Abstract's details

Cross-spectral analysis of SAR altimetry waveform tails

Marcel Kleinherenbrink (Delft University of Technology, Netherlands)


Frithjof Ehlers (Delft University of Technology, Netherlands); Sergi Hernandez (IsardSAT, Spain); Ferran Gibert (IsardSAT, Spain); Frederic Nouguier (Ifremer, France); Ourania Altiparmaki (Delft University of Technology, Netherlands); Florian Schlembach (DGFI , Germany); Bertrand Chapron (Ifremer, France); Paco Lopez Dekker (Delft University of Technology, Netherlands)

Event: 2023 Ocean Surface Topography Science Team Meeting

Session: Instrument Processing: Measurement and Retracking

Presentation type: Type Oral

Contribution: PDF file


Satellite radar altimetry has provided observations of sea-surface height (SSH), significant wave height (SWH) and wind speed (WS) for a continuous period of more than three decades. The resolution of the traditional low-resolution mode (LRM) altimeters limited the use of altimeters in the coastal zone and over inland waters. The launch of CryoSat-2 with its delay/Doppler altimeter allowed for the increase of resolution in the along-track direction, providing enhanced performance in the coastal zones. It also increased the number of independent looks, which reduced the noise of geophysical parameters. The resolution was further improved to ~0.5 m for static, coherent targets by the application of focused synthetic aperture radar (SAR) processing. However, focused SAR data from delay/Doppler altimeters, like CryoSat-2 and Sentinel-3A/B, suffer from strong azimuth ambiguities as a consequence of the closed-burst mode operation of the onboard instruments. With the launch of the first interleaved SAR altimeter satellite, Sentinel-6, focused SAR processing can be fully exploited.

The enhanced resolution of delay/Doppler altimeters makes them sensitive to backscatter modulations caused by long ocean waves. Often regarded as a nuisance, recent studies have suggested that these modulations may be exploited to retrieve swell-wave parameters. Altiparmaki et al. (2022) attempted to describe the backscatter modulation in the waveform tails at the hand of a spectral transform, but the full behavior is still poorly understood. Particularly, the non-linear range bunching is not described and a left-right modulation difference remains an open issue. Therefore an inversion of SAR altimetry spectra, or modulations, into swell-wave spectra is yet to be performed.

In this article we extend the work of Altiparmaki et al. (2022) to give a synoptic description of the modulations in SAR altimetry waveform tails. We numerically model SAR altimetry spectra from buoy spectra and validate them against Sentinel-6 data. Then we will extend the closed-form SAR spectral transform and discuss its validity for nadir SAR altimetry and swath altimetry. Eventually, we will introduce the first cross-spectral analysis and compute a stack from eleven sublooks. The stack of cross spectra will show that the left-right modulation difference will vary with look angle and can therefore be exploited to provide additional information about swell system. Additionally, cross-spectral analysis allows to remove at least two of the four spectral ambiguities.

Oral presentation show times:

Room Start Date End Date
Grande Beach Room (#208) Wed, Nov 08 2023,09:20 Wed, Nov 08 2023,09:40
Marcel Kleinherenbrink
Delft University of Technology