| During the last decade, numerous Low Earth Orbit (LEO) satellites, including TOPEX/POSEIDON, CHAMP and GRACE, have been launched for scientific purposes at altitudes ranging from 400 km to 1300 km. Because of highly complex dynamics of their orbits, coming from the Earth gravity field and the atmospheric drag, accurate and fast LEO orbit determination has been a great research challenge, especially for the lowest altitudes. To support GPS meteorology that requires an accurate orbit in near realtime, efficient LEO orbit determination methods were developed using the triple-differenced GPS phase observations, as presented in this dissertation. These methods include the kinematic, dynamic, and reduced-dynamic approach based on the wave algorithm.; To test the developed algorithms, 24 hours of CHAMP data on February 15, 2003, which amounts to 15 revolutions, were used for each method. The EIGEN2 geopotential model was used with degree and order up to 120. Precise IGS orbits are used for the GPS satellites, and 43 IGS ground tracking stations were chosen using the algorithm developed in this study, based on the network optimization theory.; The estimated orbit solutions were compared with the published Rapid Science Orbit (RSO) and the consistency testing was performed for the dynamic solution. In addition to the comparison with other orbit solutions, the SLR residuals were also computed as an independent validation of the methods presented here.; The kinematic orbit solution depends on the satellite geometry and data quality. The absolute kinematic positioning solution, with an RMS error of +/-26 meters in 3D, was used as an initial approximation for the kinematic orbit determination. Because of the inaccuracy of the initial approximated orbit, there is a bias up to a few hundred epochs in the kinematic solution. This bias is effectively removed with the backward filter by fixing the last epoch from the forward filter solution. After the forward and backward filtering, the kinematic approach shows accuracy better than +/-20 cm in 3D RMS for a half day arc compared to the reference RSO.; The dynamic approach requires careful modeling of the atmospheric drag force which is the most dominant nonconservative force at LEO's altitude. In addition, the empirical force modeling, which is similar to the stochastic process noise in the reduced-dynamic approach, absorbs most of the remaining unmodeled forces. The two frequencies of the empirical forces, that is, once- and twice-per-revolution, are modeled in this study. (Abstract shortened by UMI.)... |
