bedrock 
igeV 
lenetrated 
; from bed- 
.ogical data 
'ts of Aer- 
>f Presinian 
■anite, res- 
, the rock 
l is gneiss, 
less than 
> much as 
,e causes 
> the rock. 
in from Ba- 
100th appea- 
i the MSS 
:k constitutes 
; because of 
.c constant, 
the long 
a lot of 
and blom 
if the space 
r sis to 
>AR, tney 
■ied Deneath 
,h alluviate, 
im JPL buried 
ia of Mevada 
1 SIR-B flight 
isor has a 
lenetration 
mditions: 
of the sand; 
' covering 
.ekuwula and 
:evere arid 
.fall is be- 
,d even lower 
., the aridity 
2 in Badain 
iobao and the 
> times of 
that it is 
■ areas. 
ie above dis- 
,as penetrated 
and sand in 
activity to 
WO KINDS OF 
characteris- 
d SIR-B in- 
ism of dunes 
mmon ground 
tation. In 
ectric con- 
■y and meto- 
alue is 6.67 
There is no 
e roughness 
se of ripple- 
is means that 
trie constant 
nes and other 
t on radar 
gnificant' 
orosity of 
incident 
dered an im- 
ent returns, 
the geome- 
sition rela- 
It is well known that the typical slipface 
angle of crescentic dune is approximately to 
a constant (32’) while the one of gentle face 
is far less than this value. Most of gentle 
face angles are less than 10° . It has been 
proved that the strongest return will be met 
when radar wave illuminates on a uniform 
medium perpendicularly. The side-looking 
radar can meet this condition when it works 
on zero incident angle (£), or when the sum 
of the operating depression angle (£) and 
the slope angle (o() is 90 degree . It. is 
also known that the depression angle of ra 
dar is the complementary angle of its view 
ing angle, so when SIR-A. depression angle is 
43° , then, ot plus /3 i s equal to 73°. 
Two possible explanations to this 13° de 
gree difference compared with therotical va 
lue of 90 degree are given. Firstly, the 
overlapped ripple-mark and secondary dunes 
on slipface of the dunes with slope angle of 
32 degree make it more of actual scatterring 
surface angle than normalized 32 degree, and 
therefore is more approximate 90 degree ; 
secondly, the 90 degree is the value at which 
the strongest scatterring will occur. In fact, 
the echo energy changes in a range of inci 
dent angles with 90 degree, as its center. 
It is easy to explain the mechanism from 
three deserts with the above theory. The 
reason of why there is the strongest return 
slipface vertical to radar's illuminatung 
direction is that the sum of and (S is the 
greatest in the situation; the reason for 
the recieving of weaker energy by radar when 
it illuminates slipface in slanting angle is 
that the sea tterring surface is reduced by a 
lower value; when radar wave illuminates 
at the gentle surface, the latter becomes 
a mirror reflector because of a too low 
value and therefore no echo takes' place. 
The penetration theory of radar is a very 
complicated problem and many researchers 
have been engaged in its study for quite a 
long time. We take the radar scatterring 
modle by Elachi et al. to explain the pene 
tration of radar wave to the surface of Ala- 
shan Plateau. 
The arid and uniformed sand layer like that 
of Tiekuwula and Aertengaobao areas is a kind 
of low loss layer which leads to the inci 
dence of microwave pulse towards lower layer 
of sand and the refrection between the sur 
face of air and sand layer. As the result of 
refraction, the incident angle becomes lower, 
which is the incident angle when wave reaches 
the plane between sand and bedrock. Just as 
mentioned above that the lower the incident 
angle, the stronger the return is, the geo 
logical body covered by dry sand can, on the 
contrary, produce an enhanced imaging effect 
on radar images. The study indicates that 
the penetration of radar wave is deeper pro 
vided that there are higher incident angle 
and HH polarization, which is the condition 
SIR-A can meet. 
7 SUMMARY AND CONCLUSIONS 
SIR-A and SIR-B images of the Alashan Plateau 
have been analyzed and compared to Landsat 
images and other data. Based on observations 
and interpretion of the imagery, the fol 
lowing conclutions can be made. 
The bright return from non-vegetation sand 
dunes on two radar images is caused by dunes 
slipfaces oriented to the radar beam. The 
slipfaces oriented perpendicular to the radar 
beam have a maximum echo. The return strength 
decreases with a reduction of the angle be- 
tween the radar beam and the long axes of 
dunes' slipface. Sand dune in which the slip' 
faces are away from radar have no return. 
Secondary sand dunes that have the slipfaces 
oriented towards the radar beam also produce 
a high radar return that is clearly shown on 
radar images. Radar illumination direction 
is a critical factor to the delineation of 
non-vegetated sand dunes. Radar return from 
vegetation around interdunal lakes is also 
easily detectable and can be used to iden 
tify these arid land featurea. 
SIR-A and SIR-B radar penetration capabi 
lity is demonstrated with the detection of 
Precambrian metamorphic rocks beneath a thin 
cover of sand on the alashan Plateau. The 
same features are not visible on Landsat im- 
gery. Therefore it is deemed possible to 
find other subsurface geological features 
using SIRt-A and SIR-B radar images in the 
arid areas of China. 
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Walker,. A.S. 1982. Deserts in China. American 
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Covault, C. 1984- Challenger.crew obtains 
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Elachi, L.E., L.E.Roth and G.G.Schber 1964. 
Spaceborne radar subsurface imaging in hy- 
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