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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