passive. An active system would De one in which a signal leaves the aircraft and records a
return echo like in side-looking airborne radar (SLAR).
NOTE:
Both the AN/AAS-24 IR mapping system and the AN/AAD-5 IR reconnaissance set
do not have a standoff capability. They must De used with discretion by the
commander, taking into account the safety of the air crew and the aircraft.
11. IR peculiarities. It was previously stated, all objects having temperatures above absolute
zero emit IR energy. However, IR detectors are not able to receive all of the energy emitted
by an object for the following reasons:
a.
Atmospheric attenuation. IR radiation Is attenuated by atmospheric particles such
as rain, fog, snow, sleet, and dust. Under such conditions, the particles reduce the radiation
received by the airborne IR detector. As the size of atmospheric particles approaches a size
near the wavelength of the radiation, detection becomes Increasingly difficult. For this reason
it Is preferable to fly missions at "low' altitudes (less than 1,000 feet(ft)). At low altitudes there
is less atmospheric attenuation between the target and the sensor. IR energy also attenuates
with distance. Higher sensor altitudes coupled with atmospheric attenuation can reduce IR
radiation to a point beyond the sensor's capability to detect it. Figure I-3 shows how the
transmission of various IR wavelengths is affected by the atmosphere. The shades areas
depict regions of best IR transmission through the atmosphere. The regions of high
transmission of IR energy are called "atmospheric windows." The largest "window' is between
7.5 and 14 microns at mean sea level (MSL). An increase in elevation (not altitude) results in
larger window because of the decrease In the density of the atmosphere.
NOTE:
Astronomers locate their observatories on mountain tops where the atmosphere is
less dense and pollutants are negligible.
b.
Thermal crossover. The amount of IR radiation is directly related to the temperature
of an object.
(1) As temperature Increases, the energy level of emitted IR also increases and
the hotter surfaces emit shorter IR wavelengths. As solar energy strikes the earth during the
daylight hours, different objects heat at different rates. Thus, during the day, land is warmer
than water. At night, cooling also occurs at different rates leaving water areas warmer than
land surfaces.
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