SAR specialists from CNES and Cesbio have developed a nice tool named S1Tiling, to calibrate, ortho-rectify Sentinel1 data onto Sentinel2 MGRS2 grid, and to filter speckle with a spatio-temporal filter.
S1Tiling is an open source python code (developed by Thierry Koleck) available on github, that uses the radar ortho-rectification application from OrfeoToolbox (developed by J.Michel). It is quite efficient and fast.
S1Tiling is also available as an on-demand software on PEPS collaborative ground segment (without filtering)
You will find more info from Thierry Koleck’s post in CESBIO multitemp blog.