Three pass differential interferometry is described in . It is a method to remove the topographic induced phase from an interferogram containing topography, deformation, and atmosphere. This module thus can also be used to study atmospheric effects in interferograms, if no deformation is expected.
This step can be performed if an unwrapped topography interferogram (topo pair) and a complex deformation interferogram (defo pair) are present (with a common master, perform 2 seperate runs of Doris to achieve this). The interferograms have to be corrected for the phase of the 'flatearth' (see step SUBTRREFPHA), and sampled on the same grid (see step RESAMPLE). The files must have the same multilook factors and the same dimensions (i.e. overlap exactly). The perpendicular baseline of the topo-pair should be larger than that of the defo-pair, to prevent that noise is blown up, but this cannot always be controlled of course.
This step is performed in the defo-pair processing tree. First create a directory to run the topo-pair processing until a unwrapped interferogram is obtained (keep the master, slave, and products result files). Then perform the defo-pair processing. After interferogram generation and 'flatearth' subtraction, start this step (DINSAR), specifying the location of the result files of the topo-pair processing with input cards. For 3 pass, use a common master. For 4 pass, coregister the complex interferogram on the complex interferogram of the defo-pair, and then unwrap, or first coregister master and slave on the master of the deformation pair.
To geocode the differential phase values, geocode the topo-interferogram and use the latitude/longitude matrices for the differential grid.