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Sahara dunes

The image shows a remote area in Libyan Desert, but on Egyptian territory, over 300km south east of the Oasis of Siwa and more than 400km west of the Nile river.
In the image very dark appearing long dunes and sand filled depressions can be observed. They are alternating with higher plateaux such as the one towards the right, called El-Quss Abu Said, 1990m a.s.l.

The subscene is taken from the mid-bottom part and includes an area of 13x13km with a 25m pixel spacing. Here the bright lines run parallel to the dunes become better visible. This phenomena needs to be explained. One can assume that it is where the dune slopes are around 23 degrees steep and therefore total reflection towards the sensor occur.
However, microwaves penetrate into dry sand and with increasing depth will be more and more attenuated. For little depth backscatter can reveal the structure of the underground. In a deeper sand-layer no microwave is returned and this will result into a totally black area. But since sand surfaces are also rather smooth some energy will also be reflected, but in most cases away from the sensor.
As the scene is illuminated from the west (ascending orbit), total reflection of 23 degrees slopes will occur somewhere on the western side of the dunes. The very peculiar straight line suggests that this happens with great regularity due to the perfect shape of the dunes, which is maintained even over more than hundred kilometer.
Technical information:
Acquisition date and orbit reference :
4 June 1994 20h34 GMT - Orbit 15093/531 of. centre 26-55N 27-13E
ERS Data Utilization Section, ESA/ESRIN

Keywords: ESA European Space Agency - Agence spatiale europeenne, observation de la terre, earth observation, satellite remote sensing, teledetection, geophysique, altimetrie, radar, chimique atmospherique, geophysics, altimetry, radar, atmospheric chemistry