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German mapping satellites enter orbital formation

Posted: October 17, 2010

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Two German radar mapping satellites have moved into close formation to collect unprecedented three-dimensional imagery of Earth for scientific and commercial users, officials announced Friday.

Artist's concept of the TerraSAR-X and TanDEM-X satellites in orbit. Credit: DLR
The TerraSAR-X and TanDEM-X satellites are now orbiting just 350 meters, or about 1,150 feet, apart more than 300 miles above Earth.

After a few months of tests, the dual mission will begin three years of radar reconnaissance to assemble a detailed digital elevation model of Earth, a valuable resource for scientists and the commercial sector.

The fruition of a public-private partnership between the German Aerospace Center and Astrium, the satellites will work like a set of human eyes to create a 3D map of Earth's topography.

The combined mission's data will produce gridded maps with a spatial resolution of 12 meters, or 39 feet. The maps will show elevation with a precision of less than 2 meters, or 6 feet.

Launched June 21 from Kazakhstan, TanDEM-X was built as an add-on to TerraSAR-X for 3D mapping. TerraSAR-X conducted solo topograhic observations for more than three years since launching in 2007.

TanDEM-X previously flew about 20 kilometers, or 12 miles, behind TerraSAR-X. Over three days last week, ground controllers in Germany moved the satellite closer to TerraSAR-X, eventually reaching a point 1,150 feet apart Thursday, according to the German Aerospace Center, or DLR.

At such close distances, two 16-foot-long satellites will work together as a single unit.

"This is completely uncharted territory," said Manfred Zink, project manager for the TanDEM-X ground segment. "Never before have two satellites worked in such close formation over a period of several years."

One satellite's X-band radar will emit signals toward Earth, the beams will bounce off the surface, and both spacecraft's receivers will collect the radar reflections. Positioned at slightly different points in space, the satellites will recognize 3D scenes with stereo vision.

Each satellite's radar system is capable of accumulating enough data in three years to fill 200,000 DVDs, equivalent to 1.5 petabytes, or one-and-a-half quadrillion bytes.

The tandem uses GPS navigation data to keep from colliding and stay out of exclusion zones to avoid electronics damage from the satellites' powerful radar signals. The spacecraft must also stay in the right position because a millimeter error in range translates to a meter error in elevation data, according to scientists.

The satellites can also check each other's health through a dedicated communications link.

DLR and Astrium split the cost of TanDEM-X, which is valued at 165 million euros, or about $230 million at current exchange rates.

The operational mission is scheduled to begin in January, according to a DLR press release.

DLR is responsible for processing raw data streaming down from the satellites and distributing the information to scientific users. An Astrium subsidiary named Infoterra will disseminate data to commercial customers.

Officials say Germany's digital elevation model, the first of its kind in the world, could be valuable in hydrology, oceanography, geology, disaster response planning, resource exploration, and national security applications.