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23. Introduction to Interferometry
Accurate surface topographic data is required to investigate a wide variety of geophysical processes. An exciting and promising technique for the application of remote sensing data has emerged in recent years: Interferometric SAR, also called INSAR.
Using SAR interferometry, it is possible to produce directly from SAR image data detailed and accurate three-dimensional relief maps of the Earth's surface.
In addition, an extension of the basic technique, known as differential interferometry, allows detection of very-small (in the order of centimetres) movement of land surface features. Both of these possibilities open up many new potential application areas of spaceborne SAR data in the areas of cartography, volcanology, crustal dynamics, and the monitoring of land subsidence.
Interferometry allows the measurement of high resolution topographic profiles of terrain from multiple-pass SAR data sets. For the interferometric technique to be applicable, these data sets must be obtained when the sensor is in repeat orbit, such that the scene is viewed from almost the same aspect angle for each of the passes.
The figure shows the interferometric imaging geometry pointing out the two passes with range vectors r1 and r2 to the resolution element. The look angle of the radar is θ, the baseline B is tilted at an angle ξ measured relative to horizontal.
Each pixel of a SAR image contains information on both the intensity and phase of the received signal. The pixel intensity is related to the radar scattering properties of the surface, and the pixel phase to the satellite to ground path length, or distance.
Intensity images are the form of SAR data that is most frequently presented and probably most familiar to the public.
However, it is the phase information only (and not the image intensity) that is exploited by interferometric techniques, it contains informations about heights orthogonal to the SAR image plane.
An interferometer is a device that superimposes or mixes wave phenomena from two coherent sources.
First, the two SAR images are registered to each other to identify pixels corresponding to the same area of the Earth's surface.
Then, for each pixel, the phase values are subtracted to produce the phase difference image known as an interferogramme. This phase difference is a measure of the difference in path length from a given pixel to each antenna of the SAR interferometer.
The resulting interference effects are well-known both in optics (e.g., Newton's rings formed when a convex lens is placed on a plane surface) and sonics (e.g., beating formed by two similar frequency sound waves). Radar interferometry is the analogous phenomenon in the microwave region of the electromagnetic spectrum.