1181 
4.3 The Combination of Optical and Radar Remote Sensing 
In this section, LAI estimates from the three CAESAR recordings and the two AIRSAR recordings are integrated 
and, with their appropriate weight factors, used for calibrating SUCROS. Results are given in table 2 for the 
three CAESAR recordings in combination with L-band HH and C-band VV radar data. The comparison between 
estimated and actual yield is given in figure 6. On the average, the simulation error of (fresh) beet yield was 
3.0 t/ha (4.2% error) for L-band HH and 3.5 t/ha (4.8% error) for C-band VV, respectively. This error is clearly 
smaller than the one obtained for the three CAESAR recording dates only and of the same magnitude as the errors 
obtained using time-series of CROPSCAN (optical) measurements during the whole growing season. These results 
indicate a synergistic effect (see table 2) by using both optical and radar data for crop growth monitoring. However, 
under practical conditions only very few optical data during the growing season will be available. For instance, 
when no optical data from July 4th would be available it is to be expected that radar data from the beginning 
of July offer a significant improvement to the monitoring of crop growth, particularly at the beginning of the 
growing season. 
As mentioned before, another potential advantage of radar measurements lies in the possibility of obtaining 
information about crop structure changes. The latter may be related to important transitions in crop growth stage. 
(b) 
estimated beet yield 
actual beet yield (tons/ha) 
Figure 6. Comparison between estimated yield and actual yield for three CAESAR recording dates and for two 
AIRSAR recordings in L-band HH polarization (a) and C-band W-polarization (b). 
5 - DISCUSSION AND CONCLUSIONS 
For simultaneous (contemporary) observations no synergy occurred in the estimation of LAI. Optical data were 
most suitable. Calibration of the Cloud model at one date (contemporary) is possible using optical data if enough 
fields are available for the calibration and the between-field variation is large. 
For operational applications the assumption of non-simultaneous observations is most realistic. For sugar 
beet, radar data can only be used for estimating LAI early in the growing season (before crop closure). This may 
be called a model-based approach. After crop closure, radar backscatter is determined by crop architecture (leaf 
angle distribution). However, this still may yield important information for crop growth monitoring. Using the 
latter information may be called a feature-based approach. 
Results for sugar beet indicated that, when a time-series of optical recordings is available, LAI can be monitored 
well and a good estimate of sugar beet yield at the end of the season is possible by using a calibrated crop growth 
model. When only a few recording dates with an optical sensor are available, radar recordings at L-band HH- 
polarization or C-band VV-polarization gave a slight improvement of the results of crop monitoring and yield 
estimation in comparison to the optical data only. This confirms that the main advantage of radar lies in the possibility 
to acquire information on crop growth when other techniques (in particular optical techniques) fail. 
Different scenarios with various combinations of optical and radar data at various dates during the growing 
season for sugar beet still must be evaluated further in order to obtain an accurate picture of the significance of 
radar data (in a model-based and feature-based approach) for crop growth monitoring. The technique to calibrate 
a crop growth model with remote sensing data was developed for sugar beet in Flevoland. The behaviour of other 
crops (other varieties, other locations) can be quite different. The developed techniques, therefore, have to be