Status and Perspectives for Wave Height estimation from altimeter measurements

Pierre Thibaut (CLS, France)

CoAuthors

Thomas Moreau (CLS, France); Laiba Amarouche (CLS, France); Pierre Rieu (CLS, France); Fanny Piras (CLS, France); François Boy (CNES, France); Nicolas Picot (CNES, France); Franck Borde (ESA/ESTEC, The Netherlands); Constantin Mavrocordatos (ESA, The Netherlands)

Event: 2018 Ocean Surface Topography Science Team Meeting

Session: Instrument Processing: Measurement and Retracking

Presentation type: Type Oral

In the last years, huge efforts have been done by different teams to improve the retracking algorithm for both conventional and Delay Doppler echoes. All the research has been mainly focused on the improvement of the Sea Surface Height estimation, especially to better access the small scales of the ocean topography. However, the retracking algorithm provides, in addition to the sea surface height, an estimation of the wave height which properties can be closely correlated to ocean currents [Ardhuin & al, 2017].
In 2008, a paper was published [Mailhes & al, 2008] on the estimation of Cramer Rao bounds (CRB) for conventional radar altimeter waveforms showing that there was some space for improving the estimation of the sea surface height on the one hand but even more for improving the significant wave height, knowing that the variance of any unbiased estimator of the altimetric parameters is bounded below by its Cramer-Rao bound (CRB).
To improve the retracker performances for LRM measurements, CLS developed and successfully validated a solution called "Adaptive Retracker", implementing a new waveform model and a Nelder Mead optimization method with exact likelihood criterion. Dramatic improvements in the estimation performances over ocean have been shown (respectively 10% and 60% of noise reduction on range and significant wave height).
And consequently, similar developments have been conducted at CLS for Delay Doppler measurements (Cryosat-2 and Sentinel-3A). But, while assessing Cryosat-2 and Sentinel-3A SAR-mode data quality over ocean, some discrepancies between Pseudo-LRM and SAR-mode in range (few cms) and mostly in SWH (10 to 20 cm) have been evidenced especially above 2 m wave height (and in low sea-state conditions too) with a significant wave height dependency. In order to guarantee continuity of wave height estimates over time (including LRM and SAR altimeters), it was thus important to investigate these differences and to propose corrections. A special focus will be done on the so called “range walk effect“ (deformation of the Along track impulse response due to satellite motion during the burst) and on strategies to account for it. The relative impact of long swells in the direction of the satellite track which affect differently significant waveheight estimation in SAR and LRM will also be illustrated.
This paper aims at presenting a review of the current significant wave height estimation performance with both LRM and SAR altimeters

 

Oral presentation show times:

Room Start Date End Date
Teatro Auditorium Thu, Sep 27 2018,17:15 Thu, Sep 27 2018,17:30
Pierre Thibaut
CLS
France
pthibaut@cls.fr