January 17, 2017

New survey of planet earth

					New survey of planet earth

Over the last years, two satellites flying in close formation have re-surveyed our planet. The result is a 3D map, accurate to the meter, which scientists from various fields can’t wait to get their hands on.

It was a mission of superlatives. The TerraSAR­X and TanDEM­X satellites created almost half a million extremely precise elevation models of the earth’s surface. During their five-year formation flight 500 kilometers above the planet, the satellites gathered more than 500 terabytes of data and sent it back to earth. Experts at the German Aerospace Center (DLR) merged the raw date into a 3D map with a precision never before achieved. The elevation values are exact to one meter—the requirement was up to two meters deviation. The map, with a data volume of 350,000 DVDs (2.6 petabytes), is now available. Scientists from all over the world just can’t wait to get their hands on the data. A follow-up mission will continuously update this data in order to document the topographical changes on the earth’s surface and the consequences of climate change.

The key to the accuracy of the map was the extremely accurately coordinated formation flight of the satellites, which had to constantly change their formation and, in some cases, be within just 120 meters of each other. This was the only way to guarantee that simultaneous data recording with two radar satellites could provide such precise results. The measurements themselves were made with a new radar process—synthetic aperture radar (SAR) interferometry: From their respective positions, the two satellites transmitted microwaves to the same area and received the reflected waves by radar. Based on the difference in runtimes and the shifts of the waves in relation to each other, the DLR team was able to calculate the elevation information for their 3D mapping.

Photonics delivers the necessary precision for radar measurements

During the formation flight of the two satellites at a height of 500 kilometers the rule of thumb was: one millimeter position deviation equaled one meter measurement deviation. Thanks to accurate calibration and extremely precise laser measurements of the distance between the satellites deviations could be avoided. To ensure this, special laser retroreflectors developed by the German Research Center for Geosciences (GFZ) in Potsdam were used. They reflect ultrashort laser pulses, which were sent to the satellites from satellite laser ranging (SLR) ground stations distributed around the globe.

It’s not only the satellites that needed precise calibration. The data also had to be aligned to a reliable reference, which was also determined by laser. Or to be precise: with laser altimetry—in other words, pointwise altitude measurements by laser in the course of NASA’s ICESat earth observation. Ultrashort laser pulses are sent from the earth’s surface and are reflected, the distance is then calculated from the time it takes for the photons to return.

The precision of photonics and the robustness of light and weather-independent radar technology thus worked hand in hand. In the next step, as a result of the change from X band (3.1 cm wavelength) to L band (24 cm) it will also be possible to obtain views up to 60 meters deep into the ice layers of glaciers and polar ice caps and to penetrate and survey thick vegetation.

Data transfer with lasers

Another highlight of the TerraSAR­X mission is in-orbit verification, a technology of the future: Laser communication in space. TerraSAR-X and the US satellite NFIRE exchange data several times each day via laser. This works at rates of up to 5.6 gigabytes per second over distances up to 5,000 kilometers—in both directions. For this to function at speeds of around 25,000 km/h, the laser beam must be aligned exactly to the laser communication terminal of the receiving satellite and be fixed there for several minutes. According to Tesat Spacecom, the manufacturer of the laser terminals, this is like “aiming at a single window of an airplane flying at a high altitude and tracking its course”. Since this is now quite possible, the company is convinced that laser communication is a new basis for high performance monitoring missions. In the future, it will play a key role for realtime data connections from satellites to the earth.