generating an electrical impulse which varies in intensity according to 
+he amount of infrared energy emitted from the object on the ground. 
This impulse is then recorded on black and white film as a shade of 
grey: the more energy, the stronger the impulse and the darker the 
shade of grey on the negative film, In this manner a thermal picture 
of the ground is pieced together, line by line, as the mirror spins at 
rates proportional to the forward speed of the aircraft or spacecraft. 
Multispectral Scanners 
Multispectral scanners operate on the same principle as the infra- 
red scanners except that, by adding additional detectors and a special 
defraction grating to separate the energy coming from the scanning 
mirror, information in the ultraviolet and visible portions of the 
spectrum can also be recorded. By recording the impulses coming from 
each detector on a separate piece of film or magnetic tape, the same 
ground scene can be recorded in several so-called channels. Scanners 
are now capable of dividing the spectrum into as many as 24 channels. 
This allows analysts the opportunity to determine which part of the 
Spectrum is best for recording each type of environmental data. The 
use of this instrument also allows the researcher to compare and/or 
combine several channels to determine more precisely the spectral 
characteristics of an object. 
Side-Looking Airborne Radar (SLAR) 
Side-looking airborne radar is an all weather, day or night sensor 
which is particularly effective in imaging large areas of terrain. It 
is called an active sensor because it generates its own energy, which it 
sends out in short pulses from a transmitter on the side of the air- 
craft. This energy travels to the Earth, bounces off the ground or 
objects on the ground, and is returned to the aircraft where a receiving 
antenna collects it, converts it into an electronic impulse, and 
displays it on a cathode ray or television tube. The cathode ray tube 
is then photographed, and a permanent record of the illuminated terrain 
is made. The imagery resulting from this sensor is similar in appear- 
ance to a black and white photograph, but contains much less detail than 
a camera produced picture. The black areas on the imagery represent 
either radar shadows or voids created when the energy leaving the air- 
craft strikes a smooth surface such as a body of water, and "skips" off 
at the same angle at which it arrived. If the energy strikes a large 
object, however, a bright or white return will be recorded because a 
large part of the energy is reflected back from the object to the 
receiving antenna. 
The principal advantages of using SLAR are speed and scope of areal 
coverage, enhancement of the Earth's physical features, and the fact 
that it can be flown day or night in any type of weather. In addition, 
some SLAR systems can penetrate certain types of vegetation, thus pro- 
viding a picture of the surface below the trees or bushes. 
The scale at which SLAR imagery is usually collected is approxi- 
mately 1:240,000. Thus, the scale is too small to record most types of