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RADAR. One of the widely used detection system.

These consist offending bursts of radiation 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. 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.

Passive radar emitters[]

Another way of detecting aircraft are passive radar emitters, these detect aircraft passively, as they just receive the radar waves given off by an aircraft. Ways to prevent being detected by passive radar emitters are LPI (Low Probability of Being Intercepted) radars, featured in aircraft such as the F-22 Raptor or F-35 Lightning II. The APG-77 of the F-22 Raptor changes frequency over 1000 times a second and does only use as much energy, as necessary to detect the enemy, thus it can avoid detection by passive radar emitters effectively.

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.

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.

Heat Detection[]

Another way of detecting if an aircraft is flying somewhere is by measuring the heat it radiates. Normally this heat


InfraRed Detection

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.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.

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.

Visual Detection[]

One of the most simple ways to detect an aircraft is visual detection with the naked eye. 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. 

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...