An Analytical Theory for Orbit Determination

Abstract

The QuikScat spacecraft was launched in June 1999 and is currently measuring vector winds over the global oceans with a spatial resolution of 25 km and a temporal resolution of one day. The University of Colorado is responsible for mission operations, including production of the ephemeris used to produce the science data records. The navigation solutions from the onboard Motorola Vice Roy receiver are smoothed in a post-processing mode to produce the operational ephemeris.

The objective of this study is to develop a simple yet effective method of smoothing the navigation solutions from the onboard GPS receiver in a post-processing mode to provide a medium accuracy continuous ephemeris. The navigation solutions prior to Selective Availability being turned off in May, 2000 had noise at the 100 m level. Currently the noise level is around 10 m. However, this noise is high frequency relative to the dominant orbit perturbations periods. Hence, orbit determination accuracy with SA off is not significantly different from that presented here. The approach used is to augment a J₂ analytical theory for near-circular orbits to include additional frequencies due primarily to tesseral harmonics. Accuracy of the theory was tested through fits to simulated and actual QuikScat GPS receiver data.