Abstract
The Surface Water and Ocean Topography (SWOT) altimeter mission can measure high-resolution wide-swath sea surface heights (SSHs) that may greatly improve the current accuracy and spatial resolution of marine gravity from nadir-looking altimeters. To investigate the potential of SWOT in recovering high-quality marine gravity and how SWOT observation errors should be treated to optimize the accuracy of gravity anomaly from SWOT observations, we create high-wavenumber SSH components from multi-beam depths in the northern South China Sea (SCS) and simulate SWOT SSH errors. To cross-validate gravity signals and avoid gravity errors from SWOT, we use two computational methods (inverse Vening-Meinesz formula, IVM and inverse Stokes' integral, ISM) and recommend separate optimal data processing strategies when using geoid gradients (GGs) and geoid heights (GHs) for gravity recovery. The use of GGs (for IVM) effectively eliminates systematic errors in gravity derivation. If GHs (for ISM) are used in gravity recovery, the tilt in wide-swath SSHs should be removed before gravity computation, and the recovered gravity must be filtered (post-processed) to avoid artificial gravity signals due to the SSH errors. Our assessments using mgal-accuracy shipborne gravity anomalies in the northern SCS show that multiple-cycle SWOT observations can deliver high-quality marine gravity anomalies. IVM is more robust than ISM in resisting random and systematic errors in SWOT. Our processing strategies can be used for the gravity validation of SSHs from SWOT's fast-sampling and science phases.
Highlights
With the ever-increasing data quality and volume of satellite altim eter data, marine gravity recovered from altimetry is well into a new era
Because the Surface Water and Ocean Topography (SWOT) mission can provide two-dimensional elevations at land and sea at spatial scales not achievable by current nadir-looking altimeters, the objective of this paper is to investigate the potential of SWOT sea surface heights (SSHs) observations in recovering marine gravity anomalies, and recommend the data processing strategies when geoid gradients (GGs) or geoid heights (GHs) are used for gravity recovery
Because the focus in this paper is to show the potential of SWOT in deriving high-frequency gravity signals, optimizing the nu merical differentiation of GGs is not discussed here
Summary
With the ever-increasing data quality and volume of satellite altim eter data, marine gravity recovered from altimetry is well into a new era. The nominal grid intervals of these fields are 1×1 min, but the actual signal resolution (half-wavelength) may exceed 6 km (Andersen and Knudsen, 2019; Sandwell et al, 2019). These problems in altimeter-derived gravity may soon be alleviated with the launch of the Surface Water Ocean Topography (SWOT) altimeter mission in February 2022 (Fu and Ubelmann, 2014). According to Morrow et al (2019a), SWOT’s wide-swath spatial resolution from the Level 2 ocean products is 2 km, enabling a high-precision and high-accuracy mapping of surface elevations for hydrological, oceanographic, and geophysical studies. SSHs from SWOT can lead to a marine gravity field with a uni form (omnidirectional) and unprecedented (< 2 km) spatial resolution that cannot be achieved using along-track SSHs from conventional al timeters, including recent synthetic aperture radar (SAR)-based altim eters such as Sentinel-3A, Sentinel-3B and Cryosat-2
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