GPS, Dual Frequency, SA Free Satellite Navigation

Patrick W. Binning

Abstract:	A software utility is under development that will estimate a spacecraft state using GPS pseudorange and carrier phase measurements. This tool imple- ments a Kalman filter along with a smoothing algo- rithm. In addition, a measurement simulator is being developed to test the algorithms and methods designed for the filter. However, simulated measurements can give misleading results and it is desirable to process real measurements from a real satellite. It is of interest to determine the maximum accuracy with which one can estimate a spacecraft state by only using measure- ments taken at the user vehicle. For this reason dual frequency data that is not SA corrupted is desirable. A specific TOPEX/Poseidon GPS data set is used as the basis for this study because it fits the two above stated requirements. In 1993 the GPS constellation was not yet considered fully operational and therefore Anti-Spoofing was not on. This allowed all users to receive both the Ll and L2 frequencies. In addition, over the period spanning November 17.5 - 19 of that year, Selective Availability (SA) was also off on a lim- ited number of GPS satellites. This allowed civilian users access to the most precise GPS measurements. The estimation of TOPEX’s state is done using both the GPS broadcast navigation information as well as post processed GPS states that are available from the Jet Propulsion Laboratory and the Defense Mapping Agency. Accuracy of the estimated TOPEX orbit is found by comparing the filter’s estimate to that found by JPL. The Precision Orbit Ephemeris (POE) com- puted at JPL is stated to have a 3-D RMS error below 15 cm. This gives an excellent tool for determining the accuracy of our estimates. It will be shown that navigation in real time can be achieved to better than 7 meters, 3-D, RMS over one day. Use of post processed GPS orbits has shown that estimates of a user orbit can be generated that are accurate to less than 50 cm RMS without the use of carrier phase measurements nor direct ground station data.
Published in:	Proceedings of the 52nd Annual Meeting of The Institute of Navigation (1996) June 19 - 21, 1996 Royal Sonesta Hotel Cambridge, MA
Pages:	803 - 812
Cite this article:	Binning, Patrick W., "GPS, Dual Frequency, SA Free Satellite Navigation," Proceedings of the 52nd Annual Meeting of The Institute of Navigation (1996), Cambridge, MA, June 1996, pp. 803-812.
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