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
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
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,