TanDEM-X

The TanDEM-X mission (TerraSAR-X add-on for Digital Elevation Measurement) is based on two almost identical earth observation satellites: TerraSAR-X and TanDEM-X. Both are equipped with a modern, powerful radar system, a Synthetic Aperture Radar (SAR). It can be used to monitor the earth not only during the daytime but also at night and under cloud cover.

TerraSAR-X was launched on June 15, 2007 with a Dnepr rocket from the Baikonur space facility in Kazakhstan. TanDEM-X followed on June 21, 2010. Both satellites fly in formation at 514 kilometers altitude. The distance between them is slight, sometimes even under 200 meters. With this second radar sensor in space it is for the first time possible to derive a global digital elevation model with a vertical resolution of two meters. The horizontal grid is 12 x 12 meters.

A 3D global elevation model possible with three years of tandem flight

The two satellites are also the first configurable SAR interferometers in space. In bi-static mode one satellite functions as a transmitter and both receive the echo of the signals reflected from the earth’s surface. For global coverage, three years of parallel operation in formation flight are necessary. During this time the satellite pair can measure the entire land surface of the earth, 150 million square kilometers. The amount of data this generates, about 1.5 petabytes, is unique to date. On DVDs they could be piled up in a tower over 430 meters high—which is100 m higher than the Eiffel tower.

The TanDEM-X project is the next logical step after the international radar missions X-SAR (X-Band Synthetic Aperture Radar) and SRTM (Shuttle Radar Topography Mission), and the successful realization of the national project TerraSAR-X. Already in April and September 1994, the X-SAR instrument completed two successful, ten-day missions on the space shuttle. It was subsequently redesigned as an interferometer and in February 2000 again employed on a SRTM mission. As part of SRTM, land surfaces were recorded with two radar systems, and an elevation model of the earth’s surface was derived from the data.

TanDEM-X: A public-private partnership mission

TanDEM-X is being realized in a public-private partnership with the financial participation of industry: DLR is responsible for the overall management, the ground segment and operations. EADS Astrium is responsible for the development, construction and launch of the satellite.

An important payload on the TanDEM-X satellite is the Tracking, Occultation and Ranging Experiment (TOR), provided by GeoForschungsZentrum Potsdam (GFZ). It consists of a dual frequency GPS receiver capable of determining the satellite’s orbit with high precision down to a few centimeters. The receiver is also used for radio occultation measurements in the atmosphere and ionosphere.