AIRBORNE SAR ROUTE WITH MINIMUM LOSS BY THE SHADOWS 
IN THE MOUNTAINOUS TERRAIN 
M. Bajic”, S.C. Bajic 
* Scientific Project 036-028, Ministry of Science and Technology ofthe Republic of Croatia 
"" Postsecondary Technical School Zagreb, Croatia 
Commission VIL, Working Group 6 
KEY WORDS: SAR, DEM, Slope, Aspect, Shadows, Airborne route, Classification 
ABSTRACT 
No generally approved guide-lines exist for the optimal airborne SAR route with respect to the applications in the mountainous 
areas, in which, due to strong distortion of the images, the subjective interpretation and the quantitative automatic analysis of the 
spatial relationship are hampered and even inhibited. The increased application of the airborne SAR systems, enhances the 
problem: how select the flight route, with the minimum information loss caused by distorting relief influences. An approach was 
developed for the determination of the airborne SAR route with minimum loss by shadows, based on the statistical analysis of the 
relief ‘s aspect, slope and enhanced intrinsic features of the relief structure responsible for the shadows. Applications: the 
airborne SAR is to be used for mine field detection, in the Open Skies mission (in both cases loss of the information is not 
acceptable), or SAR was loaned and a minimum loss is desirable. 
1. INTRODUCTION 
The increased application of the airborne SAR systems, 
enhances the problem: how to select the flight route, with 
the minimum information loss caused by the distorting 
relief influences. No generally approved guide-lines exist for 
the optimal airborne SAR route with respect to the 
applications in the mountainous areas, in which, due to 
strong distortion of the images, the subjective interpretation 
and the quantitative automatic analysis of the spatial 
relationship are hampered and even inhibited (Raggam J., 
et. al, 1985). Relief is the cause of shortening the slopes 
and overlapping higher parts of terrain on the images as well 
as of losing the parts of area on the image due to shadows. 
(Domik G., et. al., 1984). Shortening and overlapping can be 
corrected by additional processing (Teillet P. M. , et. al, 
1985, Raggam J., et. al, 1985, Domik G., et. al, 1984). 
Apart from the SAR image geometric degradation due to 
relief, very pronounced are radiometric degradation's 
decreasing reliability and accuracy of the scene contents 
classification based on such images. 
Radiometric correction of the SAR image starts from digital 
model of the terrain elevation (DEM), simulated and real 
radar image, enabling substantial improvements (Raggam J., 
et. aL, 1985, Domik G., et. al, 1984, Bayer T., et. al, 
1991). Beside the undesirable effects, relief shadows can be 
useful, e.g. in determining the terrain elevation (Frankot 
R.T., Chellappa R., 1990 ). Regarding afore mentioned 
unwanted effects of the terrain, there are three different : 
cases: a) if the route is to be selected for airborne SAR 
imaging, i.e. there is no SAR image, b) if the SAR satellite 
image is to be selected (RADARSET, ERS, J ERS), so that 
loss is minimum, on the bases of quick - look image (poorer 
resolution than that of original) and description, c) if there is 
SAR image and shortenings and overlaps should be 
corrected and the shadows areas excluded form the image. 
For the case c) there are many published solutions, most of 
them apply SAR image simulation based on terrain elevation 
digital model (DEM). Since the intensity of airborne (e.g. 
DoSAR, E-SAR, PHARUS, Kras) and satellite 
(RADARSAT) remote sensing by SAR is increasing , the 
following methods are necessary to develop: a) to elect route 
with minimum loss caused by shadows, b) to select satellite 
SAR scenes. The need for such a solution is the greatest if 
the airborne SAR is to be used for mine field detection 
(Scheerer K., 1996) or in the Open Skies missions. In these 
cases loss of the information is not acceptable. The need to 
define the route also exists in case the SAR is hired for other 
tasks of remote sensing, so only low percentage of loss due 
to shadows is permitted. One approach to select the airborne 
sensor route is given in (Kovac T. 1997, Bajic M. 1997) but 
is not matched to the SAR characteristics. Concerning the 
above mentioned, this paper starts from general knowledge 
of the route influence on the discussed degradation at SAR 
imaging in mountainous terrain according to previous 
researches. This is followed by the discussion on proposed 
method. 
2. TOPOGRAPHIC FEATURES AND THE SAR 
FLIGHT DIRECTION 
Based on the results of recent research of the mountainous 
terrain influence on imaging by SAR, several conclusion 
could be derived. Topographic features of relief, especially 
lineaments can be best imaged by SAR if the flight 
direction is nearly parallel with them while the radar 
imaging is lateral. General suggestion that imaging should 
be parallel with lineaments and vertical to them can be 
derived from this fact. If SAR is used in stereoscopic 
imaging, the best results are achieved if the area with 
lineaments is imaged from two routs, the first route being 
parallel and the second route inclined for about 25 degrees 
compared to the first one (Raggam J., et al. 1985). Of course, 
the relief elevation and the altitude of the platform and the 
distance from the platform to the imaged area determines the 
imaging angles in vertical plane. Steeper imaging generally 
gives higher percentage of shortenings and overlaps, which 
510 International Archives of Photogrammetry and Remote Sensing. Vol. XXXII, Part 7, Budapest, 1998 
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