Stress tests in space and rivers of sand
As ground teams prepared for the formation flight of TanDEM-X and TerraSAR-X, TanDEM-X was put through its paces in the last week before approach. The instrument team ran a set of hot/cold tests to check the instrument’s performance limits by first allowing the radar system to cool down and then running it at full load. This was followed by tests in which a large number of randomly targeted radar images of the Earth’s surface were used to test the reliability of the reception and processing systems. The images acquired during these tests include a number of very puzzling pictures.
The TanDEM-X instrument came through with flying colours and provided the instrument team with valuable data while operating under extreme conditions, which will be used for resource scheduling in future imaging missions. Although the radar system was active during the tests, it was not delivering images, but instrument readings. The data was used to optimise the instrument and ground control settings.
For us in the processing team, the resumption of the flood of images after a couple of days of peace and quiet was naturally an exciting moment. The statistical images combine every possible imaging mode and polarisation setting with a variety of angles of view. They serve first and foremost as service trials for the instrument and the ground control system, rather than for acquiring images of interesting areas of the Earth’s surface. The instrument settings have been confirmed as optimal for the start of the formation flight phase.
Even though the images are randomly distributed, they still yielded fascinating views of the Earth’s various landscapes. Some of them are really astounding, especially because during normal, very precisely targeted TerraSAR-X imaging missions, one would seldom expect to use exotic operational combinations such as dual polarisation spotlight imaging of a desert. One image of this type in particular, of a nondescript desert area in the Arabian Peninsula about 180 kilometres south of Riyadh, highlights how differently optical satellites and TanDEM-X radar see the world. The images are not only beautiful, but they give an immediately comprehensible view of the area’s surface formations.
TanDEM-X dual polarisation image of the Saudi Arabian desert – montage with a Google Earth image. Credit: DLR/USGS/Digitalglobe.
When an optical sensor is used to image the area, it looks all the same sandy colour; but the radar images show the underlying structures. This is not due to some kind of X-ray vision – although the radar waves do penetrate the dry sandy ground by about one centimetre – but rather to the properties of the sand itself. Smooth sandy areas reflect the radar signal away from the transmitter like a mirror and not much is returned to the instrument. This means that where sand and gravel have accumulated in dry river beds (wadis) between the rocky ground, the radar image appears black; whereas the stony structures reflect the radiation back to the instrument and appear light yellow. The different polarisation properties of the sandy soil and vegetation highlight the two circular irrigated areas, so that they seem to float over the ‘rivers of sand’.
Images like this have already been used by TerraSAR-X to support archaeological investigations. Here too, the radar can reveal hidden structural features in the sand. The precise vertical measurements provided by the TanDEM-X mission will add a further dimension to such images.
But there is not much time to admire the images, because all systems have to be readied for the next calibration phase and the first imaging missions with the two satellites flying in formation. That is when the interferometry processing chain will be used for the first time and the first altitude data from the combination of the two satellites’ signals will be obtained.