¿Why should we process Fully-Focused Radar Altimetry data in near real time? Improving the computational efficiency of FF-SAR using Omega-K based algorithm.

The Copernicus Sentinel-6 mission was planned to keep studying the sea-surface height and ocean state measurements and since April 2022 its first satellite, the Sentinel-6 Michael Freilich (S6MF), has become the current reference altimetry mission.

One of the key design aspects of the S6MF altimeter is the “interleaved” chronogram pattern, which increases the number of received pulses, reducing the ambiguities in the along-track dimension, and increasing the energy obtained from the surface. Thanks to the continuous surface illumination, the echoes from the same target can be coherently integrated making a significant improvement in the along-track resolution leading to more detailed understanding of the ocean, the polar zones and inland and coastal waters dynamics. Measurement parameters such as swell state, lead and iceberg detection or inland water level changes can take advantage of these advances.

Nowadays, unfocused azimuth steering methods, such as Delay Doppler, provide along-track resolutions around 300 meters. However Fully-Focused SAR (FF-SAR) algorithms can improve it to the order of sub-meter.  In 2017, Alejandro Egido and Walter H.F. Smith published the FF-SAR method description, based on the backprojection approach. Compared to the unfocused steering, it requires more computational time, making it difficult to be implemented operationally, keeping the data generation rate in the same order of magnitude as the unfocused chain products. In 2018, Pietro Guccione et. al. published the FF-SAR 2D Frequency Domain algorithm, based on the Omega-K (WK) algorithm from SAR radar. In this paper, it is shown that the two dimensional frequency domain can be used to decrease the number of operations needed to focus the data, under some circumstances that depend on the type of orbit and the emitted signal. FF-SAR WK algorithm achieves similar results in terms of along-track resolution for the CryoSat-2 mission, but notably improving the computational efficiency. Although Omega-K algorithm intrinsic assumptions can impact negatively in the accuracy estimation of parameters such as the sea surface height, there are applications like sea ice related activities that could benefit of faster execution times. In this presentation, an adapted and redesigned Omega-K algorithm for Sentinel-6 is presented. FF-SAR Omega-K and Backprojection have been used to process Sentinel-6 data over the Crete transponder, evidencing that both FF-SAR methods are capable to achieve the expected theoretical along-track resolution. Moreover, open ocean data from Sentinel-6 has been processed for both algorithms and results have been compared.