A decoy (false target) shifts the fight from the sensor to the choice of target: offer the weapon something that looks more real than the real target. Where jamming erases or falsifies information, a decoy adds a complete, physically present alternative target. Decoys exist for every domain — inflatable tanks with corner reflectors and heat sources, naval decoys (chaff/corner reflectors on rafts), air decoys — and in two main active forms for aircraft self-protection, worked out below.
The design principle is always the same: in the threat's measurement dimensions (RCS Radar Cross Section, Doppler, IR signature, position), the decoy must score as more attractive than the platform — and at the same time the platform must suppress its own signature (Stealth). After all, a seeker simply chooses the best target it sees.
Towed decoy
A towed decoy is towed on a cable tens to hundreds of metres behind the aircraft. The decoy carries a DRFM-controlled transmitter (Digital Radio Frequency Memory); the "brain" is in the aircraft and controls it via the (fibre-optic) cable — hence the common term fibre-optic towed decoy.
Why tow? Because it solves the fundamental weakness of the self-protection jammer: an SPJ sits on the target. Any deception transmitted from the platform itself comes from the same angle as the platform — and a weapon with home-on-jam (Home-on-jamming) or good angle tracking still hits. The towed decoy physically moves the transmitter away from the object being protected:
- the decoy transmits stronger, more convincing "echoes" than the aircraft reflects → seekers and trackers choose the decoy;
- at seeker range the angle between decoy and aircraft is large enough for the missile to detonate at the decoy — hundreds of metres behind the aircraft;
- home-on-jam becomes pointless: in that case too, the weapon flies to the cable, not to the cockpit.
Limitations: the decoy restricts manoeuvrability (the cable!), is an expendable item that is lost in hard manoeuvres or when hit, covers mainly the rear hemisphere, and still has to get its power and waveforms from the parent platform. Well-known examples in the open literature: the AN/ALE-50 and the fibre-optically controlled ALE-55.
Expendable Active Decoy
An expendable active decoy (EAD) is a throwaway mini-jammer the size of a flare cartridge: ejected from the countermeasures dispenser, with its own battery, receiver and DRFM transmitter. During its short life (seconds to tens of seconds, falling freely or slowed by a parachute or small rotor) it behaves like a credible radar target: it receives the pulses of the threat radar and sends back amplified, manipulated copies.
A comparison with its passive cousin (chaff, Chaff) and its towed brother:
| Chaff | Expendable active decoy | Towed decoy | |
|---|---|---|---|
| Mode of operation | passive reflection | active retransmission (DRFM) | active retransmission (DRFM) |
| Doppler | slows down immediately → suspicious (Challenges for chaff) | can transmit any desired Doppler | follows the aircraft |
| Duration | seconds – minutes (cloud) | seconds – tens of seconds | as long as it is towed |
| Number | dozens of cartridges | several cartridges | 1–2 per mission |
| Strength | cheap, proven | defeats Doppler discrimination | continuous protection |
The strength of the EAD lies exactly where chaff fails: a pulse-Doppler radar or modern seeker filters out chaff on velocity (Challenges for chaff), but an EAD transmits its Doppler and can therefore mimic a kinematically perfect aircraft — if need be, one that gradually "breaks away" from the real platform to take the track with it (an outsourced RGPO/VGPO). Naval variants (active off-board decoys on rafts or drones) do the same against anti-ship missiles.
Employment is a matter of timing and combination: EADs are typically launched on an RWR/MAWS warning (EW: Electronic Support Measures), combined with a manoeuvre and possibly chaff — in the same second the seeker is offered a better target and loses the real one from its gates.