Radar satellite looks through the clouds

The satellite TerraSAR-X has been observing Earth for more than two years now. Terra, from the Latin, stands for Earth, SAR for the radar technology – Synthetic Aperture Radar – and X for the frequency range in the X-band. Using the radar technology, the satellite can also collect data from the Earth’s surface in cloudy or dark conditions. The images are in great demand among scientists and commercial users.

“TerraSAR-X is Germany’s first radar satellite and simultaneously serves as the first national remote sensing satellite implemented in public-private partnership between the German Aerospace Center (DLR) and the company Astrium GmbH from Friedrichshafen,” explains DLR Chairman of the Board, Professor Dr. Johann-Dietrich Wörner. “Germany is now a pioneer in collaborative space missions involving the public sector and industry,” he emphasises. The DLR is responsible for the mission’s planning and realisation, for operating the satellite and radar instrument as well as for the scientific use of the TerraSAR-X data.

Since TerraSAR-X was launched, the DLR Mission Control Center has produced more than 35,000 radar images of the Earth’s surface and processed these into some 50,000 highquality products. Due to the high spatial resolution and a geometric accuracy which is well below one metre, it is also possible to fully automatically overlay two time-delayed images of a scene down to the last pixel. And this produces exciting pictures in which changes become visible.

The radar instrument’s radiometric precision and excellent stability continues to inspire the users, whether their applications are in the field of agriculture or forestry, or are needed for land use or vegetation assessments, or involve the observation of urban areas or are used in the field of cartography. Ice research or maritime applications also profit from these data. In fact, the information gained so far has already been used on several occasions to provide crisis management support for international authorities following natural disasters.

Flooded areas have been mapped or post earthquake damage assessed. When disastrous floods led to a million people being left homeless after weeks of rainfall at the beginning of November 2007 in the Mexican federal states of Tabasco and Chiaps, the DLR Center for Satellite Based Crisis Information (ZKI) supported the Mexican civil protection agency with satellite image maps of the flooded areas. Users can also draw on the TerraSAR-X data for traffic observation tasks. The DLR has already taken images of selected motorway sections in Germany, Austria, Switzerland, and of highways in California.

In contrast to stationary measurement methods, satellites gain traffic information without needing any ground-based installations, and what’s more, regardless of the weather conditions and even across borders. Applications are not only limited to recognising traffic jams. “We can also measure the mean speed along motorways and so calculate the current travel time between traffic intersections,” explains Hartmut Runge from the DLR’s Remote Sensing Technology Institute (IMF). These data help traffic information service providers to verify the data collected with conventional methods and so they can close any gaps in their information network.

The example of a spectacular image taken of the Golden Gate Bridge at the entry to the Bay of San Francisco clearly highlights the exceptionally high localisation precision of the TerraSAR-X images. It even shows the bridge mirrored on the water surface as a result of the radar signal reflecting between bridge and water surface. Even the two primary cables with a diameter of 0.92 metres and the suspender at a distance of 15 metres can be recognised. By overlaying three different colour coded images taken at different times, the changes produced between these images appear in colour. All the unchanged areas are grey.

Spring 2010 will see a second almost identical German radar satellite, TanDEM-X, launched into space from the Russian launch pad at Baikonur. The German double will then fly in close formation with distances of between several kilometres and 200 metres. This will involve the new satellite, TanDEMX, practically dancing around TerraSAR-X. Together the two satellites will deliver three-dimensional data products that lead to a global, digital altitude model of all the land masses of the Earth’s surface with a previously unattained level of precision.