Kufra

The image shows an area in the Libyan Desert with Kufra, the oasis, famous already in ancient times positioned about 24-10 North and 23-30 East.
Thanks to deep groundwater, irrigation allows the cultivation of millet, winter-wheat and alfa-alfa. The irrigation scheme is arranged in circles of about 1120m diameter.
The image is composed of data from three acquisitions:
- 19 Nov. 92 displayed in red
- 24 Dec. 92 displayed in green
- 6 Aug. 92 displayed in blue.
The colours of the fields reveal some type of activities between the acquisition dates.
Since the cultivated crops mentioned do not cause a major backscatter for the radar, the colors in the image represent more the change in the amount of soil moisture between the acquisition dates. In general moisture on ground or on the plants increases the backscatter. Thus red fields were irrigated on the 19 Nov. 92 (or shortly before) while yellow would indicate moist field both, on the 19 Nov. and on the 24 Dec., but dry on 6th Aug 92.
Some fields have bright lines in range directions .
This has to be explained. It is assumed that also ploughing is circular. Also, it is well known that fields with furrows perpendicular to the range direction (parallel to the flight direction) give an unusual bright response. In our image the bright pixels appear only in just that narrow angular part where furrows are close to rectangular to the radar illumination. This also means that those fields have been prepared recently, probably for millet.
The circular furrow-pattern allows to assess this angular influence, which in the practical use of SAR data is very disturbing. From the backscatter values in the data it results that the influence is present up to 26 degrees on either side of the range direction. Circles or concentric rings without bright lines have not been ploughed for some time and are used most probable for alfa-alfa fodder-crop.
Dark circles indicate either newly prepared, fallow or abandoned fields. The irregular bright lines around the irrigation pattern and the one crossing the black appearing, sandy (so-called fech-fech) desert (Erg) indicate car tracks only in use in winter time.
The bright areas are rocky plateaus (Reg) which remain unchanged. The slightly reddish tone in the upper part of the image indicate some soil humidity at the November acquisition date.

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