Investigation of SWH bias in SAR Altimetry mode

Thomas Moreau (CLS, France)


Jérémie Aublanc (CLS, France); Pierre Thibaut (CLS, France); Pierre Rieu (CLS, France); François Boy (CNES, France); Nicolas Picot (CNES, France); Constantin Mavrocordatos (ESA/ESTEC, The Netherlands); Franck Bordes (ESA/ESTEC, The Netherlands)

Event: 2017 Ocean Surface Topography Science Team Meeting

Session: Instrument Processing: Measurement and Retracking

Presentation type: Type Oral

The satellites, Cryosat-2 and Sentinel-3A, equipped with a synthetic aperture radar altimeter are currently in service on a near-polar orbit, performing range measurements over different types of surfaces (ocean, sea ice, ice sheet and in-land waters). Advantages brought by this radar mode to retrieve geophysical ocean parameters with much higher resolution and precision than conventional pulse-limited altimeters are now well acknowledged. However, even though it offers improved ocean measuring capability, there are still remaining issues around the interpretation of some SAR-mode ocean data along with some questions about the model used to fit the waveforms.
While assessing Cryosat-2 and Sentinel-3A SAR-mode data quality over ocean, it is found that some discrepancies between Pseudo-LRM and SAR-mode in range (few cms), but mostly in SWH (10 to 20 cm), occur above 2 m wave height (and in low sea-state conditions too), and that their difference depends on the significant wave height. As long as the P-LRM and SAR mode data are co-localized, one might have expected that the two radar modes retrieve same significant wave heights. Yet, the differences observed between the two radar modes are not understood, and need to be analysed and worked out.
Recent analysis of SAR-mode altimeter data processed with the application of a Hamming window in azimuth direction (which reduces effectively the side lobe effects), has revealed a better match between SWH retrievals. This suggests a possible inconsistency between the model and data, and particularly raises some concerns about the reliability of the square cardinal sine function which is used to describe the along-track point target response (resulting from the synthetic aperture processing) in the SAR altimeter backscattered waveform model. To tackle this issue, we conducted different analyses: characterization of the azimuthal PTR using transponder, evaluation of the effects of the secondary lobes based on simulations, and a long period data analysis with and without the Hamming weighting function (using the CNES Sentinel-3 prototype processing) whose results are compared with findings from the simulation study.
This activity conducted by CLS in a ESTEC contract and with the CNES support, aims at improving the current SAR-mode data ground processing to further stimulate the generation of higher level quality data for the upcoming Sentinel-3 C/D missions.

Oral presentation show times:

RoomStart DateEnd Date
Symphony Ballroom IV Tue, Oct 24 2017,09:30 Tue, Oct 24 2017,09:45
Thomas Moreau