View from the wide camera
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time lapse video


TandemX and TerrasarX (image: DLR)
In the video you are seeing the laser ranger tracking a pair of satellites (the white dots, the smaller dots are stars), TandemX and TerrasarX, that are working in tandem to measure the shape of the surface of the Earth using a radar technique called synthetic aperture radar (SAR). The two satellites, each of whose sun-reflected images you can see in the video near the 'point' of the laser beam, are separated from each other by just a few hundred meters. To maximize the use of the laser range measurements, SGF have developed a technique that allows us to range to each satellite alternately in a repeating cycle of 15 seconds, the telescope automatically directing the laser first to one and then to the other one. Hence the images (dots) appearing to jump rapidly relative to the laser beam. You will also have noticed that the satellite images vary in brightness during the pass, becoming very bright near mid-pass. The changing geometry between the station, satellites and the Sun (below our horizon, of course) drives these brightness changes, with for some angles, the flat surfaces of the satellite acting as mirrors to reflect sunlight our way.