April and May). Figure 4 shows the development of the 
radar backscatter coefficient throughout the growing 
season for the 4 most important croptypes. Although 
the digital radar images have known intensity scales, 
an absolute calibration lacks in these measurements. 
Therefore the average backscatter coefficient of the 
sugarbeet fields was determined in the images and 
compared to calibrated ground based measurements, 
which were always taken at the same date and in the 
same area. The resulting correction factor was applied 
to the whole image. The data in figure 4 is for 
horizontal polarization and 15° grazing angle. The 
frequency is 9.4 GHz (X-band). 
Nowadays, a more direct approach to the calibration 
problem of the SLAR system is available. The radar is 
fitted with an internal delay line calibration. The 
radar data is calibrated in the PARES preprocessing 
phase (ref. 2). This algorithm takes care of the 
complete geometric and radiometric correction and 
calibration of the radar images. The calibration 
accuracy is in the order of 1 dB or better, as was 
tested with the aid of corner reflectors. 
From figure 4 it can be seen that a large contrast 
Figure 4. E 
the growing 
horizontal 
exists bet 
April and 
while all 
July a goo 
types, whe 
backscatte 
the one fo 
The larg 
croptypes 
low grazin 
the winter 
winter and 
other crop 
their biom 
ground covi 
backscatte 
because tb 
amount of ' 
leafs Stic' 
ably to tb 
angles say 
is much im 
is smaller 
between tb 
Thus we : 
winter- am 
May, and s: 
wintercrop 
identify a 
histogram t 
coefficienl 
figure it : 
be comp let« 
by applyinj 
Now that 
to classify 
This demons 
contrast wi 
(Ref. 1) wl 
scatter thr 
discriminat 
Sofar the 
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