TanDEM-X's first zoom and wide-angle images
Now that in addition to control and mission planning, the Kiruna and Neustrelitz ground stations have also taken on important elements of the normal operation of the ground segments as part of commissioning, the various radar modes of the TanDEM-X are undergoing instrument and processing tests. This includes the high definition ‘Spotlight’ zoom mode and the ‘ScanSAR’ wide-angle mode.
Of almost 200 images sent to Earth by TanDEM-X so far, many have been taken in these two modes. The processing team carefully examines the products of its multi-mode SAR processor, which has to deal with a huge range of combinations of control modes, lens angles and polarisation modes. Experimental control modes are also being tested by the instrument team and processed by us, in order to test the performance of TanDEM-X and the ground-based systems.
TanDEM-X Spotlight image with areas coloured to indicate statistical characteristics
Alongside its standard ‘Stripmap’ mode in which the TanDEM-X takes 30-km long strip-like images at a resolution of approximately 3 metres, there are also a variety of Spotlight and ScanSAR imaging modes. Spotlight mode zooms in to a 5- to 10-km patch of Earth’s surface in which the antenna switches hundreds of times in a few seconds in order to keep the radar signal targeted precisely. The resulting image only covers a small area of the surface, but at a resolution of up to 1 meter.
The ScanSAR mode also uses a complicated electronic antenna switching sequence – but this time to illuminate a wider strip of the Earth’s surface. The ScanSAR strip is more than 100 km wide, but resolution is reduced to 18 meters.
TanDEM-X ScanSAR image of the southern tip of Greenland (North is to the left, East to the top)
The measured resolution of the images is a decisive indicator of whether the instrument and processor are handling their complex tasks correctly. One normally measures these parameters using reference targets, called corner reflectors, which are placed in the scene and provide a calibrated signal in the SAR image. Since the final orbit in the vicinity of TerraSAR-X has not yet been achieved, TanDEM-X has not yet imaged these targets. So for the time being we have to make do with natural features as references in order to measure the images. The analyses so far indicate that TanDEM-X has achieved its expected performance here as elsewhere.
TanDEM-X dual polarisation image of the Norwegian fjords.
While the colours of the images shown so far are determined by statistical analysis of the radar reflection or overlaying of two separate images, very different type of information can be represented in colour in the SAR images: in dual polarisation mode, the instrument switches in just milliseconds between horizontal and vertical illumination. The returned radiation is in variously rotated planes of polarisation, depending on the properties of the imaged surfaces. This enables determination of the orientation and volume of plants on the ground, for instance. For ease of comprehension, the horizontal signal has been coloured red, and the vertical green. Here too TanDEM-X gives us wonderful information, and the fast switching between polarisation modes does not miss any steps.
Over the next few days, there will be fewer images and more instrument tests. But we in the processing team now have lots of test data to keep us busy for the time being.