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Air-Launched Decoy (ALD)

Decoys in all forms have constituted and still constitute an important component for deceiving the many sensor systems of war, anything from the eyes of the individual soldier to the ground or air-borne radar system.

Great efforts have been devoted especially to decoys for deceiving radar systems since the object to be protected, in many cases an aircraft, is of considerable military value. Chaff (bundles of strips) has previously been used as decoy for deceiving radar. If the metallised strips are of a length which is suitably adapted to the radar frequency of the radar that is to be misled, a strong resonance is obtained. The strips that are dispersed from aircraft in bundles then cause echoes that can mislead the radar or conceal the aircraft.

The introduction of pulsed Doppler radar dramatically reduced the capability of chaff to influence the radar. A pulsed Doppler radar uses the Doppler effect (phase variation from pulse to pulse in the radar echo) to distinguish reflecting objects moving fast in relation to the radar station and stationary objects. As a result, ground clutter and also chaff that is almost immobile in relation to the ground can be rejected. The use of Doppler radar systems for rejecting ground echoes therefore renders the capability of the bundle of strips of effective misleading impossible.

Other passive methods for confusing radar use reflectors of different kinds, for instance corner reflectors or Luneburger lenses to produce powerful echoes from small objects. To produce the necessary Doppler frequency that permits detection in a Doppler radar, these must then be hauled or accommodated in small decoy aircraft which can separate from the object to be protected. This requires aerodynamically well designed units and, moreover, in many cases restrictions in the flight appearance.

Modern decoy solutions often consist of active jamming transmitters which are launched from the aircraft or hauled thereby. A pure amplification and transmission of the radar pulse cannot be carried out with isotropic transmitting and receiving antennae owing to insufficient insulation (results in so-called feedback). Other active solutions using, for example, microwave memory and delayed transmission result in distortion of the pulse shape. Narrow band jamming as well as wide band jamming are known. Equipment for jamming by narrow band noise is sensitive to a frequency change of the radar and requires equipment for searching over the frequency band for the new frequency. Wide band noise requires high power output. All in all, active decoys will necessarily be relatively expensive and complicated equipment.

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