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Unfortunately there exist often some practical limitations 
as 
- not all of the influencing parameters will be found out 
or can be measured (a), 
- only one radiometer is available so that synchronous 
measurements will not be possible (b), 
- the measuring distance is rather low if a cherry picker 
or similar devices are used as instrument platform (c), 
- the instrument requires a large aperture to collect 
enough radiation energy (d), 
- the fields are sometimes not easily accessible and the 
preparation time for the installation of the equipment 
at the test site is long in the case of mobile stations 
(e, f), 
- the meteorological conditions don't often allow field 
measurements (e, f). 
Ins tramentati oÓun 
The mobile station we are using for the field measurements 
is shown in Figure 2.