Abstract's details
SWOT KaRIN Calibration Algorithm: principle, updates and analysis of the absorption of geophysical corrections residues such as tide and sea state bias
Event: 2025 SWOT Science Team Meeting
Session: Oceanography: Calibration and Validation
Presentation type: Poster
The images provided by KaRIn can be biased or skewed by a few centimeters to tens of centimeters. The main source of these errors is an uncorrected satellite roll angle, which explains why KaRIn images are mainly composed of a linear variation based on cross-track distance. There are various other sources of errors such as interferometric phase biases or thermo-elastical distortions in the instrument baseline and antennas. To mitigate these topography distortions, a calibration mechanism is applied.
Two variants of KaRIn calibration have been developed: the SWOT mono-mission or Level-2 algorithm, and the multi-mission or Level-3 algorithm. The L2 algorithm is used in the SWOT ground segment and is based on SWOT data only, because a ground segment cannot depend on external satellites. In contrast, the Level-3 algorithm was designed to leverage better algorithms and external satellites: not only Sentinel-6, the so-called climate reference altimeter, but also all other altimeters in operations (Sentinel-3A/3B, HY2B/C, SARAL, CRYOSAT-2). The L3 correction is generally more robust and stable than the Level-2 variant thanks to the thousands of daily multi-mission crossover segments provided by the constellation. The L3 algorithm is updated regularly with the different product versions (v1.0.2, v2.0.1, v3): the addition of a static component improves calibration for geodetic applications; the reduction of the number of parameters allows less residual signal to be absorbed; a change of interpolation methodology improves calibration in polar areas and on continents for hydrology.
A drawback of calibration is the absorption of geophysical correction residues. Indeed, the harmonic analysis of the calibration highlights the absorption of tide residues in the calibration. This absorption by the calibration is problematic for users who want to analyze the tide signal in coastal areas and for users who are subject to the projection of these errors via calibration; for example, in hydrology, coastal tide error is projected onto continents. A study is conducted to analyze the strongly impacted regions and the amount of signal absorbed for the different calibrations available (Level-2 calibration, Level-3 v1, v2 and v3 calibration) and for degraded calibrations that absorb less signal but that correct systematic errors less effectively. In addition, a study on the absorption of sea state bias by the calibration has been carried out: the dependence of calibration on wind and sea wave height highlights the absorption of sea state bias in the calibration. The use of a calibration that includes only a harmonic roll correction avoids the absorption of sea state bias.
Back to the list of abstractTwo variants of KaRIn calibration have been developed: the SWOT mono-mission or Level-2 algorithm, and the multi-mission or Level-3 algorithm. The L2 algorithm is used in the SWOT ground segment and is based on SWOT data only, because a ground segment cannot depend on external satellites. In contrast, the Level-3 algorithm was designed to leverage better algorithms and external satellites: not only Sentinel-6, the so-called climate reference altimeter, but also all other altimeters in operations (Sentinel-3A/3B, HY2B/C, SARAL, CRYOSAT-2). The L3 correction is generally more robust and stable than the Level-2 variant thanks to the thousands of daily multi-mission crossover segments provided by the constellation. The L3 algorithm is updated regularly with the different product versions (v1.0.2, v2.0.1, v3): the addition of a static component improves calibration for geodetic applications; the reduction of the number of parameters allows less residual signal to be absorbed; a change of interpolation methodology improves calibration in polar areas and on continents for hydrology.
A drawback of calibration is the absorption of geophysical correction residues. Indeed, the harmonic analysis of the calibration highlights the absorption of tide residues in the calibration. This absorption by the calibration is problematic for users who want to analyze the tide signal in coastal areas and for users who are subject to the projection of these errors via calibration; for example, in hydrology, coastal tide error is projected onto continents. A study is conducted to analyze the strongly impacted regions and the amount of signal absorbed for the different calibrations available (Level-2 calibration, Level-3 v1, v2 and v3 calibration) and for degraded calibrations that absorb less signal but that correct systematic errors less effectively. In addition, a study on the absorption of sea state bias by the calibration has been carried out: the dependence of calibration on wind and sea wave height highlights the absorption of sea state bias in the calibration. The use of a calibration that includes only a harmonic roll correction avoids the absorption of sea state bias.