Impact of different attitude modes on Jason-3 precise orbit determination and antenna phase center modeling

Global Navigation Satellite Systems such as the Global Positioning System (GPS) are a unique tool for deriving very precise orbits of Low Earth orbiting (LEO) satellites equipped with onboard GPS receivers. LEO precise orbit determination (POD) requires the proper modeling of antenna phase center variations (PCVs) for both the GPS transmitter and the LEO receiver antennas. While for the GPS antennas the nadir-dependent values from the official absolute antenna phase center model igs14.atx of the International GNSS Service (IGS), consistent with the underlying GPS orbit and clock products, are used, official PCV maps are usually not available for the LEO receiver antennas. If these variations are not considered, however, this may result in systematic errors in the derived LEO orbits. LEO PCV maps can be determined and exploited in different ways. One possibility is to use the PCV maps from ground calibrations provided by the manufacturer, which usually do not reflect, however, the influence of error sources which are additionally encountered in the actual spacecraft environment, e.g., near-field multipath. Alternatively, one can make use of GPS measurements and POD results to estimate the PCV map empirically, as it is done in this study.

In this study, the influence of different attitude modes on Jason-3 POD using GPS observations and PCV map estimation is investigated. As Jason-3 in an altimetry satellite, its main objective is to measure global sea-level rise. Therefore, it is of particular importance to precisely determine the radial component of the orbit and proper PCV modeling is of high importance. As Jason-3 is experiencing different attitude modes, yaw-steering and fixed-yaw attitude with either the positive or negative x-axis pointing in the direction of flight, PCV maps are expected to be better disentangled from other error sources. In this study, we are analyzing PCV maps determined from residual stacking using GPS data from the different attitude modes and from different orbit parametrizations. First results indicate that PCV maps estimated from time spans of different attitude modes differ and systematic orbit differences are occurring in a reduced-dynamic POD.