Thomas Moreau (CLS, France); Laetitia Rodet (CLS, France); Claire Maraldi (CNES, France ); Lionel Zawadzki (CNES, France)

A key instrumental analysis performed during an altimeter’s commissioning is the in-flight characterization of the antenna’s pointing, an essential step to assess its accuracy and stability in the post-launch period. This is traditionally carried out through the so-called cross-maneuver (X-man), during which the platform is actively rotated in pitch and roll, independently, in both positive and negative directions. The antenna’s response is measured by the squared-mispointing angle that is fitted to the LRM waveforms. By a differential analysis of the negative and positive maneuvers, one can estimate the pitch and roll biases between the antenna boresight and the platform pointing. SAR altimeter observations offer a different way to characterize these biases. Indeed, the power distribution of the SAR altimeter beams is a proxy for the platform pitch (e.g. [1]). When the satellite is rotated by 90° in yaw, it then becomes a proxy for the platform roll. This special yaw 90° maneuver was successfully carried out for the first time for Sentinel-6A, giving similar results to the traditional X-man, and became the default pointing maneuver for Sentinel-6B.
A second, related, commissioning analysis is the estimation of the antenna aperture, which is characterized on ground but requires refinement once the altimeter is in orbit. So far, this parameter has been estimated based on the LRM observations following the pointing maneuvers, by minimizing the difference between the altimeter-derived squared-mispointing and the platform pointing (corrected for the pitch and roll biases to align with the antenna boresight).
Here we present the analysis of the yaw 90° pointing maneuver for Sentinel-6B along with the estimation of the antenna aperture. The novel approach is the use of the complete information provided by the SAR Delay Doppler maps: we use the CLS-CNES Sentinel Processing Prototype (SPP, [2]) and apply a newly developed numerical two-dimensional retracking ([3]) to fit for both the pitch/roll and the antenna aperture, in addition to the wave and topography parameters (in order to limit possible misinterpretation between antenna characteristics and sea surface features, whether static or dynamic). The present method provides the pointing biases and the antenna aperture simultaneously, as opposed to the indirect LRM-based approach as well as the one-dimensional fit to the SAR range integrated power. Additionally, it can provide information on the antenna’s shape (circular or elliptical), which is not obtainable from LRM data. The LRM outputs are on the other hand sensitive to the parameters of the current retracking algorithms and tend to give antenna apertures that are further from the ground-estimated value compared to the SAR-based method. In this regard, we investigate how the recently proposed optimal LRM estimator ([4]) could be used to bring the different results together.

References:
[1] Galin, N. et al. (2014). Measuring the pitch of Cryosat-2 using the SAR mode of the SIRAL altimeter, IEEE Geoscience and remote sensing letters, 11, 8
[2] Dinardo, S. et al. (2022). Sentinel-6 MF Poseidon-4 radar altimeter: Main scientific results from S6PP LRM and UF-SAR chains in the first year of the mission, Advances in Space Research, 73, 1, 337-375
[3] Buchhaupt, C. et al. (2023). Towards the Mitigation of Discrepancies in Sea Surface Parameters estimated from Low- and High-Resolution Satellite Altimetry. Remote Sensing, 15, 4206.
[4] Mangilli, A. et al. (2025). Optimal estimation of the geophysical parameters over ocean from the Sentinel-6MF reference mission, IEEE Transactions on Geoscience and remote sensing, 63, 1-25