An acronym for interferometric synthetic aperture radar, an instrument (and by extension a method) for microwave remote sensing of the topography, velocity field and other characteristics of a surface. A synthetic aperture radar (SAR) consists of a side-looking radar system that takes advantage of the forward motion of the radar platform to synthesize a very long antenna, enabling a much higher ground resolution than in ordinary radar altimetry. Each SAR acquisition contains information on the amplitude and phase of the radiation reflected from the target and received at the antenna. Interferometric SAR requires the calculation of differences in phase between two co-registered SAR images obtained with slightly different viewing geometries, either at the same time from two antennae, or at two different times from one antenna. These phase differences yield fringe patterns (interferograms) that are an expression of both surface topography and surface motion. If the surface is not in motion, or the time between images is sufficiently short, phase differences can be converted to surface elevations with knowledge of the attitude and orbital position of the interferometer; more specifically, the baseline length, or distance between the two orbital positions, must be known. Using insar to detect motion of the surface requires imagery from two different epochs (repeat pass interferometry).In this case topographic effects are removed using an independently derived digital elevation model.