| A Global Positioning System (GPS) receiver was launched with the Extreme Ultraviolet Explorer (EUVE) in June 1992. Since the EUVE mission requires that the satellite rotate at a rate of 3 revolutions per orbit, two GPS antennas were placed on either side of the spacecraft for continuous GPS coverage, with each tracking up to 6 satellites at the single L1 frequency. EUVE is the first mission to carry a GPS receiver at this relatively low altitude ({dollar}sim{dollar}500 km), which places it in much of the Earth's atmosphere, and makes dynamic modelling difficult.; The purpose of the analysis of the EUVE data is to determine the error sources that would limit real time GPS navigation, and to develop techniques to minimize those errors. GPS data from this mission has been analyzed along with data from a globally distributed ground network of receivers to remove the effects of Selective Availability, with the goal of 1 m level GPS orbit determination in a post-processing environment. These solutions were completed with the most complicated dynamic models available for EUVE, along with a single frequency ionosphere calibration technique, and a method to reduce the effects of dynamic model error by estimating additional stochastic accelerations in radial, cross track, and along track directions. The analysis was completed with the Jet Propulsion Laboratory's GIPSY-OASIS II software. These full differential solutions then become the truth to which simulated real time solutions are compared.; The software was then modified to allow a simulated real-time estimation strategy, as could be used on-board the spacecraft. A comprehensive tuning of the reduced dynamic technique at EUVE altitude was completed, and a linear perturbation analysis was done to reduce the size of the gravity field for best performance. Solutions were completed in the presence of selective availability, and when it was deactivated. The EUVE dynamic models are shown to smooth SA error to a level of a few meters in real time studies.; With the techniques developed, full differential solutions are accurate to approximately 1 m in a 3-D sense, while real time orbit accuracy is at a level of 10 to 15 meters 3-D, with most of the error in the along track component. |
