Optimized processing of SWOT fast-sampling data for marine gravity recovery: lessons for the 21-day science phase

The new Surface Water and Ocean Topography (SWOT) altimeter mission, launched on December 16, 2022, marked a significant leap in ocean topography measurement by transitioning from traditional nadir-looking altimetry to wide-swath interferometric radar altimetry. During its 3-month fast-sampling phase from March 29 to July 9, 2023, SWOT operated on a 1-day repeat orbit focused on calibration and validation. Currently, SWOT is operating in its science phase on a 21-day repeat orbit. This study uses the Version C Level 2 Ka-band Radar Interferometer data in the South China Sea, collected during the SWOT fast-sampling phase, to validate SWOT's performance in recovering marine gravity fields. The 95 cycles of fast-sampling phase data are equivalent to about 5.5 years of data from the science phase, offering sufficient repeat data to evaluate the potential of SWOT for deriving long-term, stacked marine gravity fields. We outline optimal processing strategies for refining SWOT sea surface heights (SSHs) to derive marine gravity anomalies. These strategies include rejecting outliers, mitigating ocean variability by removing sea level anomaly, correcting tilts across the entire SWOT swath segment using a three-dimensional plane, and averaging geoid gradients to reduce random errors. This study introduces a new method, the least-squares adjustment (LSA), for estimating the north and east components of the geoid gradient from SWOT SSH data. The gravity anomalies derived from the gradient components determined using the LSA method show comparable accuracy to those using least-squares collocation (LSC), with values of 2.32 mgal versus 2.28 mgal, respectively. Moreover, the LSA method is more computationally efficient than LSC. SWOT demonstrates the capability to derive north and east gradient components and marine gravity fields at finer scales compared to conventional altimeters, particularly over rough seafloor areas where SWOT can observe high-wavenumber SSHs. SWOT reveals continental shelf margins more clearly than conventional altimeters. Marine gravity anomalies from 3 months of SWOT data have an accuracy of 2.28 mGal, which is 12% better than the accuracy achieved with 14 years of data from conventional nadir altimeters. The gravity recovery procedures developed using SWOT fast-sampling phase data can be extended to derive optimal local and global marine gravity fields during the mission's science phase.