In-flight GNSS phase map calibration modelling with Zernike polynomials

GNSS processing for POD suffers from systematic errors when the location of the instantaneous phase center is not knownwith a few millimeter accuracy. The traditional approach to model the GNSS phase center origin and variations is todetermine antenna phase maps iteratively from the residuals. A more direct approach consists in modeling the antennaphase map through an expansion in well-chosen Zernike polynomials, in order to reduce potential correlations with dynamicmodeling errors. This strategy has been applied to derive the GNSS antenna phase maps of several altimetry missions suchas Sentinel-3A, Jason-3, Sentinel-6 MF and SWOT. The derived phase corrections were tested on CNES POE-G GNSS-onlyreduced-dynamic orbit solutions to assess their impact and validate their benefits owing to independent SLR observations.