Quality assessment of onboard GPS receiver and its combination with DORIS and SLR for Haiyang 2A precise orbit determination

The GPS, DORIS, and SLR instruments are installed on Haiyang 2A (HY2A) altimetry satellite for Precise Orbit Determination (POD). Among these instruments, the codeless GPS receiver is the state-of-art Chinese indigenous onboard receiver, and it is the first one successfully used for Low Earth Orbit (LEO) satellite. Firstly, the contribution assesses the performance of the receiver through an analysis of data integrity, numbers of all tracked and valid measurements as well as multipath errors. The receiver generally shows good performance and quality despite a few flaws. For example, L2 observations are often missing in low elevations, particularly during the ascent of GPS satellites, and the multipath errors of P1 show a slightly abnormal pattern. Secondly, the PCO (Phase Center Offset) and PCV (Phase Center Variation) of the antenna of the GPS receiver are determined in this contribution. A significant leap for Z-component of PCO up to -1.2 cm has been found on 10 October 2011. Thirdly, the obtained PCO and PCV maps are used for GPS only POD solutions. The post-fit residuals of ionosphere-free phase combinations reduce almost 50%, and the radial orbit differences with respect to CNES (Centre National d'Etudes Spatiales) Precise Orbit Ephemeris (POEs) improve about 13.9%. The orbits are validated using the SLR data, and the RMS of SLR Observed minus Computed (O-C) residuals reduces from 17.5 to 15.9 mm. These improvements are with respect to the orbits determined without PCO and PCV. Fourthly, six types of solutions are determined for HY2A satellite using different combinations of GPS, DORIS, and SLR data. Statistics of SLR O-C residuals and cross-comparison of orbits obtained in the contribution and the CNES POEs indicate that the radial accuracy of these orbits is at the 1.0 cm level for HY2A orbit solutions, which is much better than the scientific requirements of this mission. It is noticed that the GPS observations dominate the achievable accuracy of POD, and the combination of multiple types of observations can reduce orbit errors caused by data gaps and maintain more stable and continuous orbits.