Impact of the oversampling method on the radar altimeter estimates

Thomas MOREAU (CLS, France)


Pierre Thibaut (CLS, France); Jérémie Aublanc (CLS, France); François Boy (CNES, France); Nicolas Picot (CNES, France)

Event: 2014 Ocean Surface Topography Science Team Meeting

Session: Instrument Processing: Measurement and retracking (SAR and LRM)

Presentation type: Type Poster

In some situations where the surface targets are very specular (e.g. sea ice or at very low sea states in coastal zones), the squaring step in the SAR power processed data may lead to some aliased power echo waveforms and then to the loss of some of the resolution. To address the poor representation of the peak over specular surfaces (typically in polar ocean but also in coastal zone and over hydrological river basins) the processing of CryoSat-2 data uses a factor 2 oversampling of the waveform resulting in an apparent doubling of the range resolution (broadening the leading edge of the waveform). This method consists, as suggested by Jensen et al. (1999), to extend the frequency domain to 256 bins by padding the 128 existing ones with zeros, and then do the Fast Fourier Transform to obtain 256 complex gates (with twice the resolution) in time domain. However, although the sampling has changed, the resolution has not since it is fixed by the instrument impulse response.
The over-sampled method has been applied to real CryoSat-2 pseudo-LRM data within the CPP chain over a 1 month period of time and the large Pacific SAR-mode area. The estimates have been analyzed through standard diagnoses in comparison with the CPP products, allowing to determine the accuracy and reliability of this method over open ocean. Simulation of over-sampled waveforms has also been performed to help in the assessment of this process. This paper presents these results.