Abstract's details

FFSAR replica removal algorithm for closed-burst data

Samira Amraoui (CLS, France)

CoAuthors

Thomas Moreau (CLS, France); Maxime Vayre (CLS, France); François Boy (CNES, France); Sophie Le Gac (CNES, France); Nicolas Picot (CNES, France); Franck Borde (ESA, Netherlands)

Event: 2022 Ocean Surface Topography Science Team Meeting

Session: Instrument Processing: Measurement and Retracking

Presentation type: Type Oral

The SAR-mode processing in altimetry as it is currently operated in ground segments does not exploit the full capabilities of the SAR system in terms of spatial resolution. The so-called unfocused SAR altimeter (UFSAR) processing performs the coherent summation of pulses over a limited number of successive pulses (64-pulses bursts of a few milliseconds in length) reducing the along-track resolution down to 300 m only. Since recently [2], the concept of coherent summation has been extended to the whole illumination time of the surface (typically more than 2 seconds) allowing to increase the along-track resolution up to the theoretical limit (approximately 0.5 m) and thus improving the SAR-mode capability for imaging reflective surfaces of small size. The benefits of the fully-focused coherent processing have already been demonstrated on various surfaces to differentiate targets of heterogeneous surfaces (like sea-ice, inland-water and coastal) and to achieve the maximum effective number of looks available from SAR altimetry on homogeneous surfaces (like ocean) [2].

The limitations of the FF-SAR in closed-burst mode has already been reported in [2], creating very harmful artificial side lobes in the along-track dimension due to lacunary chronogram. It is extremely challenging to separate real signal from its replicas when they are superimposed, considering that every reflecting focalization point on ground creates its own replicas. Both Sentinel-3 and Cryosat-2 SAR altimetry missions have been designed with a lacunary chronogram, one exception is for the quasi-continuous pulse transmission Sentinel-6 inter-leaved mode. Over heterogeneous targets, replicas interference creates peaks and troughs pattern, with overflow of power outside of the water body boundaries and destruction of power inside the water body boundaries. This clearly jeopardizes confidence in data and its use for large water body detection like lead detection a major goal in SAR altimerty sea-ice application.

At level-2, impact of replicas on the estimated geophysical parameters are not completely understood yet. Even at crossing point between Sentinel-3 and Sentinel-6, it might be tricky to compare the results due to not identical footprint and overflight angle, but also altimeters differences apart from chronogram (like the sampling frequency, deramping/match-filtering and SNR). A new methodology of comparison has been developed and implemented at CLS taking Sentinel-6 data and emulating the sparse closed-burst mode chronogram of Sentinel-3 by removing pulses. Thus on same acquisition points open-burst and closed-burst can be compared each other by isolating only replica effect. More than 700 hydrological targets (including narrow rivers, larger rivers, lakes and dam) have been already processed. First results showed as expected global differences in amplitude but more surprisingly a higher range variability of 1.5cm in closed-burst mode compared to open-burst mode.

Next progress is replica removal, a very important topic if we expect to exploit the full potential of FF-SAR processing with Sentinel-3 and Cryosat-2 data. We propose a deconvolution technique to recover the open-burst radargram using optimization method starting from Wiener filtered first guess [3] and a model that takes into account replicas. A new model of multi-scatterer FF-SAR impulse response function, based on LRM inland-water model approach in [1], has been developed and validated over diverse rivers data acquisition. This model supposes water presence a priori knowledge, which might be relevant for inland-water (by exploiting water surface masks), but turns out to be completely irrelevant for sea-ice lead targets permanently in movement. To tackle this problem, the replica model is first optimized to determine the position and specularity of water presence that fit at best real data. Once the model fixed, deconvolution is validated by comparison of reference Sentinel-6 open-burst geophysical parameters with deconvoluted degraded Sentinel-6 closed-burst data. Different surfaces captured by Sentinel-6 will be deconvoluted including rivers, lakes, leads and open-ocean.

[1] R. Abileah, A. Scozzari, and S. Vignudelli. Envisat RA-2 Individual Echoes: A Unique Dataset for a Better Understanding of Inland Water Altimetry Potentialities. Remote Sensing, 9(6):605, June 2017.
[2] A. Egido and W. H. F. Smith. Fully Focused SAR Altimetry: Theory and Applications. IEEE Transactions on Geoscience and Remote Sensing, 55(1):392–406, Jan. 2017.
[3] A. Monti-Guarnieri. Adaptive removal of azimuth ambiguities in SAR images. Geoscience and Remote Sensing, IEEE Transactions on, 43:625–633, Apr. 2005.
 

Oral presentation show times:

Room Start Date End Date
Sala Grande Tue, Nov 01 2022,09:35 Tue, Nov 01 2022,09:50
Samira Amraoui
CLS
France
samraoui@groupcls.com