Separation of Coherent and Incoherent Scattering Components from Delay/Doppler Altimeter Waveforms over Sea-Ice

Abstract:

One of the main benefits of the delay-Doppler altimeter (DDA) is the improved resolution of the system along the satellite track. By means of an unfocussed Synthetic Aperture Radar (SAR) processing technique, the altimeter footprint along the flight direction can be reduced by an order of magnitude with respect to conventional altimeters. However, with the delay-Doppler processing the resolution improvement occurs only on the along-track direction, while the across-track direction remains pulse-limited. The result is an elongated footprint perpendicular to the satellite flight path.

The combination of the effects of several scatterers within the footprint can lead to random variations of the DDA waveforms, preventing conventional retracking techniques from retrieving geophysical parameters from altimeter data. This is particularly significant in the case of sea ice, where the coherent response from leads can completely exceed the response from the actual ice surface.

We have developed a processing technique that allows the separation of the coherent and incoherent scattering components from SAR altimetry waveforms. The technique is similar to the one used in imaging SAR systems, and is based in the exploitation of the phase history of coherent targets during their illumination period with the antenna beam.

For the development of the technique we have used the CryoSat-2 SAR Mode data. The starting point of our processing is the full bit rate (FBR) I/Q complex echo samples. By accounting for the phase evolution of the static targets in the scene, it is possible to correct the phase of the FBR complex echoes along the aperture, which allows to perform an inter-burst coherent averaging, potentially, as long as the target illumination time. This reduces the incoherent components of the radar signal, which results in a radar waveform that contains only the coherent scattering component. The coherent component can later be removed from the original delay-Doppler waveform.

For the case of sea-ice studies this technique can be used to separate the contribution from leads and ice floes, which can potentially be used to determine the sea-ice freeboard. In this paper we present the first results obtained with CryoSat-2 data over the Arctic, that show the potentialities of this technique.