and to the spectral attenuation. For the landscape areas, other than the 
reference ones, the order of the spectral reflectance may be established and 
comparison of the spectral response may be done between the bands. 
Referring to the atmospherical effect and to other variable 
factors we may use the following reference material: 
- simultaneous available recordings Nimbus, ERTS; 
- known premarking recordings - as position, form, colour, 
spectral reflectance (including reflexion mirrors /13/); 
- recordings of the natural and artificial sampling targets, 
judiciously chosen as settling and characteristics. 
In this sense the experience acquired during many years has a 
special importance so that the above mentioned procedures do not represent 
but possibilities of intra and extrapolation controlled within some 
sufficiently restricted values as long as we know very well the photographic 
recording systems, the field conditions and the laboratory processing ones. 
Even if we consider the preliminary processing transformations 
as a whole, including the diurnal or even hourly changes regarding the 
surrounding medium, observational and of sensors conditions, there is a 
technique of experimental organization of the data which allows the operations 
and results control. 
Within this, the problem of simultaneous calibration of the data 
is approached not only from the geometrical but also from the spectroradio- 
metrical point of view. 
CALIBRATION-HOMOGENIZATION FROM SPECTRORADIOMETRICAL 
POINT OF VIEW 
This kind of calibration is considered in principle and in 
practice in close correlation and simultaneously with the geometrical one. 
To observe the quality of the recordings i.e. their depreciation 
as an information value, the quantification of the space information content 
of the collected data is necessary. Thus the space frequency content of the 
image by means of the FTM-modulation, that is Fourier's double transform 
/10,14/ may be used as unit. 
Thus we shall write the transfer functions of the MTF-modulation 
for all the stages and for each recording starting from object-landscape- 
phenomenon, passing to atmosphere, platform, sensor-captor, up to so called 
recording with the successive depreciations from the different generations. 
The aim is to determine MTF in flight for each recording. This is difficult 
but using sampling targets, field samplings and some artifices for the prac 
tical constant or known consideration of some values, the problem is