October 17, 2010
German Satellites ‘Dance’ In Space
Two German radar satellites that are being used to create the most detailed 3D map of the Earth's entire surface, have moved to within 1150 feet of each other in what scientists are calling a "tight formation."
The pair of satellites, TanDEM-X and TerraSAR-X, are sweeping around the globe at about 4.5 miles per second. They will soon begin an intense observation campaign that will pin down the variation in height across the Earth's surface to a precision of better than 7 feet.
The digital elevation model (DEM) will be put to numerous uses, such as improving the safety of aircraft navigation and understanding better which areas of the planet are more at risk during flooding.
TerraSAR was launched in 2007 and TanDEM in June of this year. The two have been brought closer together as time passed. The very close proximity maneuvers were conducted step-by-step over the course of the past week.
Manfred Zink, of the German space agency, told BBC News that the maneuvering was tricky and everyone was "a bit nervous."
The two satellites fly on ever-so-slightly offset orbits, which means they trace paths across the sky that look like strands of a double helix. This orbit ensures the two never bump into each other.
This operational flight feat it believed to be unique in civil space history.
The satellites will spend the next three years entwining around each other as they travel the heavens above Earth. But the current separation distance is not even the shortest planned; that will be 650 feet.
Dr. Zink said the closest distance will be pushed fro once they are through the commissioning phase. "We hope that by Christmas we can finish all the various tests that are still outstanding, and to start the acquisition of the global DEM data."
The unique orbital dance will give the two satellites "stereo vision," by enabling them to operate in interferometric mode in which one will act as a transmitter/receiver and the other as a second receiver.
The space agency expects the fully finished DEM of Earth should be available in 2014.
The US Shuttle Radar Topography Mission (SRTM) of 2000 was the best-known, near-global, space-borne DEM prior to the German venture. The SRTM produced a 100 feet by 100 feet spatial resolution with a vertical resolution that varied from 62 to 32 feet.
The TanDEM mission should be able to reach a spatial resolution of 40 feet by 40 feet with a vertical accuracy of less than 7 feet (2m).
While laser instruments (lidars) flown on planes can achieve better resolutions, their products are regional and they are not seamless maps of the whole Earth. TanDEM will achieve that.
"The SRTM is the most used Earth observation dataset ever. There is a version of it which is freely available on the USGS website. They have record hits. You have to imagine that all the people doing science around the Earth - be it geology, hydrology, whatever - for them, topography of the terrain is a key input," said Dr. Zink.
"I think for the scientists, we expect that the TanDEM-X dataset will replace SRTM," he said.
The TanDEM-X/TerraSAR-X venture is operated on the basis of a public-private partnership. EADS Astrium builds the satellites, which are owned by the German space agency, and Infoterra GmbH has exclusive rights to commercialize the data produced by them.
The German Space Operations Center, in Oberpfaffenhofen, is responsible for controlling the satellites.
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