A game of mirrors - TerraSAR-X
2153 mirrors twist and turn at the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR) Experimental Solar Thermal Power Plant in Jülich, directing sunlight onto a 22-square-metre receiver.
TerraSAR-X, the German radar satellite operated by DLR, can also detect the mirrors as they follow the Sun - from more than 500 kilometres above Earth. The reflections of the radar signals make the tower and mirror array appear as bright spots of light.
Row upon row of mirrors stand in the 10-hectare field, automatically aligning themselves with the position of the Sun. The surface area of the mirrors totals 18,000 square metres, all used for converting solar radiation into power in the solar tower. But the smooth mirror surfaces also reflect most of the signals from TerraSAR-X back to the satellite. Metal components on the edges and the top of the 60-metre tower reflect the radar signals particularly well. The result is that, while many of the surfaces remain invisible to TerraSAR-X, the reflections are still sufficient for showing the contours of the tower and the majority of the mirrors in the radar image.
The houses in the nearby town of Jülich, with their reflective rooftop air conditioning units and ventilators, can also be seen as illuminated points in the radar image. "The black areas seen in the image are the industrial park. The walls face away from the satellite, meaning that none of the radar signals make it back to the receiver – so of course the satellite is not receiving any data," explains Christian Minet, a scientist at the DLR Remote Sensing Technology Institute. "But we can combine the cast shadows with data on the orbit and orientation of the satellite to calculate the height of the buildings." The appearance of the surrounding fields depends on their surface roughness; is TerraSAR-X looking at the fields before or after the harvest? Which crop is being grown in the field? Colouring the black and white radar images in different shades of red brings out the differences.
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