Impact of the ocean waves motion on the Delay/Doppler altimeters measurements

Laïba Amarouche (CLS, France)

CoAuthors

Ngan Tran (CLS, France); Damien Herrera (CLS, FRANCE); Charles-Antoine Guerin (MIO, France); Pierre Dubois (CLS, France); Jérémie Aublanc (CLS, France); François Boy (CNES, France)

Event: 2019 Ocean Surface Topography Science Team Meeting

Session: Instrument Processing: Measurement and Retracking

Presentation type: Type Oral

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 wondering if ocean phenomena with smaller scales are observable. Nevertheless, before we can fully interpret the observed small scales phenomena, it is important to fully understand the content of the altimeter signal in SAR mode.
Among the phenomena likely to alter the performance of the measurements, as long as they are not considered in the processing, we can mention the surface motion. Indeed, Doppler altimeters, which use the motion of the platform 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 In the event that the surface movements are not negligible, the processing assumptions are no longer valid and the final performance is likely to be impacted.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). In this work statistical solutions have been developed to estimate the means and standard deviations of the Doppler shift induced by the surface motion. Considering the average values and associated standard deviations, we were able to highlight the possibility of encountering situations where the performance is significantly impaired.
To complement this work, a new analysis has been performed to accurately assess the impact of these Doppler shifts on the altimetry performance. This analysis is based on a theoretical development allowing to describe the long and short waves time evolution and then on the use of simulation tools to assess the impact on the performance. Finally, real Sentinel-3 SAR mode data are used to validate the theoretical conclusions.
The results showed that the high positive biases observed on the SAR mode significant wave height (SWH) estimations of real Sentinel-3 data, in comparison to the low-resolution mode, are explained by the waves orbital velocities that affect the Doppler signal. The waves orbital velocity is the higher for high significant slopes. This phenomenon is not linked to swell and doesn’t depend on the wave propagation direction. At the same time, SWH is underestimated in the case of swell with high wavelengths propagating in the same direction as the satellite. In this case very high noise is observed on SWH and range estimates.
This talk will give an overview of the analysis performed on the time evolution of the short and long wave scales and how this analysis has been used to assess the impact on the delay/Doppler altimeters performances. The results using real SAR mode data will be also presented.
 

Oral presentation show times:

Room Start Date End Date
The Forum Tue, Oct 22 2019,10:00 Tue, Oct 22 2019,10:15
Laïba Amarouche
CLS
France
lamarouche@groupcls.com