Radar satellites emit microwave pulses from an on-board antenna which, scattered back from the Earth's surface, are received again by the satellite. The runtime of these pulses is used to compute distances to the surface, which may be developed into a terrain model. SAR interferometrie is an advanced version of this technology. It involves scanning an area from two different antenna positions. The principle remotely resembles the way in which humans see with two eyes. For points at various elevations, various path length differences result. Ultimately, we obtain a phase-difference pattern called an interferogram. Having further refined this technique, DLR scientists have evaluated the back-scatter from more than four million permanent scatterers in numerous images. This method permits detecting elevation changes in the centimetre range.