An Analysis of GPS Navigation Solutions for Shuttle Mission STS-69.

The NAVSTAR Global Positioning System GPS has provided a quantum leap in real-time autonomous navigation capabilities. NASAs Space Shuttle will be receiving an integrated GPS capability in the near future, and the orbiter Endeavour has been equipped with a stand-alone GPS receiver. Although much data is available regarding spacecraft GPS receiver performance at higher altitudes, little information is available for spacecraft at Shuttle altitudes of approximately 400 km where drag and gravity effects are more pronounced GPS receiver navigation solution data from Shuttle mission STS-69 was made available by NASA and provided an opportunity for evaluating GPS performance in low Earth orbit. NASA ground tracking network and Tracking and Data Relay Satellite TDRS data for this mission provided a reference for comparison. Analysis of the data was accomplished using Satellite Tool Kit STK for visualization and Matlab routines for data comparison. GPS navigation solutions were available for approximately 63 percent of the STS-69 mission, and they generally coincided with the reference track. Differences between the GPS navigation solution state vectors obtained using the Standard Positioning Service SPS and the reference state vectors produced RMS position differences between the data sets of about 1500 m. One sigma position accuracies of 54 m in the vertical direction and approximately 1400 m in the downtrack direction were experienced. Velocity vector magnitude differences during this period were generally or - 1ms, with a RMS velocity difference of less than 9 ms. One sigma velocity accuracies of approximately 4.2 ms in the vertical direction, 2.3 ms in the downtrack direction and 1.5 ms in the crosstrack direction were experienced. A firm conclusion regarding Shuttle GPS accuracies could not be drawn because all sources of error were not identified.