f Flying direction Flying direction 
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a «p 
No radar shadow 
  
a zz a »p 
No radar shadow in 
Radar shadow 
brushed back slope 
Fig.2 Illustration for generation principle of radar shadow 
The physical electric characteristics of culture, such as 
dielectric constant or relative permittivity. is an 
another effective factor to the hue of radar image. 
When radar ray comes to the surface, part of it was 
reflected or scattered, others were transmitted to the 
subsurface and scattered again. The scattering or 
transmission of radar ray is influenced by the physical 
electric characteristics | ( mainly dielectric constant ) 
of the culture. For those high permittivity cultures, the 
back wave is intensive, they show light hue on radar 
image; Low permittivity cultures show dark hue on 
image. The dielectric constant of solid object usually is 
less than 10. For example, marble is 8 — 10, granite is 
7 — 9; Most moisture-free objects are less than 6 in 
permittivity, for instance, asphalt is 2.7; The dielectric 
constant of micro-wave for water is up to 60 — 80. The 
most important influencing factor to the dielectric 
constant of cultures is the moisture of the cultures. The 
dielectric constant varies with the moisture in a 
positive linear function in determined material. More 
moisture in soil or plant, bigger the dielectric constant 
is, so the moisture-contained material become 
intensive micro-wave scatter. Wheat is flourishing in 
May, it has more moisture and more intensive back 
wave. Irrigated land shows lighter hue than dry land. 
The conductivity of culture and the wave length of 
radar ray also affect the penetrating ability of radar ray 
as well as the scattering for micro-wave. Metal culture 
( such as railroad ) which has high conductivity is 
the best scatter for its intensive scattering, so it shows 
light hue on image; Cement and rocky material ( such 
as buildings ) are intermediate scatters; Dry wood is 
the worst scatter because of its high penetration and 
absorption to radar ray. Shorter is the radar wave 
length, lower is the effective penetration. More 
moisture is in soil, weaker is the penetrating ability of 
radar ray. For forest, the radar back wave mainly come 
from crown. However, for long wave radar, the back 
wave not only come from crown, but also from ground 
surface because of its intensive penetrating ability. As 
an important basic remote geological studies, more 
attention has been paid to the dielectric constant of 
rocks. 
The roughness of the culture surface obviously affects 
the back wave of radar ray. Smooth surface ( such as 
stable water ) can reflect all incidence ray, incidence 
angle is equal to reflect angle, opposite direction 
causes a mirror reflection. The back reflection is 
minimum, so images show dark hue; rough surface 
( such as waved sea ) scatter the radar ray in 
different directions, intensive back wave causes lighter 
hue on images; Intermediate roughness surface 
produces weak back wave, so different gray hues are 
shown on images. The roughness of culture surface 
could indicate the scattering of working area, it is 
related to radar wave length and the depression angle 
of antenna. Based on the calculation by Rayleigh, for 
50° incidence angle and 23.5cm wave length SIR-A, 
592 International Archives of Photogrammetry and Remote Sensing. Vol. XXXII, Part 7, Budapest, 1998 
  
  
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