Long-Term Analysis of Sentinel-6A Orbit Determination: Insights from Three Years of Flight Data

Abstract

The Sentinel-6A mission extends the set of satellites dedicated to continuous ocean altimetry measurements, starting with TOPEX/Poseidon in 1992. To perform these measurements, high accuracy orbit solutions with radial position errors of less than 1.5 cm (RMS) are required. For precise orbit determination (POD), a dedicated GPS/Galileo triple frequency receiver (PODRIX) is available. Complementary to this, the TriG receiver provides GPS-only observations for POD and radio occultations. Previous research has successfully shown that reduced-dynamic orbit solutions with GPS/Galileo measurements from PODRIX meet the radial position requirement of less than 1.5 cm. However, baseline estimation between the PODRIX and TriG antennas revealed obvious inconsistencies in along and cross-track positions. In this study, we present a comprehensive reprocessing of orbit solutions using three years of Sentinel-6A flight data from both receivers (November 2020 to December 2023). Cross-comparison between both receivers showed that inconsistencies can be removed by applying a yaw bias and a timing error correction. A comparative analysis of macro-models demonstrates strong correlation of cross-track empirical accelerations with beta angle in the manufacturer model. More advanced models improved performance at the expense of requiring increased surface areas or reflectivity coefficients. In addition to improved radiation pressure models, newly estimated antenna patterns and a time-varying gravity field were used in the analysis. The reprocessed data confirm the agreement of both receivers, and the yaw bias correction reduces previously observed systematic differences in the estimated empirical accelerations. Satellite laser ranging results based on SLRF2020 station coordinates demonstrate a 7 mm (1D RMS) accuracy of the GNSS based POD solutions across the IGS14/IGS20 transition in November 2022.