ıntainous
is of the
ces the
oach was
sis of the
ons: the
on is not
satellite
ing , the
lect route
t satellite
reatest if
detection
. In these
e need to
| for other
f loss due
> airborne
1997) but
Tning the
nowledge
n at SAR
previous
proposed
R
untainous
sonclusion
especially
the flight
the radar
ng should
m can be
ereoscopic
area with
jute being
5 degrees
Of course,
m and the
rmines the
; generally
ips, which
can be compensated in additional processing . Unlike this
one, at more shalow imaging, these two sort of degradation
decrease while the percentage of loss in shadows increase
due to the terrain masks.
When selecting the satellite RADARSAT images, sharper
angles of imaging can be selected to obtain acceptable loss
due to the shadows in the terrain masks. At airborne
imaging by SAR, the altitude of the aircraft is limited. The
necessary and acceptable imaging angle in vertical plane is
3. THE ENHANCEMENT OF THE TERRAIN
FEATURES RESPONSIBLE FOR THE SHADOWS
We take digital terrain elevation model (DEM) as the basic
source of the data on terrain. The maximum aircraft altitude
limits the greatest distance at which the imaging is possible.
The refraction in troposphere is included in model.
Additional limit to SAR imaging of plane objects is that
grazing angle of imaging must be bigger than several
degrees (usually 4^) since backscatterring for smaller angles
not possible to provide at greater distances. Another solution
is imaging from the other side of lineaments and covering
the areas in shadows. At RADARSAT, it is possible to
select the route from the two sides, lateral to some area but
from opposite flight direction. At aircraft imaging, there is
the option to select the route enabling imaging from
different directions compensating in this way limitation due
to small imaging angles in vertical plane.
drops abruptly so that it can be neglected. This fact
introduces additional limit to solution of the discussed
problem. To analyze the shadows influence, the situation is
defined according to the Fig. 1. as follows: SAR is imaging
the Earth surface and the main limitations are the platform's
altitude, the lowest depression angle of SAR's antenna in the
vertical plane and the elevation of the terrain.
5
4 w | Depression angle
M degrees
3 "le ~
N.
a 3 +
E gu
= ‚x.
= nth
2 2] XM
n o
= den,
d man,
TN,
1 gos
o ~
ya“ x
Pau 2 : v
0 JU. «b B8 ie. A : > be I re Sr ^ X Series
-— SHADOW —| | " rere ane
0 10 20 30 40 50 60 70 80
GROUND DISTANCE km
Figure 1. Model for the analysis of the shadows. The top T of the hill produces a shadow for the SAR flying e.g. at the altitude
2000m (line A-A). The line B-B is a tangent on the backslope of the hill. An angle greater than 4°, between the backslope's
tangent B-B and the new SAR's position, line C-C, enables the imaging of the backslope by the SAR, but at greater aircraft's
altitude, ca 3.3 km in the example. Minimum depression angle of the SAR antenna in the vertical plane, measured from the
horizontal line, limits the maximum achievable range for the smooth terrain (for SAR at the altitude 5000 m it is ca 76 km). The
standard troposphere refraction was included in the model.
According to the Fig.1, if the increase in the airborne SAR
altitude is allowed (in the example from 2000m to 3300m),
it is possible to image the backslope. This manner of
decreasing the loss of image due to the shadows of relief
Will be possible only to some small distance since the
necessary aircrafUs altitude increases with the growing
distance. If smaller, i.e. narrower areas are imaged, e.g.
mined areas, it is possible for lower relief elevations to
reduce or exclude the loss due to the shadows of the relief
masks by the change of imaging altitude. Altitude of the
aircraft H, variations in the relief elevation, imaging
distance d define the width of the shadow S,
S=d(h/(H-h)), (1)
where h -elevation, d - obstacle distance.
International Archives of Photogrammetry and Remote Sensing. Vol. XXXII, Part 7, Budapest, 1998 511
