Radar interferometry and its application to changes in the earth's surface

Geophysical applications of radar interferometry to measure changes in the Earth's surface have exploded in the early 1990s. This new geodetic techniq ue calculates the interference pattern caused by the difference in phase be tween two images acquired by a spaceborne synthetic aperture radar at two d istinct times. The resulting interferogram is a contour map of the change i n distance between the ground and the radar instrument. These maps provide an unsurpassed spatial sampling density (similar to 100 pixels km(-2)), a c ompetitive precision (similar to 1 cm), and a useful observation cadence (1 pass month(-1)). They record movements in the crust, perturbations in the atmosphere, dielectric modifications in the soil, and relief in the topogra phy. They are also sensitive to technical effects, such as relative variati ons in the radar's trajectory or variations in its frequency standard. We d escribe how all these phenomena contribute to an interferogram. Then a prac tical summary explains the techniques for calculating and manipulating inte rferograms from various radar instruments, including the four satellites cu rrently in orbit: ERS-1, ERS-2, JERS-1, and RADARSAT. The next chapter sugg ests some guidelines for interpreting an interferogram as a geophysical mea surement: respecting the limits of the technique, assessing its uncertainty , recognizing artifacts, and discriminating different types of signal. We t hen review the geophysical applications published to date, most of which st udy deformation related to earthquakes, volcanoes, and glaciers using ERS-1 data. We also show examples of monitoring natural hazards and environmenta l alterations related to landslides, subsidence, and agriculture. In additi on, we consider subtler geophysical signals such as postseismic relaxation, tidal loading of coastal areas, and interseismic strain accumulation. We c onclude with our perspectives on the future of radar interferometry. The ob jective of the review is for the reader to develop the physical understandi ng necessary to calculate an interferogram and the geophysical intuition ne cessary to interpret it.