What is "tandem-x"

TanDEM-X (TerraSAR-X add-on for Digital Elevation Measurement) is the name of TerraSAR-X's twin satellite, a German Earth observation satellite using SAR ( Synthetic Aperture Radar) - a modern radar imaging technology. Implemented in a Public-Private-Partnership between the German Aerospace centre ( DLR) and EADS Astrium (now Airbus Defence and Space), it is a second, almost identical spacecraft to TerraSAR-X. TanDEM-X is also the name of the satellite mission flying the two satellites in a closely controlled formation with typical distances between 250 and 500 m. This unique twin satellite constellation will allow the generation of the WorldDEM™, the seamless global digital elevation models available from 2014.