Addressing Inaccurate Phase Center Offsets in Precise Orbit Determination for Agile Satellite Missions

Incorrect offsets between a satellite’s center of mass and the mean phase center of its GNSS antenna pose challenges to the Precise Orbit Determination of many current Earth Observation missions. While extensive efforts have been undertaken to identify and mitigate these errors, they have been based on satellites with slowly varying attitudes. The errors in the offsets, which primarily arise from inaccurate phase center offsets, can usually be derived when highly precise non-gravitational forces are employed. However, with the advent of agile missions, where the satellites perform frequent attitude maneuvers around all three degrees of freedom, inaccurate offsets might affect the POD differently, thereby warranting additional investigation. Based on hardware-in-the-loop simulations, this paper explores the effects of inaccurate offsets on the POD performance of agile satellites. The findings demonstrate that the impact of incorrect offsets is even more critical for these types of missions and the methodologies used for non-agile missions are not as easily applicable. An observability analysis shows that the lever arm, which is typically unobservable in the reduced-dynamic POD of non-agile satellites, becomes directly observable when the satellite performs rapid attitude maneuvers. This property is exploited within the agile POD, where these offsets are additionally estimated. The results demonstrate both a consistent and significant performance improvement in the POD in the presence of biases in the lever arm. These findings further hint at the feasibility of estimating the lever arm for both agile and non-agile satellites in dedicated calibration maneuvers, without the need to rely on highly precise non-gravitational force modeling.