Resolving GPS carrier phase ambiguities for a low Earth orbit spacecraft.

摘要

The application of GPS carrier phase integer ambiguity resolution to low Earth orbit spacecraft missions has been a topic of great interest in recent years. This advanced processing technique may make it possible to achieve higher orbit accuracy for spacecraft that carry GPS receivers. The research described here addresses a number of aspects of GPS carrier phase ambiguity resolution as applied to the Jason-1 altimetric mission. When GPS carrier phase ambiguities are correctly resolved, the phase measurements will effectively act as very precise pseudorange measurements. This leads to improvement in the GPS derived orbit solutions for Jason-1. The advanced-codeless BlackJack GPS receiver, onboard Jason-1, that enables the retrieval of pseudorange and carrier phase observables on the L1 and L2 frequencies promises the possibility of applying such a technique.; Both the orbits of Jason-1 and the GPS satellites are estimated simultaneously in the orbit determination process. Carrier phase integer ambiguity resolution is applied to the best determined reduced-dynamic Jason-1 and dynamic GPS satellite orbit solutions. The ambiguity resolution approach first resolves the wide-lane integer phase ambiguities. The narrow-lane phase ambiguities are then constrained to integers using the resolved wide-lane integer phase biases. This approach does not require a search process. Instead it uses the error covariance matrix to select the best determined set of double-differenced phase biases. A confidence test procedure is implemented to resolved the wide-lane and narrow-lane phase biases to correct integers.; The performance of the GPS carrier phase ambiguity resolution method is evaluated through a few orbit accuracy assessment tests. These assessment tests include the orbit overlap differences, intercomparison with SLR-DORIS based solutions, the high elevation SLR bias analysis and the sea surface height crossover residuals. An error budget study is created to investigate the effects of incorrectly fixed phase biases and the GPS orbit errors on Jason-1 orbit solutions.; The results from 29 days of data analysis show modest improvement of 11% in Jason-1 radial orbit accuracy after resolving carrier phase ambiguities. The crosstrack and alongtrack orbit overlap components exhibit slightly better improvement of 25% and 12% respectively. The orbit offset with the SLR-DORIS based orbits show sub-centimeter level influence after ambiguity resolution. The overall effect is hardly noticeable which probably reflects the dominance of SLR-DORIS orbit errors in the differences. In analyzing the geocenter offset in the Terrestial Reference Frame (TRF), the mean offsets in the x, y and z-axis also show sub-centimeter (less than 6%) improvements. As for the high elevation laser range bias and the sea surface height residual analysis, both demonstrated sub-millimeter improvement after resolving phase ambiguities.; With the Jason-1 radial orbit accuracy reaching 1 cm, it is reasonable to achieve millimeter or sub-millimeter improvements in each orbit assessment test. Furthermore, the performance of carrier phase ambiguity resolution can be partially hampered by the presence of incorrectly fixed phase biases. With a short observation time span of 29 days, it is rather difficult to confidently deduce the impact of GPS carrier phase ambiguity resolution on the Jason-1 orbit accuracy and the orbit centering along the Earth's spin axis.