Conventionally, to estimate backscatter (\sigma^o, \gamma^o or \beta^o ) for one single Sentinel-1 SLC data, we apply Radar -> Radiometric -> Calibration. In my case, I want to estimate backscatter for 6 consecutive Sentinel-1 aquisitions at my study area to construct a time series of \gamma^o. To achieve such goal, the stupid and tedies way I know is to estimate backscatter for individual SLC data, then stack together the estimated \gamma^o . However, I wonder if it possible to estimate backscatter directly for a stack of co-registrated images (DN) which is already prepared. But such funtionality seems not supported in SNAP 7.0 yet? As I inspected the metadata of the stack, calibration vector which is used for calibration is missing at this stage. Is it possible to estimate backscatter for each band simutaniously in a stack of co-registrated Sentinel-1 DN images with ESD technique applied?
You can try to calibrate each S-1 SLC product first but save it in complex. This will give you sigma0 in complex. Then coregister them using S-1 Backgeocoding. This will give you the coregistered sigma0 in complex.
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Thanks for your helpful advice! It works. I forgot that there is such an option to save image in complex format.