between -180° and +180 during the simulator scenario, correspond-
ing to the cold start commands. This asserts the validity of the results, giving reliable informa-
tion.
                                                                 CS navigation solution acquisition test - GSS: Aeolus scenario
                                                                      DW=11.5kHz - PDOP mask=10 - waitsecs=3min
                         4




                         3
 COLD START [COUNTS]




                         2




                         1




                         0
                         0

                                5

                                       0

                                              5

                                                     0

                                                            5




                                                                                                               20

                                                                                                                    35

                                                                                                                         50

                                                                                                                               65

                                                                                                                                    80

                                                                                                                                         95
                                                                    0

                                                                           5

                                                                                  5

                                                                                         0

                                                                                                5

                                                                                                       0

                                                                                                           5




                                                                                                                                               0

                                                                                                                                                    5

                                                                                                                                                         0

                                                                                                                                                               5

                                                                                                                                                                    0
                          8

                                 6

                                        5

                                               3

                                                      2

                                                             0

                                                                 -9

                                                                        -7

                                                                               -5

                                                                                      -4

                                                                                             -2

                                                                                                    -1




                                                                                                                                              11

                                                                                                                                                   12

                                                                                                                                                        14

                                                                                                                                                             15

                                                                                                                                                                   17
                       -1

                              -1

                                     -1

                                            -1

                                                   -1

                                                          -1




                                                                                      ARGUMENT OF LATITUDE [DEGREES]


Fig. 4.4 Coverage of the LEO orbit from the simulation.


Fig. 4.5 represents the TTFF values as a function of argument of latitude; no evident correla-
tion between these two parameters can be seen from this graph.
                                                                 CS navigation solution acquisition test - GSS: Aeolus scenario
                                                                     DW=11.5kHz - PDOP mask=10 waitsecs=3 minutes
                        30



                        25



                        20
       TFTF [MINUTES]




                        15



                        10



                         5



                         0
                          -180                 -135                -90                  -45                0              45              90             135            180
                                                                                  ARGUMENT OF LATITUDE [DEGREES]


Fig. 4.5 TTFF as a function of argument of latitude.


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Acquisition Test for the Phoenix GPS Receiver

Some comments are added below and are made from the Table 3.1. First, a PDOP mask
gives less 4 satellite-navigation solutions than without PDOP mask. Thus, no PDOP mask
means more navigation solutions and better acquisition times. Nevertheless, one has to be
careful not to neglect the PDOP value which reflects the reliability of the navigation solution.
In all tests, an average value of 6 satellites required to compute each navigation solution is
highlighted; up to 10 satellites were solicited so as to fulfill the PDOP mask criteria.
The PDOP figure for which the 3D-navigation solution is achieved in the cases of no PDOP
mask is high and reaching up to 100 which is not good in terms of navigation solution reliabil-
ity. Indeed, the parameter Position Dilution of Precision is important since it characterises the
contribution of the user-satellite geometry in the quality of the navigation solution estimate.
In order to stay under the PDOP threshold of 10, the PDOP value observed averages 5.0,
ranging from 2 to 10.




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Acquisition Test for the Phoenix GPS Receiver



Summary and Conclusions
The cold start acquisition performance of the Phoenix receiver in low Earth orbit has been
evaluated in extensive signal simulator tests. A closed-loop Matlab/Simulink test bed was
developed to control the Phoenix receiver and to conduct automated, time-delayed resets
after each acquisition of a navigation solution. A Spirent STR4500 simulator was used to
generate artificial GPS signals for a low altitude polar satellite orbit. The test bed allowed the
collection of a statistically representative set of test data over several 24h arcs and enabled a
good orbital coverage of the receivers cold starts with minimum operator intervention.
Using the standard PDOP mask of 10 and a search window of 30 kHz, an average time-to-
first-fix (TTFF) of 9 min was obtained. In 92% of all cases the cold start TTFF was less than
15 min. A slightly inferior performance was obtained for Doppler search windows of 20 kHz
and 11.5 kHz, for which the average TTFF increased to 10.5 min and 13 min, respectively. In
these cases the probability of achieving a navigation solution within less than 15 min
amounted to 88% and 65%.
Based on the test results, a Doppler window of 30 kHz is recommended for an optimum cold
start acquisition of the Phoenix GPS receiver. While smaller search windows result in an ob-
vious degradation of the overall acquisition performance, a reliable acquisition and accept-
able TTFF is even obtained with Doppler windows down to 10 kHz. The results clearly indi-
cate that it is not necessary to extend the search window to the actual range of Doppler shifts
(approx. 45 KHz) experienced by a GPS receiver onboard a LEO satellite. Instead, a smaller
window results in more frequent reallocations of the available channels. This, in turn, in-
crease the probability of acquiring a sufficiently large number of satellites with high elevation
and low Doppler shift.




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Acquisition Test for the Phoenix GPS Receiver



References
[1] Phoenix Spaceborne GPS Receiver; Datasheet; DLR/GSOC, Issue 1.0, 1 Dec. 2005.
[2] Montenbruck O., Markgraf M.; User’s Manual for the Phoenix GPS Receiver; DLR/GSOC; GTN-MAN-120,
    Issue 1.7, 6 June 2006.
[3] STR4500 GPS/SBAS Simulator with SimPLEX Software User Manual; DGP00603AAA; Issue 1.11, Jan
    2005.
[4] Van Dierendonck A. J.; GPS Receivers; chap. 8 in: Spilker J., Parkinson B., eds.; Global Positioning System:
    Theory and Applications Vol. I; American Institute of Aeronautics and Astronautics Inc., Washington (1995).




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TN 06-02                                                                                                         Dec. 31, 2006
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