TerraSAR-X, a German Earth observation satellite, is a joint venture being carried out under a public-private-partnership between the German Aerospace Center DLR and EADS Astrium GmbH; the exclusive commercial exploitation rights are held by the geo-information service provider Infoterra GmbH. TerraSAR-X was launched on June 15, 2007 and has been in operational service since January 2008. Following the launch of a second sister satellite, TanDEM-X in early 2010, the two satellites act as a pair.
With its active phased array X-band SAR antenna (wavelength 31 mm, frequency 9.6 GHz), TerraSAR-X acquires new high-quality radar images of the entire planet whilst circling Earth in a polar orbit at 514 km altitude. The orbit is selected such that the satellite flies in a sun-synchronous dusk-dawn orbit, which means that it moves along the day-night boundary of the Earth and always presents the same face to the sun, ensuring an optimum energy supply via the solar cells. TerraSAR-X is designed to carry out its task for five years, independent of weather conditions and illumination, and reliably provides radar images with a resolution of up to 1m.
Satellites with radar technology are still relatively new compared to optical camera systems. The resolution (detail sharpness) is in principle lower, but radar has other advantages: radar is independent of illumination and weather conditions, so that acquisitions can be made at any time of the day or night and independent of cloud coverage. This contributes significantly to the reliability of the system a property that is required by many applications and users.
Early radar satellite techniques were e.g. the Altimetrie (leveling over the sea), NASA's SEASAT (launched in 1978), regulation of waves/wind or soil data. Nowadays we can measure for instance the speed of other satellites to mm/sec exactly (GRACE), and the slow deformation of volcanos. The military has used radar since the late 1930s and RADAR satellites at least since since 1978.
TerraSAR X exhibits some technical-industrial novelties. One of these innovations is a kind of zoom shot, with the resolution and scanning field vice versa changeable in a 1:10 relationship, either a larger area to grasp or a small area with the highest possible resolution. Furthermore the antenna can be aligned by electronics within an angle range so that the point of view is adjustable. Earlier radar satellites could radiate the antenna only in one direction.
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