the atmosphere at (d), and finally enters the 
spacecraft instrument (e). It is this energy that 
is measured by the instrument. In summary, 
the quantity that we are seeking is the ratio of d 
to c (the reflectance - which characterizes the 
surface material), but the quantity measured is 
e (the radiance entering the instrument). Part 
of the processing that is undertaken must, by 
one means or another, derive the reflectance 
from the measured radiance taking into 
account all of the factors which have been 
noted above. We call this process reflectance 
modelling. 
Accurate modelling of the reflectance process 
is quite a complex undertaking (Woodham & 
Gray [1987]). The process is illustrated in 
Figure 4. The energy entering the 
instrument at point E has three components: 
(1) energy reflected directly from 
the target pixel (T), (2) energy 
reflected from an adjacent pixel 
(A) and scattered by the 
atmosphere, and (3) energy 
which is simply scattered by the 
atmosphere. In addition, the 
energy illuminating the target 
pixel has several components: 
direct illumination by the sun, light 
from the sun which has been 
scattered in the atmosphere, light 
which has been reflected from 
adjacent pixels and light which 
has been reflected from adjacent 
pixels and scattered by the 
atmosphere. Depending on the 
shape of the surface, and the 
relative positions of the 
illuminating source (the sun) and 
the sensor, some of the the 
pixels may not be directly 
illuminated at all because they are 
in the shadows of hills or 
mountains. These pixels are 
illuminated only by light which 
has been scattered by the 
atmosphere and/or reflected off 
adjacent pixels. All of this is in 
addition to the effects caused by 
orientation of the target pixel. It is clear that in order to model this situation adequately, a digital 
elevation model which describes the shape of the surface is necessary. 
Figure 4 Reflectance Modelling 
The Digital Elevation Model and its Importance 
The above discussion has emphasized the importance of the availability of a digital elevation 
model in accurately determining the reflectance of surface material. The digital elevation model is 
critical in other respects as well. Radar backscatter is a strong function of the orientation of the 
surface pixel relative to the incoming radiation. The ability to relate backscatter readings acquired