filtering ones in order to adjust high con
trast between the ajacent objects and give
the image a "smooth" effect. In this study,
the high contrast caused by strong scatter
ring from granite and the weak echo from
dunes is really reduced by using divided
filtering. It has proved in many studies
that this is a useful technique in the pro
cessing of radar images.
A review of the images processed with five
different filtering windows shows that the
quality of the image processed with divided
filtering and 5 by 5 window is better than
that of others. In this paper, the image
filtered with 5 by 5 window is used in image
analysis (Fig. 2a).
Landsat MSS image is important in the
matching analysis with SIR-A image. The CCT
data of June 1977 has been chosen for doing
digital processing in order to stress the
dune pattern except for optical false color
composite image of some areas. The 5 and
7 band images are considered to be better
ones in geological application. The data of
three bands are processed with linear stre
tching. The composition is made on the nigh
clarity color compositing instrument to form
false color composite image, which clearly
shows the dune patterns and they can not be
seen on the optical processed images.
3 SIR-A RESPONSE FROM SAND DUNES
3.1 Yamalik Dunes
The Yamalik Dunes are located in the center
of the Alashan Plateau, east of Badain Jaran
Desert. The Yamalik Dunes study area is shown
in Fig. 1. The dune field is about 150 km
long and 5 km wide and is controlled by do
minant wind from north-west direction. The
dune field developed along a valley on the
stony plain, forming a "sand river". In fact
the dune field is connected with Badain Jaran
Desert, the sand sources of Yamalik Dunes.
The prevailling north-west wind transports
the desert sands to these dunes. Analysis of
computer-enhanced Landsat images (Fig. 2b)
and field investigations indicate that the
principal dune types are compound crescentic
dunes. Numerous star dunes are also present
in some areas of the dune field. The Yamalik
Dunes are composed of fine-grained sand and
no other vegetation exists in the dune area.
Individual dunes are generally 20-35 ® in
height, however, occasionally some star dunes
may be as high as 70-80 m. Crescentic dunes
are oriented with axes trending along the
valley perpendicular to the sand transport
direction and the two rims of the valley.
The Precambrian metamorphic rock and gra
nite constitute the two rims which exhibit
a bright return on the radar image (Fig. 2a).
Vegetation and some geologic features, out
crops and dikes, exhibit an intermediate
bright return on the SIR-A imagery. The Yama
lik Dunes or "black zone" on radar image are °
distinguished by its low return level com
pared witn other features. The image tone or
ratar return from the dunes changes from
northwest to southeast direction of the SIR-A
image. Based on changes and to assist analy
sis, the "black zone" is divided into four
sections.
Table 1 shows the relationship of the image
tone with the slipface orientations and the
slipface angles formed between the radar beam
and the long axes of sand dunes. Section "d"
differs from the other three sections being
composed of outcrops with a sand covering.
These features have an intermediate-mottled
response in SIR-A image. Bedrock usually has
a bright return. Radar illumination direction
is not an important factor for tne radar
backscatter in bedrock area.
Sections a, b, and c are composed of the
same type of compound crescentic aunes with
occasionally some star dunes (Fig. 3)* No
vegetation exists in the dune area. The
orientations of slipfaces change with the
change in the valley's trend. SIR-A radar's
flight direction is about N80°W and illumi
nation direction is N10°E. The angles be
tween the flight direction and orientation
of the slipfaces of Sections a, b, and c are
50’, 10°and -20°, respectively.
Table 1. Imaging parameter analysis of Yama
lik Dunes.
Section Tone
Transport
direction
RIB
LAD*
Type of
dunes
a
bright-point
S30°E
50°
crescentic
some star
b
intermediate
bright-point
S70°E
10
crescentic
c
very dark
N 80° E -
■ 20° **
crescentic
some star
d
intermediate
mottled
- S30’E
outcrops
sand
* Angle between radar illumination beam and
long axes of sand dunes.
** Here "-" means radar beams illuminated on
gentle slope of sand dunes.
It is clear that radar return ana there
fore tonal changes from tnis type of sand
dune depend on the aegree of the angles be
tween radar illumination direction and slip-
face orientation direction, at a 50° angle
(Table 1, Section a) the impinging radar
energy will illuminate on most of the slip-
face producing a relatively strong return.
At a 10° angle (Table 1, Section b) tne radar
beam only illuminates tne edge of the slip-
face and a relatively low return results.
For the -20° angle of Table 1, Section c, the
radar beam illuminates the stoss slopes
(gentle slopes) and no radar return is ex
perienced except from tne pyramidal-snaped
star dunes. The star dunes in this study
tend to have two bright point of radar re
turn. From these observations we can conclude
that tne sand dunes will exhibit a maximum
in radar return when the slipface orientation
is perpendicular to the illuminating radar
beam.
3.2 Yapulai Shan Dunes
The Yapulai Shan Dunes represent the two dune
fields located on either side of tne Yapulai
Shan Mountain. The Yapulai Shan Mountains
trend northeast-southwest direction and for
reference purposes the aunes located to the
northwest of this mountain are referred to
as the NW dunes and dunes on the otner side
are the SE dunes. In fact, however, the two
dune fields belong to two distinct deserts
in China. The Nw dunes occupy the nortneast
most part of the Badain Jaran Desert ana the
SE dunes are the northwest most part of Teng-
ger Desert (See Fig. 1).
The prevailing northwest wind across the
Badain Jaran Desert area reaches the north
east trending Yapulai Shan Mountains and de
flects the northwest winds to the N30°E di-
