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- 653 - 
spot the plots from the TV-monitor in front of him, while the 
system was operated in the passenger compartment by an operator 
(Fig. 2). 
2.3 Implementation of measurements 
When implementing the reflected radiation measurements, each 
sample plot was first spotted with the 
help of the cross targets (Fig. 3). The helicopter was then 
held at 150 m height above the plot by the pilot watching the 
targets in the TV-monitor. The recording of the reflected radi- 
ation took 30 seconds (Fig. 4). During this time the grating of 
the monochromator was turned continuoug1$tough 
its spectral range. 
At the beginning and end of each measurement, 35 mm photographs 
were taken in order to determine the possible movements of the 
helicopter during the measurement. Each recording was made twice, 
and the results comprised two plotted radiation curves and four 
color photographs per sample plot (Fig. 5). 
For incident hemispherical radiation measurements made twice a 
day between 10.00 and 14.00 hours, the helicopter landed and the 
optical unit was turned 180° to point at the sky. The direct solar 
radiation and scattered incident radiation were diffused by putt- 
ing an acrylic plate (filter) in front of the lens assembly which, 
again, was extended above the rotor of the helicopter. 
A list of the measuring conditions and parameters is given in 
Table 1. In compliance with the resolutions of the ISP, Commis- 
sion VII, the following conditions are also reported: 
The observation direction was 
vertical. Occasional deviations from this, caused by 
wind, amounted to about 59. Plots were on flat terrain. 
The proportion o f direct solar 
radiation t o scattered in ci- 
dent irradiation was not measured due 
to the lack of appropriate instruments. 
The instantaneous field of 
view (IFOV) of the spectroradiometer was 
0.075 rad. 
The middle point of the wave- 
length band was continuously varied through 
400 to 1100 nm during the measurement and the