of eleotromagnetid radiation, and /4/ the energy cycle in 
the sensing process. 
THE MEDIUM OF PROPAGATION 
The medium of propagation in the case of remote sens- 
ing systems is the atmosphere. To-operate remote sensing 
systems it is necessary to select those portions of the 
electromagnetic spectrum that are transparent to the ra- 
diation to be sensed. Along the spectrum are zones where 
the atmosphere is transparent to the elebtromagnetic ra- 
diation, and zones where absorption by atmospheric gases 
makes it opaque. Absorption at a particular wavelength oc- 
curs where the electromagnetic energy at that wavelength 
is vibrating in resonance with the atoms and molecules of 
the atmospheric gases. The atmosphere is an extremely com- 
plex medium through which electromagnetic radiation must 
pass in order to be remotely sensed. A complex process of 
scattering, reflection, and absorption by the atmosphere 
alters the amount of solar energy striking the earth.Furt- 
her alteration occurs as energy reflected or emitted by a 
feature on the earth's surface travels back through the 
atmosphere and is recorded by & sensor. The properties of 
the atmosphere can be summarize as follows: /1/ tempera- 
ture, /2/ pressure, /3/ density which reflects seasonal 
and latitudinal variability, /4/ atmospheric compostion: 
Co, C0, CH, H,O, N,, N, 
absorbants, scattered along the electromagnetic spectrum, 
0, 0», 04; which are the major 
/5/ particle concentration and size distribution which 
atributes the scattering in the atmosphere, varying wide- 
ly both geographically and temporally in a given location. 
The problem becomes more complicated with the concept of 
illumination. All electromagnetic energy emitted or reflec- 
ted from the surface of the earth, with the exception of 
geothermal energy, are affected directly or indirectly by 
Solar radiation. In this respect the sun ans le is a very 
important factor because 1t affects not only the amount 
of illumination, emitted or reflected to the sensor, but 
also the spectral quality. Furthermore, the intensity and. 
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