137 
Symposium on Remote Sensing for Resources Development and Environmental Management / Enschede / August 1986 
Shuttle imaging radar response from sand dunes and subsurface 
rocks of Alashan Plateau in north-central China 
Guo Huadong 
Institute of Remote Sensing Application, Academia Sinica, Beijing, China 
G.G.Schaber & C.S.Breed 
US Geological Survey, Flagstaff, Ariz., USA 
A.J.Lewis 
ERSAL, Oregon State University, Corvallis, USA 
ABSTRACT: SIR-A and SIR-B images of three sand dunes and two bedrock regions in alashan Pla 
teau of north-central China have been studied and compared to Landsat imagery and field in 
vestigation data. The results of the study show that radar illumination direction is an im 
portant factor which results in the echo strength change for the same type and size of sand 
dunes. The sand dunes in which the slipfaces are oriented to radar beam have a bright-point 
response and the dunes in which the gentle slopes face the radar beam have a dark signature 
on the radar image. Two Precarnbrian metamorphic rock bodies buried beneath the thin layer of 
alluvial material or aeolian sand have been detected by using these two radar images. 
1 INTRODUCTION 
The Shuttle Imaging Radar-A and B (SIR-A and 
SIR-B) carried on the Space Shuttle Columbia 
in November 1981 and the Challenger in Octo 
ber 1984 acquired images of part of Alashan 
Plateau in north-central China. This very 
dry plateau covers three deserts of the main 
thirteen ones over the country. One of these 
deserts, the Badain Jaran, contains some of 
the tallest dunes in the world. 
These SIR-A and SIR-B images are the only 
orbital radar data acquired over China and 
provide a great deal of information on these 
sand dunes as well as subsurface geological 
features. In this paper five areas are stu 
died, including three ounes anu two aeolian 
sand and bedrock regions (Fig.1). The pur 
pose of this study is to understand the L- 
band radar responses from sand dunes and 
some subsurface features. 
One of the authors (Guo Huadong) conducted 
field work in several study areas covered by 
radar imagery. The Landsat images compared 
with the radar data were optimally processed 
on the VAX 11/750 computer system of the U.S. 
Geological Survey in Flagstaff, Arizona. 
Fig. 1 Index and simplified geologic map 
of the Alashan Plateau showing location 
of the five study areas.I--Yamalik 
Dunes; II--Yapulaishan area; III—Badain 
Jaran Desert; IV—Shuangjinzhi study 
area; V--Aertengaobao study area. 
R--Bedrock 
N--Neogene 
Q--Quaternary 
2 COMPUTER ENHANCEMENT TO THE SIR-a AND MSS 
IMaGES 
SIR-A is a L-band synthetic aperture radar 
with HH polarization and is imaged witn op 
tical recording method. The radar on ooard 
recieves the information on phase and ampli 
tude of the echo from the ground target, 
which represents the change of the scatterring 
strength. The information is recorded on the 
data film and then processed with optical 
relative processor to form image film. 
The SIR-A image is processed with digital 
enhancement technique in order to study the 
radar scatterring characteristics from the 
desert region more effectively; the Landsat 
MSS CCT data is also processed with the same 
technique to be used as the data for matching 
analysis. In this study, the radar image 
covering the test region is digitized to form 
an image matrix consisting of 2439 by 46OO 
pixels. The high pass filtering technique is 
considered to be an important one in the pro 
cessing of SIR-A data because it has the same 
defect with other on-board radar images, that 
is, there are too many speckles on the image, 
which are easily confused with speckle-shaped 
echoes from the dunes. The speckle noise may 
be lowered by filtering the image. 
A selection of the filtering window is im 
portant to the processing. The window of 101 
by 101 has been accepted a traditional one 
in the filtering process for the MSS data. 
But there is no such an optimum window for 
the space radar image in this dune area. So, 
four windows, 3 by 3, 5 by 5, 23 by 25 and 
101 by 101 are chosen respectively. Anotner 
kind of filtering, the divided filtering, is 
also used. The back scatterring strength of 
radar changes sharply with different back 
ground formed by different ground objects. 
Due to the higher dielectric constant as well 
as the effect of corner reflector , the ob 
jects such as bedrock, vegetation and buil 
ding usually have stronger echoes; and the 
echoes from the objects such as smootn ground, 
water body and dry sand are much weaker. The 
obvious difference between the DN values of 
these two types of objects makes a high con 
trast on the image. This is, of course, an 
unfavourable factor to image interpretation. 
The principle of the divided filtering is to 
divide the original data by its low pass