During the coregistration, the master is aligned with respect to the DEM based on the simulated amplitude image, producing a single offset for the whole image. Because the resolution of a DEM is typically coarser (e.g., SRTM3: 90 m) than that of a radar image and the radar (ground) resolution differs between azimuth and range direction (e.g., 4 m in azimuth and 20 m in range for ERS1/2 and Envisat), the sensitivity in coregistration differs between the two directions (measured in resolution cells). As a result, a non-square correlation window may be more suitable. For example, for ERS1/2 we apply a 256 lines by 128 pixels correlation window.
This step updates the master acquisition azimuth and range times, respectively. Therefore, it affects steps whenever the master timing is used such as the coregistration using a DEM (DEMASSIST step) and the computation of reference phases (COMPREFPHA and COMPREFDEM steps).