The Earth in 3D from space

With the TanDEM-X satellite, the German Space Agency is to create a 3D map of the entire planet.

 

The TanDEM-X and TerraSAR-X satellites are now orbiting in formation a little more than 500 km above our planet. Credit: DLR

The Russian-Ukrainian Dnepr launch vehicle, once a ballistic missile now converted into a small satellite launcher, has made two successful blast-offs in just a few days! On 15 June, the former SS-18 tasked with nuclear destruction during the Cold War, peacefully placed the Picard and Prisma satellites (see this article) in orbit by blasting off from Yasny, in the south of Russia, right next to the Kazakhstan frontier. And it was from the famous Baikonur cosmodrome in Kazakhstan that another Dnepr soared up into the sky to launch the German TanDEM-X satellite on 21 June.

The Dnepr launch vehicle blasting off from the Baikonur Cosmodrome in Kazakhstan on 21 June. It successfully placed the TanDEM-X satellite in orbit. Credit: DLR

Two relief satellites
ven its name explains its mission since TanDEM-X stands for TerraSAR-X add-on for Digital Elevation Model, that is an addition to another spacecraft, the TerraSAR-X which has been in orbit since 2007, with the aim of gathering data in order to generate a relief map of the Earth. The TerraSAR-X and TanDEM-X satellites associate the DLR, German Space Agency, and the European manufacturer Astrium in an Earth observation programme, Infoterra. It is based on radar technology which was notably tested during missions aboard the American space shuttles (STS-59 and 68 in 1994, then STS-99 in 2000). The radar makes it possible to gather data independently of the weather conditions over the observed regions: the waves used go through clouds. TanDEM-X is to work in tandem (another reason for its name) with TerraSAR-X; both satellites flying in formation at an altitude of just over 500 km. On the ground, their measurements are to be combined to deduce the relief in the analysed regions. This procedure is similar to that of our two eyes with which we get a stereoscopic view of the world around us.

The orbit of TanDEM-X and TerraSAR-X takes the satellites over our planet’s poles (known as polar orbit). The Earth turns as they fly over so that they end up by surveying it completely, passing over the same regions every 11 days or so. The satellites are flying in formation at a distance varying between 200 and 500 m. Credit: DLR

Taking stock of situations and changes
In 3 years, TanDEM-X and TerraSAR-X will have generated the equivalent of 200,000 DVD of data! With a vertical accuracy better than 2 m for a 12 m grid, this relief map of our entire planet will be what is known as a digital elevation model, the actual uses of which are as numerous as they are varied. With such information, it is possible, for example, to hope to be able to improve detection of areas at risk during natural disasters (identification of land liable to flooding) and thus give the relevant bodies an urban growth management tool. The use of natural resources, the optimisation of the setting up of transport infrastructures as well as the determination of the best sites for communication transmitters (telephone and television networks, etc.) are other non-comprehensive examples.

The digital elevation model generated as a result of TanDEM-X and TerraSAR-X will be based on a 12 m grid. In relation to measurements at 1 km, 90 m and 30 m, the gain in accuracy will be significant. Credit: DLR

In addition to this extremely useful inventory, TanDEM-X and TerraSAR-X survey the whole of the Earth from their orbit in about 11 days. Therefore, they come back over a given area in the same periodicity and are then, as a result, in a position to detect changes in the short and long term. Movements of the surface of our planet, even slight, caused by natural events (earthquakes, landslides, etc.) or by mankind (construction, drilling, etc.), can be observed and assessed. With the radar, satellites are thus capable of measuring the few centimetres of subsidence, invisible to the passer-by on the ground, that result from tunnelling in town for a new underground line!

Published on 21 June 2010