|Paper title||FFSAR replica removal algorithm for closed-burst data|
|Form of presentation||Poster|
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 , 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) .
The limitations of the FF-SAR in closed-burst mode has already been reported in , 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
 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 , 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.
Keywords— FFSAR, closed-burst, replica, deconvolution
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