Already, with its first image acquisitions, TanDEM-X (TerraSAR-X add-on for Digital Elevation Measurement) has surpassed its twin satellite, TerraSAR-X. On 24 June 2010, only 3 days and 14 hours into the mission, the satellite sent its first image data back to Earth. The transmission was received by the German Aerospace Center ground station at Neustrelitz and processed to produce images.

TanDEM-X looked down from an altitude of more than 500 kilometres above northern Madagascar, Ukraine and Moscow.

Even the ups and downs of the waves in the Indian Ocean - coloured pale yellow on the image - are charted by TanDEM-X as it flies over at a speed of seven kilometres per second. The change in the waves at the entrance to the Diego Suarez Bay is clearly visible.

The water in the bay itself, on the shore of which the provincial capital, Antsiranana, can be recognised, is very flat - in contrast to the undulating ocean - and reflects the radar signals from TanDEM-X more uniformly. The area of valleys to the south drains the volcanic cone of Ambre-Bobaomby into the Indian Ocean.

The data were received yesterday afternoon at around 16:55 hrs. "We have broken the world record that we set with TerraSAR-X," says project leader Manfred Zink from DLR's Microwaves and Radar Institute. At that time, the first reception occurred after four and a half days.

For TanDEM-X, the team was ready to download data just three days and fourteen hours after the launch, which took place at 04:14 on 21 June 2010.Eight gigabytes of data reached the Neustrelitz ground station of the German Remote Sensing Data Center (Deutsche Fernerkundungsdatenzentrum). Before this, the flight dynamics team had exactly predicted the route of TanDEM-X and had derived the commands for controlling the attitude of the satellite, which were used by the instrument operations team.

Mission milestones
"That's a neat milestone," says a satisfied Michael Bartusch, TanDEM-X Project Manager at the DLR Space Agency in Bonn. He accompanied the satellite during transportation to Baikonur; for the launch, he sat in DLR's German Space Operations Center (GSOC) in Oberpfaffenhofen. "With the image, we have the proof that the radar satellite works without problems." "But of course, now we have had experience with TerraSAR-X.

With the end of the launch and early orbit phase this weekend, the TanDEM-X team begins the first part of the commissioning phase, during which the satellite is 'put through its paces'.

"It takes about three months to prepare it for operational use," says Bartusch. By the end of July, the two satellites will be brought within 20 kilometres of one another. In October, is will be another, unique milestone: the satellites will fly in formation with a distance of only some 200 metres separating them as they orbit the Earth. This marks the second part of the commissioning phase, during which the approach and control of both satellites is in focus.

Teamwork by the two satellites
As soon as the satellites begin to record data as a combined 'pair of eyes', the 'complex interactions' begin, says project leader Michael Bartusch. The design of their orbits prevents the satellites from colliding but they must also be prevented from illuminating one another with their radar signals.

"If TerraSAR-X and TanDEM-X illuminate one another during formation flight, there is a possibility of damaging the instruments at this short range." The first official three-dimensional image acquisition by the twin satellites will occur in January 2011. "Then we will begin with measurements of the entire Earth and the generation of the elevation model."