The Onboard Gps Orbit Determination Of Leo Satellite Geometry And Dynamics Smoothing Method

GPS-based orbit determination is nowadays one of the hotspots in satellite geodesy studies. The thesis deals more deeply with the geometric orbit determination and the dynamic orbit smoothing of geometric orbits for low-orbit satellite equipped a GPS receiver onboard, based on precise orbits and clock offsets of GPS satellites. In this thesis experiments are implemented on the CHAMP orbit determination using onboard GPS measurement data, by means of the geometric method and dynamic smoothing. The main work and conclusions of this study are summarized as follows. The error sources in GPS-based orbit determination and the accuracies of GPS satellite ephemerides are analyzed. The usage of GPS clock offsets in GPS-based orbit determination is discussed. It is found through the comparison of the 5 min clocks interpolated from 15 min IGS precise clocks with the 5 min IGS clocks that the precision of interpolation clocks is 0.046m, which can meet the requirement for LEOs orbit determination. The noise level of the CHAMP's onboard GPS measurements is analyzed. The results show that the pseudorange noise is 0.236m, and it is found that the noise level becomes high markedly as the elevation angle of satellites is decreases below 30? It is proposed that in precise orbit determination the accuracy of pseudorange should be better than 0.3m and the accuracy of phase should be better than 3mm. The pre-processing of GPS undifferenced observation data is studied in detail. An automatic editing algorithm for undifferenced measurements, as used by JPL and BERNE, is employed to effectively detect and repair cycle slips, as well as to reject outliers. The basic theory and algorithms of geometric orbit determination are studied. The comparisons in precision of orbit determination are made between orbits obtained from original pseudoranges and smoothed pseudoranges, based on CHAMP's GPS observations. About half a month of CHAMP orbits are computed by the geometric method. The results show that the GPS pesudoranges smoothed by carrier phases can significantly improve the precision of orbit determination, and that the precision of orbits from IGS precise ephemeris and interpolated clocks is equivalent to that of orbits from JPL precise ephemeris and the 30s clocks. The dynamic smoothing method of the geometric orbits is researched. The usages of non-conservative force models and CHAMP accelerometer (ACC) data are studied. The geometric orbits are smoothed with ACC data incorporated. The results show that the dynamic smoothing can improve the precision of geometric orbits. The orbits of CHAMP are calculated using 16 day onboard GPS observation data. Results show that the geometric orbits are accurate to 0.6m,0.27m and 0.25m in radial,along-track and cross-track respectively, and that the dynamically smoothed orbits are accurate to 0.35m,0.20m and 0.222m respectively.