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FF-SAR Processing

Exploitation of the Fully focused SAR (FFSAR) processing using S6-MF over ocean and sea ice surfaces

Fully-focusing of radar altimeters is a recent concept that has been introduced in (Egido and Smith, 2017) to allow further improvement of along-track resolution in high pulse repetition frequency (PRF) radar altimeters. While in Delay/Doppler processing the coherent summation of pulses is performed over a limited number of successive pulses (i.e., bursts), the concept of coherent summation has recently been extended to the whole synthetic aperture. This is the so-called fully-focused synthetic aperture radar (FFSAR) concept, in which all the echoes within the antenna extent are coherently summed after phase compensation to increase the along-track resolution up to its theoretical limit (half the along-track antenna length) and to improve also the ENL with respect to Delay/Doppler.  

It has to be recognized that exploitation of FFSAR waveforms in science application is just at the beginning, see (Kleinherenbrink et al, 2020) and (Egido et al, 2020) as well as that the already operational high PRF radar altimeters (CryoSat and Sentinel-3) have some limitations in the FFSAR processing: 

  • The closed burst timeline implies that the along-track FFSAR Impulse Response Function is affected by grating lobes that reduce the achivables accuracy of the resulting geophysical parameters 
  • The deramping-on-receive instruments introduce along-track phase distortion that have to be properly characterized to be then compensated to achieve a sufficient quality in the FFSAR waveforms 

S6-MF Poseidon-4 instruments offer new capabilities to investigate on the real potential of FFSAR concept. In fact, due to the open burst timeline of the Poseidon-4 instrument, in the along-track FFSAR Impulse Response Function obtained by processing S6-MF L1A products the grating lobes are no more present. Additionally, the matching-filter-on-receive scheme for Poseidon-4 is expected to guarantee a higher phase coherence within the visibility time of each point target. On the other hand, the combination of the PRF and of the along-track antenna pattern in Poseidon-4 determines that Doppler ambiguities are expected to affect the Level1b FFSAR waveforms in case that the whole Doppler bandwidth is processed. 

This activity aims to:

  • Find an optimal configuration of FF-SAR on S6 data following the target type (specular or not specular targets) by an analysis over few test cases over different areas identified first. The most important configuration parameters to be determined are the posting rate (to have the best compromise noise/resolution), the doppler bandwidth (to remove aliasing), the illumination time (to possibly reduce the sea surface motion effect over dynamical targets).
  • Detect leads on sea-ice areas by collocation with S1 (and S2) images. We want to take the opportunity of replicas absence with S6-MF interleaved mode to assess the FF-SAR capability to detect leads and to provide more precise surface height estimation compared to UF-SAR.
  • Analyse large FF-SAR data set over open ocean with the optimal configuration found in the previous task and evaluate the possible interest of implementing the omega-kappa method in ground segment. Additionally, a study of sea state retrieval with FFSAR will be conducted and validated by comparison with wave model (MFWAM) over swells of different wavelengths.

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