Impact of the range walk processing in the Sentinel-3A sea level trend

Jérémie Aublanc (CLS, France)

CoAuthors

Salvatore Dinardo (CLS, France); Emeline Cadier (CLS, France); Matthias Raynal (CLS, France); Thomas Moreau (CLS, France); Claire Maraldi (CNES, France); François Boy (CNES, France); Nicolas Picot (CNES, France); Pierre Femenias (ESA, )

Event: 2020 Ocean Surface Topography Science Team Meeting (virtual)

Session: Instrument Processing: Measurement and Retracking

Presentation type: Type Forum

Sentinel-3A is a European Space Agency Earth observation satellite mainly dedicated to oceanography, launched on February 2016. It is the first of a four satellites programme, including Sentinel-3B launched on April 2018, and Sentinel-3C/D (replacing the Sentinel-3A and Sentinel-3B later on). Among the main innovations, the altimeters on-board Sentinel-3 are the first ones fully operating in SAR altimetry mode. Even if sea level rise monitoring is not one of the mission primary objectives, it remains a key parameter and the main mission application in relation to climate. Reliable long-term sea level estimations from Sentinel-3 will help better quantifying the occurring changes, in support of the Jason/Sentinel-6 constellation.

At present, the first Global Mean Sea Level (GMSL) trends computed with Sentinel-3 acquisitions are not in agreement with the current estimations. A drift is observed compared to other altimetry missions. A slight part of this drift was quickly explained by the instability of the Pulse Target Response, affecting both SAR and Pseudo-Low Resolution Mode estimations. Nonetheless, a relative important drift remains in SAR altimetry mode, preventing the GMSL estimation from any long-term climatological applications.

The current SAR mode processing chain implemented in the Sentinel-3 ground segment makes some approximations to save complexity and computation time. In particular, the Doppler processing applied at level-1 does not compensate accurately the range variation during the burst acquisition with respect to the focused point: the so-called range walk correction. Initially considered negligible, the range walk correction was found recently to be of some interest, especially now that SAR altimetry mode data is of high quality.

In this study we analyse the impact of the range walk correction in Sentinel-3 sea surface height and wave estimations, over a 2 years' time series. We show that range walk correction prevents from side-effects occurring to the stack lateral looks, linked to the instability of the Pulse Target Response. In the end we demonstrate that the main part of the current GMSL drift observed with Sentinel-3 in SAR mode will be removed once range walk will be applied. This work is of major importance to demonstrate the validity of SAR altimetry mode for sea level long-term monitoring.
 
Jérémie Aublanc
CLS
France
jaublanc@groupcls.com