Abstract's details
Sea State Bias on SWOT measurements
Event: 2025 SWOT Science Team Meeting
Session: Oceanography: Calibration and Validation
Presentation type: Poster
Since the launch of SWOT in December 2022, astonishing bi-dimensional maps of highly resolved Sea-Surface Height (SSH) have been revealed showing the complex structure of the ocean and the multi-scaled processes occurring at its surface. The highly resolved data provided by KaRIn provides a unique opportunity to monitor, study, and quantify such structures.
One important source of error in altimetry, related to the presence of surface waves, is the so called Sea State Bias (SSB) that induces a negative shift in the measured surface height of the order of several percent of the significant wave height (SWH). The SSB is the signature of various complex physical properties of the ocean waves, among which their non-Gaussian nature and the fact that the local roughness (mean square slope) varies along the wave profile due to non-linear interactions between the waves (hydrodynamical modulation), inducing differences in the electromagnetic signal backscattered to the radar that bias the measurement (electromagnetic bias). In the context of conventional (or SAR) nadir altimetry, empirical corrections (essentially derived by tabulating differences between height measurements at cross-overs as a function of sea state parameters) are used to reduce the error due to SSB.
The measurement principle used by KaRIn (interferometry) is significantly different from that of nadir altimetry and as a result, the SSB created by waves could be different. Studies before launch using somewhat simplified wave physics (gaussian waves and a simple model for the backscatter modulation) concluded that the SSB would be similar enough to the one affecting nadir altimetry to allow the usage, as the initial correction (used in the operational processing since launch), of the empirical table derived from the AltiKa mission (also in Ka band). In this work, we revisit this issue using refined wave physics. Since KaRIn's processing uses a SAR compression to achieve its azimuth resolution, we pay particular attention to the impact of surface motion, which has recently been shown to be an important contributor to SSH errors on sensors using Doppler processing (Buchhaupt et al 2021, Marié et al 2024).
We first derive an analytical model for the bias affecting an interferometric SAR instrument like KaRIn in the presence of waves, which are characterized by their four-dimensional (elevation, slopes and vertical velocity) joint probability density functions (PDFs). We use our model to compute the SSB as a function of cross-track distance and doppler beam for a variety of sea state conditions (Significant wave height, wind speed, wind direction, ...). This allows us to quantitatively investigate the impact of instrument characteristics (PTR, pointing...) and wave physics ingredients to the SSB. Our analysis includes the effect of the non-linear interactions and couplings between the slopes and velocities of the large waves, while higher-order effects are left for future work. Specifically, we have derived and used the waves PDFs under second-order Eulerian and first-order Lagrangian approximations.
We will present the results of these analyses, comparing and analyzing the dependences of the predicted SSB on cross-track distance, wind speed, wind direction and wave doppler. By understanding the impact of each wave physics ingredient, we aim to progress toward a more complete SSB model that can be used for correction in the long term.
Back to the list of abstractOne important source of error in altimetry, related to the presence of surface waves, is the so called Sea State Bias (SSB) that induces a negative shift in the measured surface height of the order of several percent of the significant wave height (SWH). The SSB is the signature of various complex physical properties of the ocean waves, among which their non-Gaussian nature and the fact that the local roughness (mean square slope) varies along the wave profile due to non-linear interactions between the waves (hydrodynamical modulation), inducing differences in the electromagnetic signal backscattered to the radar that bias the measurement (electromagnetic bias). In the context of conventional (or SAR) nadir altimetry, empirical corrections (essentially derived by tabulating differences between height measurements at cross-overs as a function of sea state parameters) are used to reduce the error due to SSB.
The measurement principle used by KaRIn (interferometry) is significantly different from that of nadir altimetry and as a result, the SSB created by waves could be different. Studies before launch using somewhat simplified wave physics (gaussian waves and a simple model for the backscatter modulation) concluded that the SSB would be similar enough to the one affecting nadir altimetry to allow the usage, as the initial correction (used in the operational processing since launch), of the empirical table derived from the AltiKa mission (also in Ka band). In this work, we revisit this issue using refined wave physics. Since KaRIn's processing uses a SAR compression to achieve its azimuth resolution, we pay particular attention to the impact of surface motion, which has recently been shown to be an important contributor to SSH errors on sensors using Doppler processing (Buchhaupt et al 2021, Marié et al 2024).
We first derive an analytical model for the bias affecting an interferometric SAR instrument like KaRIn in the presence of waves, which are characterized by their four-dimensional (elevation, slopes and vertical velocity) joint probability density functions (PDFs). We use our model to compute the SSB as a function of cross-track distance and doppler beam for a variety of sea state conditions (Significant wave height, wind speed, wind direction, ...). This allows us to quantitatively investigate the impact of instrument characteristics (PTR, pointing...) and wave physics ingredients to the SSB. Our analysis includes the effect of the non-linear interactions and couplings between the slopes and velocities of the large waves, while higher-order effects are left for future work. Specifically, we have derived and used the waves PDFs under second-order Eulerian and first-order Lagrangian approximations.
We will present the results of these analyses, comparing and analyzing the dependences of the predicted SSB on cross-track distance, wind speed, wind direction and wave doppler. By understanding the impact of each wave physics ingredient, we aim to progress toward a more complete SSB model that can be used for correction in the long term.