A detailed analysis of S3 and S6 fully-focused SAR waveforms: Enabling SAMOSA-based retracking

Description

S6 fully-focused SAR (FF-SAR) waveforms are noticeably narrower than for S6 unfocused SAR (UF-SAR), which produces a significant bias in the geophysical estimates after applying a SAMOSA-based fitting. We demonstrate that a major part of these shape differences can be explained with the shapes of grating lobes and side lobes in the impulse response function (IRF) of FF-SAR and UF-SAR. With this insight, we can compensate for the differences.

Note that S6 has been designed in such a way that it suppresses the grating lobes in the FF-SAR IRF such that their influence is almost negligible. However, any grating lobes and sidelobes are not perfectly in focus and appear blurred. This was already noticed over a transponder target for CryoSat-2, see Figure 8 in by Guccione et al. (2018). The blurring in range direction occurs, because the applied range cell migration correction (RCMC) corrects only for the range history of the focus point, but not for the range history of targets at the grating lobe or side lobe positions.

We have developed a straightforward technique to minimise the difference between the emulated S6 UF-SAR and S6 FF-SAR waveform shapes, which preserves the full along track resolution. This result is significant, because it enables us to use the same SAMOSA-based retracking model for both UF-SAR and FF-SAR waveforms. This is especially useful for the coastal areas, where the high resolution of FF-SAR has a very high potential.

Our analysis is performed using our multi-mission FF-SAR backprojection algorithm, which allows for fully-focused processing of CryoSat-2, Sentinel-3 and Sentinel-6 MF. UF-SAR waveforms are an “emulated” by-product of the FF-SAR processing, as described in Egido et al. (2017) and Egido et al. (2021). This allows for a fully-consistent comparison of UF-SAR and FF-SAR waveforms at very high posting rates. In fact, all processing settings and input data are identical. It can be demonstrated that our emulated UF-SAR waveforms correspond well to the official L1b product.

Period	1 Nov 2022
Event title	Ocean Surface Topography Science Team meeting 2022
Event type	Other
Location	Venice, ItalyShow on map
Degree of Recognition	International