188 
directions cause 'chinook' effects during the winter 
season. In the Tarim Basin, sand storms frequently 
occur during the spring and summer months (USSR 
Academy of Science 1969, Petrov 1976). 
The floral composition is controlled exclusively by 
xerophyte and halophyte species. Although the 
vegetation cover is very sparse in general, 
concentration and diversification occurs in the 
oases, e.g. various species of poplars, willows, 
tamarisk, saksaul and thickets of reed and camel 
thorn (USSR Academy of Science 1969). 
Geologically, the formation of the Tarim Basin 
dates back to the late Paleocene. Upper Cretaceous, 
Eocene and early Pliocene sedimentary rocks are 
largely covered by Quaternary aeolian sand dune 
fields and are only exposed as gently folded 
sequences along the margins of the basin. The 
boundary of the foothill zone towards the Pamirs is 
composed of a series of alluvial fans. The eastern 
Pamirs consist of a system of NNW-SSE as well as E-W 
striking chains and intermontane depressions. 
Ordovician, Silurian, Devonian, and Paleozoic rocks 
formations were subjected to faulting and uplift 
during Cenozoic times (Geological Map of China 
1:2,500,000, 1976). 
A significant portion of the eastern Pamirs, i.e 
Mount Kongur (7,719 m), Mount Muztaghata (7,546 m), 
and the Kingata Range (6,700 m) are exposed to high 
altitude westerly winds and precipitation. 
Approximately 60 glacier fields cover an area of 596 
km 2 (Wiens 1966) with firnlines being confined to 
elevations between 5,600 and 5,700 metres (Freeberne 
1965, Field 1975, Shi and Yang 1985). The moisture 
accumulated in these snow fields and glaciers is 
partially released during the melting season. 
Since neither the annual nor the seasonal 
precipitation meet the water requirements for crop 
cultivation, the Kashgar oases have been and still 
are entirely dependent on the discharge of meltwater 
for irrigation purposes. The area is part of the 
cotton-sericulture and two year tripple cropping 
wheat and corn region of western Xinjiang, supporting 
a population of more than 2.3 million people within 
the administative district of Kashgar. 
KASHGAR 1306m, 11.6'C , 63r 
T X \ 
/ \ 
/ , / \ 
-40 
■20 
n MM 
1 MELT SEASON—} 
JFMRMjjflSOND 
WR in mm 
1000-1200 
450 - 525 
CORN » — 
375 - 450 
COTTON - 
600 - 750 
Figure 2. Climatic diagram for Kashgar, Xinjiang. 
(T = Temperature, in °C; P = precipitation, in mm) 
and water requirements (WR, in mm) for cultivation. 
3 THE SIR-A SYSTEM 
The system configuration and first results of the 
SIR-A experiment were presented by Cimino and Elachi 
(1982) and Ford et al. (1983). The space shuttle 
orbited the earth at an altitude of 259 km, at an 
inclination of 39°. During the mission the imaging 
radar directed a microwave beam at a wavelength of 
23.5 cm (L-band) toward the earth surface at a 
depression angle of 40°. In flat terrain, this 
results in an incidence angle of 50° (Figure 3) and 
varies accordingly with different slope angles and 
slope orientation. The backscattered energy from the 
earth surface is received by means of an antenna. 
The capability of radar to penetrate cloud cover 
avoids dépendance on diurnal time and weather 
conditions. Following the mission the signal film 
which contained a record of the radar returns in 
holographic form was optically correlated to produce 
the actual image. The SIR-A swath width covers an 
area of 50 km on the ground. The spatial resolution 
is 38 metres in azimuth as well as in range 
direction. 
Figure 3. The SIR-A system and viewing geometry. 
4 RADAR INTERACTION WITH TERRAIN 
The energy of the radar return signal from the 
terrain to the antenna is a function of the radar 
system configuration, its viewing geometry and the 
terrain properties such as dielectric constant, 
topography, surface roughness, and vegetation cover. 
The resulting backscatter coefficient is generally 
expressed on the image either in terms of magnitude, 
i.e. image tone, or spacial variability, i.e. image 
texture (Ulaby 1982, Evans et al. 1986). 
The complex dielectric constant is a measure of the 
electrical properties of natural material upon 
incidence of electromagnetic energy. Variations in 
the return of the radar signal may be determined by 
the actual moisture content of the rock, soil or 
vegetation. Under very arid conditions signal 
penetration of sand and soils may be considerable 
until refraction at a major dielectric discontinuity 
surface such as sand covered bedrock occurs. The 
influence of the dielectric constant, however, is 
often masked by the effects of topography and surface 
roughness. 
Expression of surface morphology is defined by the 
orientation of terrain and the illumination geometry 
of the radar. Slopes facing the radar look direction 
produce strong returns and bright image signatures. 
Slopes oriented away from the radar at an angle 
exceeding the depression angle result in radar 
shadow, i.e. no energy return. Apparent displacement 
or layover of mountain tops toward the radar antenna 
occurs where the curved front of the radar signal 
encounters the summit of a topographic feature first 
before reaching its base. 
Roughness refers to relative surface irregularities 
measured in dimensions of centimetres, and is 
dependent on the radar wavelength and incidence angle 
(Rayleigh Criterion). At L-band (SIR-A) a flat 
surface with less than 1.5 cm of surface roughness 
appears smooth and without appreciable scattering, 
since the signal is reflected at an angle equal and 
opposite to the angle of incidence. The resulting 
radar signature is dark. Surfaces with an 
intermediate roughness of 1.5 cm to 8.3 cm disperse 
the incident energy and produce a scattering response 
and intermediate grey signatures on the image. A 
surface is considered rough when local relief is 
greater than 8.3 cm. In this case, a relatively high 
amount of 
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