d covering, 
euiate-mottled 
k usually nas 
tion direction 
tne radar 
osed of tde 
c aunes witn 
ig. 3). No 
rea. The 
e with the 
IR-A radar's 
and illumi- 
angles be- 
crientation 
b, and c are 
ysis of Yama- 
B # Type of 
D* aunes 
° crescentic 
some star 
crescentic 
’ **crescentic 
some star 
outcrops 
sand 
tion beam and 
lluminated on 
ana there- 
pe of sand 
e angles be- 
ion and slip- 
a 30° angle 
ing radar 
of the slip- 
ong return, 
a b) tne radar 
of the slip- 
n results. 
Section c, the 
s slopes 
turn is ex- 
idai-snaped 
his study 
f radar re- 
e can conclude 
t a maximum 
ce orientation 
ating radar 
t the two dune 
f the Yapulai 
Mountains 
t-LOn and for 
cated to the 
referred to 
e otner side 
ver, the two 
net deserts 
he nortneast 
esert and the 
part of Teng- 
across the 
s the north- 
tains and de- 
he N30°E di 
rection parallel with the mountain trend. As 
expected the dunes formed by these re-directed 
winds are oriented to the N30 e E are different 
from the main SE oriented dunes of the Badain 
Jaran Desert. These are NW dunes mentioned 
above. The SE dunes referred to above are 
also crescentic dunes. The prevailing north 
west winds control the direction of aeolian 
transport and therefore the orientation of 
the slipfaces is S50°E (Fig. 4b). 
SIR-A radar image (Fig. 4&) shows that NW 
dunes have dark response and SE dunes have 
bright-point return. Respectively, these are 
similar to Section c and a of the Yamalik 
Dunes. Analysis of Landsat images indicates 
that the characteristics of Yapulai Shan and 
Yamalik Dunes are very similar. Both dune 
fields have crescentic dunes and there is no 
vegetation cover in the active dune area. 
Therefore the radar imaging mechanism is the 
same. The NW dune slipfaces are away from 
radar beam and exhibit no return. The SE 
dunes in which the angle between radar beam 
and long axes direction is about 60® produce 
a bright-point signature. 
3.3 Badain Jaran Desert 
Badain Jaran Desert, located in western Ala- 
shan Plateau (See Fig. 1), has an area of 
about 44>000km . Most of the sand dunes or 
"sand mounds" in this desert are very high, 
averaging 200-300 m in height with an occa 
sional dune more than 500 m high. These dunes 
are reported to be the highst in the world. 
The crest-to-crest spacing of the dunes is 
approximately 3 km. 
Effective winds in this desert, as mentioned 
in the last chapter, are mainly from the 
northwest. Fine grained sand form the com 
pound crescentic dunes--the main type of dunes 
in this area. Smaller secondary crescentic 
shaped dunes on the stoss slopes are common. 
Vegetation is very sparse. Lakes developed 
in interdune areas are runned with vegetation, 
mainly reeds and marsh grass. Linear align 
ment of these interdunal lakes, as noted on 
Landsat and SIR-A, suggests structural con 
trol. 
SIR-A radar image of east-central Badain 
Jaran Desert shows a large area of highly 
contrasting return (Fig. 5&)• Three different 
kinds of radar return patterns have been 
identified. They are as follows: 
1. Short curvilinear return pattern. The 
long axes of the return patterns are parallel 
to the radar flight direction (perpendicular 
to radar illumination direction). This is 
the most promonent radar return pattern in 
the study area; 
2. Small bright point patterns. They are 
similar to the pattern in Sections a and b 
of the Yamalik Dunes and the SE dunes of Ya 
pulai Shan Dunes. This small bright point 
pattern is generally found distributed in the 
area between two dune crests; and 
3. Circular bright return patterns. 
These large dunes of the Badain Jaran De 
sert are clearly shown on Landsat MSS ima 
gery (Fig. 5b). The Landsat indicated that 
the long axes trend of the dunes is NNE and 
the slipface orientation is SSE. This sug 
gests that information about the dunes inter 
preted from SIR-A imagery is incomplete. Com 
parison of the SIR-A image with Landsat of 
the same scale indicates that the tnree dif 
ferent-kinds of return patterns have different 
origins. The short curvilinear return pattern 
results from radar reflection from the sand 
dunes' slipfaces oriented toward the radar 
illumination source e.g., soutn facing slip- 
faces have bright return in tnis area. The 
sand slipfaces not facing toward tne radar 
source have no return. The small bright point 
pattern appears to be radar return from se- 
conaary dunes slipfaces oriented towards the 
radar beam. The size of these secondary dunes 
is comparable to tne secondary dunes in the 
Yamalik dune field and therefore have a si 
milar radar return pattern. The vegetation 
around the intertunal lakes, reeds and marsh 
grass, have a nigh complex dielectric con 
stant and surface roughness, resulting in a 
very bright return from around the rim of 
lake. 
4. SIR-B RESPONSE FROM SaND DUNES 
SIR-B radar which works more effectively is 
developed basea on the SIR-A. Table 2 snows 
the different cnaracteristics between, the 
two radar systems. It can be seen from the 
table that tne frequency and polarization 
of the two systems are same but the looking 
angle and data record method are different. 
Table 2 Comparison between SIR-A and SIR-B. 
Characteristics 
SIR-A 
SIR-B 
Altitude 
260 km 
352, 274, 225 km 
Wavelength 
23.5 cm 
23.5 cm 
Polarization 
HH 
HH 
Looking angle 
47° 
15-60° 
Swath width 
50 km 
20-50 km 
Range resolution 
40 m 
14-46 m 
Arimuth resolution 
40 m 
20-30 m 
Data record 
optical 
digital & optical 
SIR-A only has one looking angle, i.e., 47° 
while SIR-B looking angle changes from 15° 
to 60° . This advantage means that an optimum 
looking angle image can be selected. The 
range resolution, swath width, ratio of 
signal and noise and dynamic range will vary 
with tne change of the looking angle. The 
range resolution may be improved. For ex 
ample, the range resolution can increase 
from 40 m of SIR-A to 20 m of SIR-B. Another 
advantage is that SIR-B provides digitally 
processed images. All digital data acquired 
by SIR-B being sent to the ground through 
the TDRS Ku-band antenna. 
It is significant that SIR-B flight di 
rection is approximately parallel to the 
longitude over the north of China. This di 
rection is perpendicular to SIR-A direction, 
which orbited along the latitude. That is, 
the radar beam direction of SIR-A and SIR-B 
is perpendicular. These two orbits cross in 
Shuangjinzhi region to the east of Yamalik 
Dunes. 
The Yamalik Dunes are developed along a 
valley, like a sand river flowing from west 
to east. In the east end, the Shuangjinzhi 
area, the windblown sands begin to reduce 
because of influence of the NNE strike Lang- 
shan Mountain. The sands form a sand sheet 
and it is very smooth on the surface. About 
15 kilometers far away from the Longshan 
Mountain, the sands form again the crescentic 
dunes and the slipface orientation is S60°E. 
These features can be easily recognized on 
the digital processing MSS imagery. 
Like the sand sheet to its west, DN value 
of the Shuangjinzhi crescentic dunes tend to 
the minimum on SIR-A radar imagery and does 
not have any indication of dunes (Fig. 6a). 
139