Synthetic aperture radar (SAR) interferometry, or simply InSAR, is a remote sensing technique utilising the phase difference between two or more complex-valued SAR images. Most modern SAR systems are capable of measuring both the intensity and the phase of the reflected signal, where the latter carries information about the distance travelled by the signal. Consequently, the different of phase information of two successive SAR images over a specific area contains a distance information.
The phase difference measured between two SAR images is called the interferometric phase. The interferometric phase image is an interferogram. The interferometric phase is a function of the geometry and timing of the individual SAR acquisitions. Different geometric and temporal configurations enable different applications.
If the SAR acquisitions are made from different angles and without significant temporal change of the scene, InSAR can be used to create digital elevation models (DEMs) of the Earth, as demonstrated by the NASA/JPL Shuttle Radar Topography Mission (SRTM). This configuration is called across-track interferometry. If the individual SAR acquisitions are made at different times in the same geometric configuration, i.e. in an along-track or differential interferometric configuration, then InSAR can be used to measure radial velocity of targets and to assess displacements caused by, e.g., volcanoes and earthquakes. The variation of the temporal baseline allows determining velocities ranging from several meters per second to a few millimeters per year. While standard differential interferometry can be used to retrieve changes that happened between two SAR acquisitions, differential interferometric stacking techniques, such as Persistent Scatterer Interferometry (PSI) and Small Baseline Subset (SBAS), are used to monitor deformation over a longer period of time by stacking multiple differential interferograms and filtering out the atmospheric phase contribution in order to retrieve very accurate deformation of the ground and its infrastructures.