German TanDEM-X satellite seeks 3D view of Earth

By Jonathan Amos
Science correspondent, BBC News


The TanDEM-X satellite has blasted into orbit on a mission to acquire the most precise 3D map of the Earth's surface.

The German radar spacecraft will fly in formation with an identical platform called TerraSAR-X launched in 2007.

Together, the pair will measure the variation in height across the globe to an accuracy of better than two metres.

Their digital elevation model will have myriad uses, from helping military jets fly ultra low to showing relief workers where an earthquake's damage is worst.

"Our aim is to generate a model at a resolution and a quality that doesn't exist today," explained Dr Vark Helfritz, from satellite image-processing company Infoterra GmbH.

"This will be a truly seamless global product - not a patchwork of datasets that have been fitted together," he told BBC News.

TanDEM-X was carried into space atop a converted intercontinental ballistic missile from the Baikonur Cosmodrome in Kazakhstan.

The Dnepr rocket left its silo at 0214 GMT. A signal confirming TanDEM-X had separated successfully from the vehicle was picked up 31 minutes later at an Antarctic tracking station.

Image caption,
The aim is for a model with a two-metre vertical resolution on 12m by 12m grids

The new satellite was put into a polar orbit that is ever so slightly inclined to the one occupied by TerraSAR-X, some 514km above the planet.

The path of TanDEM-X across the sky means it will fly an extremely tight helix around its more established sibling.

Image caption,
The Dnepr emerges from its Baikonur silo to climb to orbit

"It is the first time that two satellites have been put in such close formation," said Brigadier General Thomas Reiter, the former astronaut and now executive board member at the German Aerospace Centre (DLR).

"Their orbits bring them together with a minimum distance of about 200m. This will be very challenging for mission controllers, as you can imagine."

The pair's radars work by constantly bouncing microwave pulses off the surface of the planet. By timing how long the signal takes to make the return trip, the instruments can determine differences in height.

The compact orbital dance will give the pair "stereo vision", by enabling them to operate an interferometric mode in which one spacecraft acts as a transmitter/receiver and the other as a second receiver.

Perhaps the best-known, near-global, space-borne data-set prior to the German venture came from the US Shuttle Radar Topography Mission (SRTM) of 2000. A decade on, TanDEM-X should provide a big leap forward in capability.

The top SRTM digital elevation model (DEM) product has a 30m by 30m spatial resolution, and a vertical resolution that varies from 16m to 10m.

With the TanDEM mission, the intention is to go down to a vertical resolution of two metres, with a spatial resolution of 12m by 12m. Airborne laser instruments (lidars) can do better than this but their DEMs are only regional.

To achieve the TanDEM level of detail on all 150 million sq km of the Earth's land surface will require three years of operation.

Radar observations already have extensive military, civil, and scientific applications.

Good examples recently include the Eyjafjallajoekull eruption in Iceland and the Gulf of Mexico oil spill.

In the former, TerraSAR-X's microwave vision was able to look through the ash plume to enable scientists to assess the developing status of the glacier volcano; and in the latter, the satellite has been able to follow the progression of the oil slick day or night from the way radar signals are reflected off the polluted water. This is possible because spreading oil has a tendency to flatten the sea surface and this effect is detectable from space.

Image caption,
TerraSAR-X looks through the Eyjafjallajoekull Volcano's ash plume to see three craters. Such views are only possible with radar satellites

The improved dataset coming from the TanDEM mission should deepen and extend the range of radar applications.

Mobile phone operators will use the elevation model to work out the best places to position their masts; the aviation sector can use the information to plan safer flight paths; city planners need the data to assess flood risks in urban areas; maritime officials can even use the information to track pirates and vessels that fish illegally.

Image caption,
The presence of oil in the water works to flatten the surface

The TerraSAR-X/TanDEM-X venture is operated on the basis of a public-private partnership. DLR owns the hardware; EADS Astrium builds it; and Infoterra GmbH has exclusive rights to commercialise the data.

Continuity is key to developing the radar applications market and already plans are in place to fly follow-on spacecraft.

The next step in development being pursued by the partnership is a high-resolution, wide-swath technology which would allow extremely detailed, large-scale images of the surface to be acquired in a single pass.

"We could decide to take a 100km swath with a resolution of one metre," Brig Gen Reiter said.

"That means two things, however. First, we can take a swath and get all the products we need straight away; but second, the disadvantage is that we get incredible amounts of data and the challenge will be to get this data downlinked. The current downlink stations are reaching their limits."

This coming bottleneck issue is one of the reasons why Germany has chosen to lead the European Data Relay System (EDRS), a European Space Agency project that would see Earth observation data bounced off geostationary satellites using laser technology.

Image caption,
TerraSAR has already brought advances in spatial resolution

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