DInSAR is a technique that employs two or more SAR images of the same area acquired at different times to generate maps called interferograms that detect and map changes in ground-surface displacement (range change) between the two time periods (Bürgmann et al. 2000; Bamler and Hartl 1998; Massonnet and Feigl, 1998).
The reference (or master) SAR image's phase difference is calculated along the LOS of the satellite. It is defined as the line connecting the antenna to the ground object being imaged (Figure 2.3), and it shares some conceptual similarities with the SAR slant range vector. The SAR antenna views a particular area on the ground at angle θ1 during the first satellite pass (SAR1). The same spot is observed from a marginally different angle θ2 during the second pass (SAR2). The distance between the satellite positions at the two acquisition times can also express the view angle difference. The physical distance is defined as the baseline.
The plane's direction of flight is parallel to the ground. The baseline is the distance B between the antenna positions on the first and second passes. The angle between the two viewing directions is proportional to baseline B's perpendicular angle. As discussed below, this is a deciding factor for coherence and topographic mapping between the two antennas B ( Figure 2.3). The distance from the satellite position in the master acquisition to the LOS of the slave acquisition is the perpendicular baseline. In an interferogram, a fringe is a line with an equal phase. An interferogram measures the number of fringes from a reference point where the surface deformation (displacement) is ostensibly zero. The interferogram must be regenerated at greater spatial resolution by lowering the look number or combining two pictures with shorter intervals to count fringes in locations with extremely fine fringes. With the range changes that affect the phase difference between two co-registered phase images, an interferogram may extract information such as topography and deformation patterns. The time between two image acquisitions can range from 0.1 seconds using single-pass interferometry to independent intervals (up to years) using repeat-pass interferometry, depending on how quickly the target is detected (Massonnet and Feigl, 1998; Zhou et al., 2009).
Figure (2. 3): Image of the InSAR geometry for two sensors located at SAR1 and SAR2 that has been modified from (Bamler and Hartl, 1998).
