city of Sydney, Australia. The chosen area had a mixture 
of residential, commercial, and industrial land uses. The 
study involved obtaining pixel values over a number of 
residential, commercial, and industrial areas of varying 
street orientation with respect to the radar azimuth angle. 
The average pixel values and standard deviations were 
then calculated for the subject areas. Although the 
sample size was small, there was a definite trend 
showing a correlation between radar backscatter and 
building size. 
The results show that the average pixel value and 
standard deviation for the residential classes were lower 
than those of the commercial classes while the industrial 
areas had the highest values (for their relative orientation 
angles). It is expected that residential land use would 
have the lowest average pixel value and standard 
deviation since residential buildings are generally smaller 
in size (therefore less area to backscatter the radar 
wave). The residential building materials were mainly 
brick or fibrous cement walls with tiled roofs. These are 
primarily dry with a low dielectric constant. A low dielectric 
constant means a large amount of the radiation will 
penetrate the surface of the building reducing the 
backscatter measured at the radar receiver. 
Areas of commercial buildings are usually more dense 
and variable than residential. A local shopping area often 
has metal clad roofs and terrace type buildings. A central 
business district (CBD) contains many large buildings, 
both in floor area and height, with a high density. There 
are more metal structures (having a high dielectric 
constant and therefore a strong reflection) acting as 
support in large buildings. Hence areas of commercial 
buildings show higher average pixel values and standard 
deviation, than residential classes. 
Industrial buildings gave the largest average pixel value 
and standard deviation. Industrial regions consist of large 
buildings mostly clad in metallic materials. Metals are 
conductors with a high dielectric constant. They can give 
a very strong backscatter at particular orientation angles. 
The residential class, with the largest sample size of the 
three, was examined further to show a relationship 
between the backscatter response (being directly related 
to the pixel value, or digital number, on a radar image) 
and building orientation with respect to radar direction 
(Figure 1). The standard deviation was also determined, 
When the angle between the normal to the street or 
building and the radar look direction (phi) is equal to zero, 
the backscatter response is at its highest. As phi 
increases to around 220. the backscatter decreases to a 
minimum. For values of phi above 20 degrees the 
backscatter varies little with phi. This result is similar to 
that obtained by Hardaway et al (1982). 
A MODEL FOR PREDICTING URBAN 
CHARACTERISTICS 
A model has been developed to give the expected 
backscatter from a group of buildings of user defined size 
(single or multiple storey), shape, material (including 
surrounding ground surface), and radar parameters. 
Existing formula, giving the backscatter for a corner 
reflector and a rectangular facet, have been adapted into 
the model. It is presently designed for a simple building of 
rectangular shape with either a flat or sloping roof. When 
the model is used to find the expected backscatter from a 
block of buildings, of either small residential size or large 
commercial size, the roof facing the radar, and the corner 
reflector effect from the front wall, are the dominant 
contributors to the backscatter (Figures 2 & 3). Figures 2 
and 3 show an example of the simulated backscatter with 
respect to phi (for residential and industrial areas 
respectively) for each component of a building, as well as 
the total backscatter. The oscillations of the backscatter 
for both buildings are due to difference in phase as the 
distance the wave travels between the extremes of the 
object leads to either constructive or destructive 
interference. These oscillations become more frequent as 
building size increases. 
As expected, Figures 2 and 3 show the backscatter is 
greatest when the radar approaches the building normal 
to the wall (when phi equals zero). The backscatter drops 
as phi increases to 459 (with predominantly diffuse 
backscatter). Around this angle the backscatter response 
from trees (essentially volume scattering) need to be 
considered. 
  
Figure 1. Average pixel values of SIR-B vs Orientation 
angle (Residential class) 
  
  
  
  
  
  
  
  
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= Std Dev 
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phi (degrees) 
  
  
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B7. Vienna 1996