Comparison of LEO GNSS antenna phase characteristics from ground and in-flight calibrations

Proper a priori knowledge of the phase center location and pattern of the GNSS antenna is an essential prerequisite for use of GNSS measurements in the determination of the terrestrial reference frame. This is well known for terrestrial GNSS stations, but likewise applies for space-borne GNSS tracking. In the context of the upcoming Genesis mission, the analysis of flight data from existing low Earth orbit (LEO) missions offering co-location of multiple space-geodetic instruments and their possible contribution to the TRF refinement has gained renewed interest. With this background, we investigate the quality of ground-based calibrations from the two most widely-used geodetic-grade LEO GNSS antenna types and compare these calibration with in-flight measurements for a range of scientific Earth observation missions. More specifically, we analyse the combination of an aviation patch antenna with JPL chokering using flight data from the GRACE, Jason-2/3, and TerraSAR-X satellites as well as the RUAG (now Beyond Gravity) patch antenna with integrated choke ring from GNSS observations of the GRACE-FO and Sentinel-3A/3B/6A satellites.Overall, the analysis covers a period of at least 10 years and makes use of pre-computed precise GNSS orbit, clock, and bias products from the International GNSS Service for precise orbit determination of the LEO satellites. The analysis period includes different reference frames (IGSR3, IGS14, IGS20) to verify the consistency of changes in the estimated phase center offsets (PCOs) with TRF scale changes. Following a discussion of conceptual problems in the definition and measurements of "the" antenna phase center, we assess the expected uncertainty of LEO force models to characterize the expected stability of the dynamical reference frame of the various LEO satellites that serves as a reference for the PCO determination from in-flight observations. The results reveal notable discrepancies between ground calibrations and in-flight results, which obviously hamper the use of existing LEO GNSS data for independent TRF scale determination. These include notable phase pattern distortions attributed to the impact of the local antenna environment as well as systematic PCO differences, which can only partly be attributed to center-of-mass uncertainties. In the context of Genesis, adequate measures need to be taken to avoid launch-related structural deformations changing the center-of-mass location from the design values and to calibrate the GNSS antenna characteristics after satellite integration in a flight-representative configuration.