This step requires a DEM. It can best be performed after the interferogram generation. SRTM can be used to obtain the topography in 3 arcseconds (Near global: 90 m. at the equator) or 1 arcsecond (USA only) resolution. The input DEM file must have the byte order of your platform in order to extract correct elevation value, see CRD_IN_FORMAT option for details. The DEM should be in the WGS84 system (same as the orbit ephemerides) The Doris distribution contains the utility construct_dem.sh to download and prepare SRTM data (see Section C.2.12).
An alternative is the USGS gtopo30 DEM. This is a global DEM with
relatively low precision and gridspacing (30 seconds, approximately
1 km at equator). There are 33 tiles covering the globe, in total
requiring about 3GB of disk space. (For more information, see for
http://edc.usgs.gov/products/elevation/gtopo30/gtopo30.html or Google for more information).
The input DEM has to be in an equiangular grid. The format is short signed integers, real4 floats, or real8 doubles (meters). The DEM should be in WGS84 system (same as orbit ephemerides). The UpperLeft pixel of the DEM matrix on file should be the most North, most West pixel, i.e., the pixel with largest latitude (between -90,90) and smallest longitude (between -180,180 (?)).