Kenn Gold

Orbit Determination for the QuikSCAT Spacecraft

An operational orbit determination system for QuikSCAT has been developed to meet the requirement for 100-m(3ae) positioning knowledge. This is nominally accomplished by processing global positioning system (GPS) position solutions in a dynamic e lter. The operational orbit determination system produced 24-h overlapping arc position errors between 15 and 25 m (root-sum-square )and 3-h arc overlaps between 5 and 6 m (root-sum-square ) for seven-day and one-day arcs, respectively. We also investigated the use of short segments of GPS pseudorange and carrier phase data and obtained results that differ by less than 10 m from the nominal orbit solutions. A third investigation considered the feasibility of a backup orbit determination system using antenna azimuth and elevation angles from three ground tracking stations. The methods and results of processing these three data types are presented.

Precision Orbit Determination for the Geosat Follow-On Satellites

The potential radial orbit accuracy of satellites in low Earth orbits has been investigated through simulation analysis and the reduced dynamic technique. Toward that end, a simulation interface has been developed so that the GIPSY OASIS II orbit determination software system may be used in simulation analyses. In simulation mode, orbits and measurement data are generated with a given set of models, after which the simulated data are processed with a different set of models. The results are compared to the simulated data, and the difference is due solely to modeling differences and data noise. By introducing model errors of appropriate size, realistic orbit error may be simulated. The simulation technique described has been applied to the GEOSAT Follow-On and TOPEX/Poseidon satellites. The results of the study, incorporating only six ground stations into the solution, show that radial orbit accuracy in the GEOSAT orbit will be approximately 4.3 cm rms. When the number of ground tracking stations was increased to 13 and the parameters for the reduced dynamic orbit determination technique were tuned, the radial error was reduced from 4.3 to 3.0 cm in the GEOSAT orbit and from 1.9 to 1.6 cm rms in the TOPEX/Poseidon orbit.