duration of illumination for a given locality varies with 
latitude north or south from the equator, season.of the 
year, time of the day, and the local topography. 
At present, researchers are concentrating their stu- 
dies in the wavelength bands between approximately 0.3 mi- 
crons to about 1 to 3 centimeters in the microwave portion 
of the electromagnetic spectrum. 
All remote sensing techniques require calibration pro- 
cedures whereby known observables are correlated with the 
remotely sensed variable. For the time being no detailed 
comprehensive radiativetransfer model has been developed 
which can be applied directly to problems in remote sens- 
ing. The need is a workable model which can be applied %o 
a wide range of atmospheric and surface conditions.The ap- 
proach to the solution should be: /a/ the creation of a 
data bank, in respect of geographical areas, and /v/ deve- 
lopment of new sophisticated self-calibrating sensors. 
SENSORS 
There are sensors that have the ability to detect 
electromagnetic energy in each portion of the speotrum. 
The sensors can be divided into six operational categso- 
ries: cameras, radiometers, scanners, spectrometers, ra- 
dars and field-meters. The sensors that operate in the 
visible part of the electromagnetic spectrum are usually 
imagers; the others non-imagers. The reason for this is 
that resolution is directly related to the wavelength of 
the energy being sensed, and in the visihle region the wa- 
velengths are in the ordre of 0.001 om. while in the mi - 
crowave region, the wavelengths are in the order of whole 
centimeters. An image is made by & multitude of small re- 
solution elements that can be obtained only with the short 
vavelength energy of the visible and near infrared part of 
the eleetromagnetio spectrum, unless extremely large ener- 
gy collectomrs are used. The equation below expresses the 
smallest ground resolution elemnt /GRE/ in relation to 
the wavelength of the electromagnetic energy being sensed, 
the platform altitude, and the diameter of the energy col- 
lector of the sensor. 
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