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1.0 Radar technology
Currently the way to detect and even identify aircraft,
is the use of radar. This system, invented during world
war II, simply works by constantly sending bursts of
radio waves of certain frequencies and measure the echo's
of each burst.
Parts of the energy of radio waves are being reflected
by objects. This can be a plane, but also a cloud or
a bird. Depending on the material the object is made
of, this echo is stronger or weaker, but there is an
echo. By measuring the reflected energy as a function
of position and time, computers can calculate what it
is that reflects the energy, where it is in 3D space
and also in what direction it moves.
To get a proper overview of an area with radar, the transmitting
and receiving antenna should rotate in angles of 360 degrees.
This is why you always see these rotating antenna's at
for instance airports and ships. To protect the antenna's
from damage, they are often mounted in a radio wave transparent
dome, which you will probably already have seen somewhere.
2.0 Detection techniques
There are a number of causes for planes or other flying
objects like missiles, giving away the fact that they
are there. Radar or in other cases laser technology enables
the searching party to detect the flying object and act
upon detection.
2.1 Direct echo's
Once radar waves hit a plane, a part of the radar energy
is bounced back to the sending source. The amount of
bounced back energy highly depends on the shape of the
object and the material it is made of.
The returned echo can be deteced, giving away the position
and speed of the object.
2.2 Jet wake
The parameter determining radar return from a jet wake
is the ionization present. Return from resistive particles,
such as carbon, is seldom a significant factor. The
very strong ion-density dependency on maximum gas temperature
quickly leads to the conclusion that the radar return
from the jet wake of an engine running in dry power
is insignificant, while that from an afterburning wake
could be dominant.
2.3 Heat detection
Another way of detecting if an aircraft is flying somewhere
is by measuring the heat it radiates. Normally this heat
is produced by the planes engines. There are two significant
sources of infrared radiation from air-breathing propulsion
systems: hot parts and jet wakes.
By using
modern heat image sensors (read InfraRed sensors)
the difference can be seen between a flying object
itself and the surrounding cold air.
This is the same for the jet engine exhaust gases. |
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The ideal case would be that the plane body and exhaust
fumes have the same temperature as the surrounding air,
making it blend with its background (seen from the detectors
point of view.
Heat detection is often used in missiles which can
lock themselves on the hot jet-engine exhaust and thus
flying themselves directly into the planes most vital
part. The Sidewinder is a good example of such a missile.
2.4 Turbulence detection
Shape also has a lot to do with the `invisibility' of
stealth planes. Extreme aerodynamics keep air turbulence
to a minimum. Rumors are heard about sophisticated laser
controlled turbulence sensors, which can measure paths
of disturbed air, generated by an aircraft which just
passed.
2.5 Visual detection
Reducing smoke in the exhaust is accomplished by improving
the efficiency of the combustion chambers. Getting rid
of contrails - the white line in the sky caused by high
flying planes - is a harder task however. More about
that later.
2.6 Acoustic detection
A very obvious source of detection is the noise, generated
by jet engines. Several systems have been designed in
the meantime to reduce the sound of jet engine exhausts
to a minimum, making them harder to detect by just measuring
sound waves. But often it is already too late if you can
hear the plane...
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