The technique that makes use of interference of electromagnetic waves that are transmitted and received by a SAR is called interferometric synthetic aperture radar, or InSAR. Very simply, InSAR involves the use of two or more SAR images of the same area—one arbitrarily chosen reference or master image and one or more additional images referred to as slave images—to extract land surface topography and deformation patterns.
But something very useful emerges when two otherwise useless SLC SAR images are combined, as explained below.
Interferometric SAR (InSAR) exploits the phase differences of at least two complex-valued SAR images acquired from different orbit positions and/or at different times.
也就是說,InSAR利用的是干涉圖(interferogram)來反演信息的。
The interferogram is calculated by co-registering two SAR imagesμ1 、μ2 and differencing the corresponding phase values on a pixel-by-pixel basis, i.e., by a pixel-by-pixel complex multiplication of the master image μ1 with the complex conjugated slave image μ2 . Due to baseline B , the distances from the antennas to the scene differ by δR , which results in a phase difference δφ in the interferogram:
s=μ1μ∗1=∣μ1∣exp(jδφ)
δφ=φ1−φ2
Under the pre-condition φscatt,1=φscatt,2 and the utilization of the same emitting horn for both images leading to Rfw,1=Rfw,2 , which is the case for single-pass measurements, the interferometric phase is just related to the range difference of the two antennas: