TanDEM-X images under the magnifying glass – first data is perfect
While TanDEM-X spent mission days four and five catching up to its twin – TerraSAR-X – which is hurrying on, 13 000 kilometres ahead, it continued to acquire numerous test images. These will be analysed by the team and compared with previously processed TerraSAR-X images to tune the processing chain to suit the new instrument.
During these early days of the mission, the team will be paying close attention to the quality of the radar signals, checking things like the modulation and the properties of the transmitted pulses. The first images in the various TanDEM-X modes to come from the production chain and will be examined closely. The quality of the images in terms of location accuracy and coverage will be studied intensively.
As yet, the orbits of the two satellites are too different to be able to acquire images from the same viewing angles for direct comparison. Also, TanDEM-X has not yet overflown the reference targets required to perform its calibration. But already, overlaying of images from the two satellites – with TerraSAR-X in red, TanDEM-X in green and the difference shown in blue – shows how well the instrument and processor are working together at this early stage.
Among the newly acquired images is the area that TerraSAR-X recorded three years ago, on the fourth day of its mission. A 'blind' overlay of the TerraSAR-X image and the new data, in which we trust the given orbit and attitude data and the illumination correction of the processor without further adjustments, shows a perfectly matching pair – the images align without distortion. This makes us confident that, after a few weeks of precision measurement work, TanDEM-X will deliver the centimetre-accurate position data for which TerraSAR-X products are famous.
The intensity of the images differs in a number of places, but this is due to changes that have taken place in the three years between the image acquisitions. The most prominent of these are the changes in the fields and the absence of the dense storm cloud (top left, in red), which the radar beams were unable to penetrate.
The second image shown here seems rather nondescript compared to the colourful image above, but represents for us something of a highlight – the comparison of an image of Neustrelitz acquired by TerraSAR-X only a few days ago with its very new TanDEM-X counterpart. In the short time between image acquisitions, very little has changed on the ground and so the scene is rather uniform in colour. This confirms that the two instruments will see the Earth in almost the same way and that the processor adaptation is already excellent. This is a perfect start to the difficult task of the next few weeks – adjusting TanDEM-X to be a precision instrument.
Top image: Heiko Breit, a member of the SAR processor team, analyses the first data. Credit: DLR.
Centre image: The first TerraSAR-X image, overlaid with an image acquired by TanDEM-X. Credit: DLR.
Bottom image: TerraSAR-X and TanDEM-X image of Neustrelitz. Credit: DLR.