Walter Smith (NOAA, USA)

The delay/Doppler algorithm implemented in CryoSat-2 and Sentinel-3 applies a coherent processing the 64 echoes within each burst (about 3.5 milliseconds of flight), which allows narrowing the footprint in the direction along the track to about 300 m. However, by accounting for the phase evolution of the targets in the scene, it is possible to focus the complex echoes along the aperture, and perform inter-burst coherent integration potentially as long as the target illumination time. This process, similar to SAR imaging systems, reduces the along-track resolution down to the theoretical limit equal to L/2, where L is the antenna length. We call this the fully focused SAR Altimetry processing. For the development of the technique we have used the CryoSat-2 SAR Mode data, but our methods could also be used with similar data from Sentinel-3 or Sentinel-6/Jason-CS.

The footprint of a fully focused SAR altimeter measurement is an elongated strip on the surface, which is pulse-limited across-track and SAR focused along-track. The technique has been demonstrated using transponder data, showing an achievable along-track resolution of 0.5 meters. Despite the asymmetry of the altimeter footprint, the fully focused technique may be useful for applications in which one needs to separate specific targets within highly heterogeneous scenes, such as in the case of sea-ice leads detection, hydrology, and coastal altimetry applications. Applying this technique on CryoSat-2 data over land and sea-ice, we can correctly measure the along-track extent of water bodies and ice-leads only a few meters long in the along-track dimension.

On a random rough surface like the open ocean, the fully focused altimeter waveform is a random realization of speckle noise. However, these single looks will be inherently uncorrelated between each other, so they can be incoherently averaged to obtain a multi-looked waveform that could potentially have significant improvements not only with respect to conventional and D/D (unfocused SAR) altimeters. Firstly, the fully focused SAR processing technique could potentially achieve an increase on the equivalent number of looks by a factor of 3 with respect to D/D. This could represent a significant speckle noise reduction in the final multi-looked waveform, and therefore an improvement in the estimation of geophysical parameters. And secondly, the multi-looked unfocused D/D altimeter waveform is a combination of several beams at different look angles. For each single beam, the impulse response widens as the square of the beam angle, so multilooking has the effect of widening the final waveform. In contrast, the fully focused SAR altimeter achieves the limiting case of an infinitesimally narrow beam at nadir, resulting in a narrower waveform, which improves the estimation of geophysical parameters and allows the retracking of the multilooked waveform by just the zeroth Doppler beam of the D/D stack model. This is relevant as it drastically reduces the computation time of the retracking.

This paper concentrates on the oceanographic applications of the focused SAR altimetry technique, and reviews the results that we have obtained so far from CryoSat-2 SAR mode observations from the open ocean, where a consistent performance improvement of square root of 2 with respect to the ESA L2 product is obtained for both sea surface height range and significant wave height. The performance improvement is lower than expected for an ideal FF-SAR, as the closed burst operation mode of the CryoSat-2 SIRAL instrument imposes a lacunar sampling of the Doppler spectrum. This results in side lobes in the full along-track point target response, which introduces correlation in the successive looks of the ocean. This effect and possible mitigation strategies are also discussed in the paper.