Signal Simulator Testing
In cooperation with the Center for Space Research, University of Texas, a generic concept for testing the accuracy of spaceborne GPS receivers has been developed, which is outlined on the left. A comprehensive description of the test setup and sample results are given in our report.
To assess the tracking performance of a spaceborne GPS receiver, artificial GPS signals are generated which closely match the signals received by a spacecraft in low Earth orbit. The raw measurements and the navigation solution obtained by the receiver may then be compared against the simulated values. Other than a ground based testing with a roof antenna, the use of a signal simulator provides a realistic signal dynamics including high Doppler shifts and line-of-sight accelerations. In addition it allows the separate study of individual error sources (e.g. broadcast ephemeris errors or ionosphere) that may affect the quality of the resulting tracking data.
The test description is based on the use of a Spirent STR4760 or STR7700 GPS signal simulator (Fig. on the right) with one (or more) R/F outlet(s) and at least 12 single frequency (L1) channels. The choice of other simulator models or brands does not affect the basic test concept but may necessitate minor adaptations or restrictions. For testing dual frequency receivers, the simulator must be equipped with an L2 option and configured to generate P/Y code as desired.
The STR4760 simulator setup is maintained in the form of configuration files, each covering a specific group of parameters. The root file (*.SCEN) provides a list of all configuration files that make up a particular scenario. It can be viewed with standard editors and printed for documentary purposes. With this exception, all configuration data are stored in binary files and should only be modified using the windows based configuration editors of the simulator software. The full set of configuration files required to conduct the is available for (download) at this web site.
The simulation is setup for Nov. 6., 2001 and the corresponding GPS constellation is based on a YUMA almanac for GPS week 1138 (=114) and reference epoch toa= 589824 s (i.e. approximately 2.2 days before the simulation epoch). It comprises a total of 28 healthy satellites (all PRNs except 12, 16, 19, 32). The modeled spacecraft is assumed to fly in a 450 km altitude near circular, polar orbit, which resembles that of the Champ and Grace satellites.
In-Flight Performance Analysis
JPL's BlackJack receiver currently represents the most widely used geodetic grade GPS receiver for space applications. Using data from the CHAMP science mission, the in-flight performance of the BlackJack receiver has been assessed and the impact of various soft-ware updates performed during the 2.5 years since launch is described. Key aspects of the study comprise the channel allocation, anomalous data points, and the noise level of the code and carrier data. In addition, it has been demonstrated that the code measurements collected onboard the CHAMP satellite are notably affected by multipath errors in the aft looking hemisphere, which can be attributed to cross-talk between the occultation antenna string and the prime POD antenna. For carrier smoothed 10 s normal points the code noise itself varies between a minimum of 5 cm at high elevations and 0.5 m (C/A) to 1.0 m (P1, P2) at 10° elevation. Carrier phase data exhibit representative errors of 0.2 to 2.5 mm. The results of the CHAMP GPS data analysis contribute to a better understanding and possible improvement of the BlackJack receiver and support the design of optimal of data editing and weighting strategies in precise orbit determination applications.
Montenbruck O., Holt G;Spaceborne GPS Receiver Performance Testing;DLR-GSOC TN 02-04; Deutsches Zentrum für Luft- und Raumfahrt, Oberpfaffenhofen (2002).
Holt G., Lightsey E. G., Montenbruck O.;Benchmark Testing for Spaceborne Global Positioning System Receivers;AIAA-2003-5666; AIAA Guidance, Navigation and Control Conference, Aug 11-14, 2003, Austin, Texas (2003).
Montenbruck O.;Performance Assessment of the NovAtel OEM4-G2 Receiver for LEO Satellite Tracking;DLR-GSOC TN 03-05; Deutsches Zentrum für Luft- und Raumfahrt, Oberpfaffenhofen (2003).
Montenbruck O., Kroes R.;In-flight Performance Analysis of the CHAMP BlackJack Receiver;GPS Solutions 7, 74-86 (2003).DOI 10.1007/S10291-003-0055-5
Markgraf M., Montenbruck O.;Total Ionizing Dose Testing of the Orion and Phoenix GPS Receivers;DLR-GSOC TN 04-01; Deutsches Zentrum für Luft- und Raumfahrt, Oberpfaffenhofen (2004).
Lux H., Markgraf M.;Thermal-Vacuum Testing of the Phoenix GPS Receiver;DLR-GSOC TN 04-07; Deutsches Zentrum für Luft- und Raumfahrt, Oberpfaffenhofen (2004).
Langley R. B., Montenbruck O., Markgraf M., Kang C.S., Kim D.;Qualification of a commercial dual-frequency GPS receiver for the e-POP platform onboard the Canadian CASSIOPE spacecraft;2nd ESA Workshop on Satellite Navigation User Equipment Technologies, NAVITEC'2004, 8-10 Dec. 2004, Noordwijk, The Netherlands (2004).
Montenbruck O., Williams J., Wang T., Lightsey G.;Preflight Validation of the IGOR GPS Receiver for TerraSAR-X;DLR-GSOC GTN-TST-0200; Deutsches Zentrum für Luft- und Raumfahrt, Oberpfaffenhofen (2005).
Montenbruck O.;Preflight Validation of the MosaicGNSS GPS Receiver for TerraSAR-X;DLR-GSOC GTN-TST-0210; Deutsches Zentrum für Luft- und Raumfahrt, Oberpfaffenhofen (2005).
Garcia-Fernandez M., Montenbruck O., Markgraf M., Leyssens J.;Affordable Dual-Frequency GPS in Space;16th International ESA Conference on Guidance, Navigation and Control; 17-21 Oct. 2005, Loutraki, Greece (2005).
Leyssens J., Markgraf M.;Evaluation of a Commercial-Off-The-Shelf Dual-frequency GPS Receiver for use on LEO Satellites;ION GNSS 2005 Conference; 13-16 Sept. 2005; Long Beach, California (2005).
Montenbruck O., Garcia-Fernandez M., Williams J.;Performance Comparison of Semi-Codeless GPS Receivers for LEO Satellites;GPS Solutions 10, 249-261 (2006).DOI 10.1007/s10291-006-0025-9
Montenbruck O.;Phoenix-S/XNS Performance Validation;GTN-TST-0120; DLR/GSOC (2007).
Renaudie C., Markgraf M., Montenbruck O., Garcia-Fernandez M.;Radiation Testing of Commercial-off-the-Shelf GPS Technology for Use on Low Earth Orbit SatellitesRADECS 2007, 9th European Conference Radiation and Its Effects on Components and Systems; 10-14 Sept. 2007; Deauville, France (2007).