InSAR Stack Processing — Deformation Mapping in the Area of ...

Chapter 3Methodology3.1 Interferogram creationThe first step of the processing is the interferogram creation. It consists of the following important steps:• coregistration of the two scenes,• resampling of the slave scene in order to correspond exactly to the master,• subtraction of the slave phase from the master phase (interferogram creation),• subtraction of the flat-Earth phase (i.e. the phase corresponding to the Earth surface withouttopography) from the interferogram. For the flat-Earth phase computation, orbital information isused.3.2 Topography subtractionA significant component of the phase information is influenced with topography. The coefficient ofproportionality is influenced by the spatial baseline between the two scenes, particularly by the componentperpendicular to the radar ray (perpendicular baseline).There are two basic methods of topography subtraction:• 2-pass method, i.e. using an external digital elevation model (DEM) as a topography information.Using orbital information, the DEM is converted into the radar system (i.e. a synthetic interferogramis created) and this is subtracted from the real interferogram. GAMMA software allowsto create also a synthetic magnitude image to be coregistered with the magnitude image from thesatellite; however, this possibility was not used for the processed scene crops. The synthetic magnitudeimage is created from the DEM information only and therefore does not contain man-madefeatures, which are most significant in the satellite magnitude image. Due to this fact the coregistrationof the small scene crops fails. However, as proven in the whole scene processing, the orbitsare in most cases precise enough such that the difference between the synthetic and real magnitudeis not higher than one pixel. We use the SRTM (Shuttle Radar Topography Mission) DEM, whichis created by InSAR method, and therefore some properties may be similar.• 3-pass method, i.e. using a different interferogram for topography subtraction. The phase of thisinterferogram must be unwrapped which may be a problem in incoherent areas. We decided not touse this method in the project.13