there is no method in xsarsea to match with sigma0 denoised values provided by ESA in Level-2 products
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agrouaze commented
Currently there is no specific methods in xsarsea
to get a NRCS (aka sigma0 denoised) value from a Sentinel-1 product as a Level-2 like. For instance, WV products computing the average value of xsar
sigma0
values lead to values up to 3 dB below compare to the value provided in official ESA Level-2 product (oswNrcsNezCorr
variable).
Looking at the ESA processor it seems that some steps are potentially* missing in xsarsea
.
Typically what ESA processor do to compute the sigma0 is:
- read digital number
- apply calibration (dividing by the sigma0_lut²)
- low pass filter the image
- keep intensity multiplying by <abs(image)²>
- remove platform (I suppose it is related to https://en.wikipedia.org/wiki/Apodization technics.
- final average sigma0 value is
mean((image* np.conj(image)).real)
- I don't know if we need to be able to reproduce the sigma0 given by ESA