Radar is a unit of measuring the distance and orientation of a target with respect to a radar signal transmitter. Radar interferometry is a technique where two SAR images of the same area are combined by matching the pixels representing the same ground feature precisely and calculating the phase or distance between the two images to produce the so called inteferogram. The improvements in these radar interferometry are the production of an ultra high resolution DTM with state of art air borne radar systems and new generation radar satellite missions such as TerraSAR-X. Some innovative techniques for mapping small changes in the geometry of the surface of the earth down to the cm-level in real time and over a very large area with a pair of radar images. These techniques work with the usage of radar satellite constellations and also a new technology called ScanSAR. A radar satellite constellation is a group or three or more satellites carrying identical SAR sensors. With the help of these constellations the time necessary for the satellite to visit the same area again can be reduced to a single day or less. These techniques map subtle changes on the earth’s surface up to mm-level using a stack of radar images.