Radar Interferometry

Radar interferometry, as the term has come to be used, actually refers to a technique for comparing the phase of radar images. While the creation of a radar image (synthetic aperture processing) can be seen as the organization of a constructive interference between the various echoes collected, this comparison is essentially the subtraction of the phases of two radar images previously superimposed geometrically, from which other systematic elements are subtracted, such as the effect of the trajectories at the origin of the two images, the topography of the terrain within the limits of our knowledge of it, or even the predictions of subsidence deformation models, earthquakes or others. This difference, the interferogram, is an image of lengths, since the phase is directly related to the wavelength used by the radar. It is ambiguous, however, as it gives only the remainder of any geometric difference observed in the two images modulo wavelength. The entire number of wavelengths present in the measurement must be restored by other methods. It is generally accurate, since signal-to-noise ratio conditions often enable phase to be assessed to within 10 degrees or better, which, depending on the radar wavelength, can correspond to accuracies in the millimetre range.

Radar interferometry, or simply radar interferometry, can calculate topography with metric or even sub-metric precision, and reveal deformations in the ground or on structures with millimetric accuracy. It also contains the difference in contributions from the variation in atmospheric thickness during the two shots, generally considered a rather annoying artifact. The latter is potentially indicative of atmospheric pressure, turbulence and atmospheric water content, but without any vertical discrimination.

The use of radar interferometry became widespread when radar satellites, notably the ERS-1 mission (European Remote Sensing 1, launched in 1991), made it possible to establish a global and homogeneous planetary archive enabling comparisons before and after an event. Dedicated missions, principally the Shuttle Radar Topography Mission (SRTM) in February 2000, have established a global reference of medium-precision terrain models.

The technique is easy to implement, involving only well-known operations (image correlations, resampling, subtractions and linear combinations) and simple geometric simulations designed to translate known experimental conditions such as carrier trajectories and terrain topography into phase differences.

We shall see that phase unwinding and other filtering techniques applied to interferograms do not deserve the importance they were given at the dawn of this technique.

On the other hand, interpretation techniques...