Laiba Amarouche (CLS, France); François Boy (CNES, France)

Since the launch of Sentinel-3 A in February 2016, the data acquired in SAR mode over ocean have opened up a very wide field of investigations. Indeed, the improvement of the along-track resolution and the reduction of the noise of the measurements have led the scientific community to wonder if ocean phenomena on a smaller scale are observable. Nevertheless, before to be able to answer this question it is important to fully understand the content of the altimeter signal in the SAR mode and thus understand whether the geophysical estimates are affected by errors that could be due to a poor modeling of the signal.
Among the phenomena likely to alter the performances of the geophysical estimates as they are not taken into account in the signal modeling we can cite the surface movement combined with the presence of waves. Indeed, Doppler altimeters, which use the satellite movement to improve the measurement resolution, assume that the surface is stationary. In the event that the surface movements are not negligible, the processing assumptions are no longer valid and the final performance may be impacted.
A first analysis has been performed in the context of a CNES/CLS/MIO PhD and resulted in a scientific paper published in IEEE TGRS (O. Boisot et al. 2017). This analysis, based on the estimation of statistics of the Doppler shifts induced by the surface motion, highlighted the possibility of encountering situations where the altimeter performances can be significantly impaired.
This first work has been completed by a second study based on theoretical developments and simulations to accurately assess the impact of these Doppler shifts on the altimetry performances. The corresponding results have been presented at the OSTST Meeting of Chicago 2019 (Amarouche et al. 2019).
This study showed that the large positive biases observed between the S3A SAR mode significant wave height (SWH) estimates and the low-resolution mode data, may be explained by the waves orbital velocities that affect the Doppler signal. This phenomenon is not linked to swell and is independent of the wave propagation direction. At the same time, swell with high wavelengths propagating in the same direction as the satellite causes a significant increase in noise on the range and SWH estimates.
A third study has been performed based on Sentinel-3 data processed with CNES prototype (S3PP) and has consisted in comparing the estimates observed from the different measurement modes as a function of external data from wave models. This later analysis confirmed the results of the previous theoretical analysis and simulations. It highlighted moreover an up-waves/down-waves effect on the range estimates bias that needs to be further investigated
This presentation will provide an overview of the results of the assessment of the impact of the ocean waves on the delay/Doppler altimeter performances using real data.