Introduction
Chaff is a cloud of thousands of wafer-thin, metal-coated fibres. Each fibre is about half a wavelength long and therefore reflects radar strongly: on the radar the cloud looks bigger than the aircraft or ship itself.
In practiceWarships fire chaff rockets to create a bigger, more attractive "ship" next to them for the radar seeker of an anti-ship missile, and then turn away. Aircraft dispense chaff in combination with a hard turn to break the lock of a fire-control radar.
Chaff is the oldest radar countermeasure (1943, "Window") and still ubiquitous: cartridges full of thousands to millions of wafer-thin, metal-coated (glass) fibres. Once ejected, the load unfolds within a fraction of a second into a cloud that reflects radar energy powerfully.
The physics: each fibre acts as a dipole resonator — maximum reflection when the fibre is about half a wavelength long. By mixing fibres of different lengths, one cartridge covers several radar bands. A few hundred grams of fibres can thus produce an RCS of tens of m² (Radar Cross Section): on the display, larger than the aircraft or ship that ejected it.
Forms of employment:
- Self-protection / break-lock: a salvo of chaff just behind the platform + a hard manoeuvre: the tracker's range/velocity gate (Target tracking by radar: the Target Tracking Radar) is suddenly offered a much stronger target and stays on the cloud while the platform turns away — the passive counterpart of RGPO.
- Corridor/saturation: laying long ribbons of chaff to blot out whole sectors or routes for surveillance radars; masses of false targets saturate the processing.
- Distraction/seduction (naval): laying clouds next to the ship that offer an anti-ship missile's seeker a larger, closer target — combined with the ship turning away.
Challenges for chaff
Chaff is cheap and reliable, but its weaknesses have been systematically exploited by radar designers since the 1970s — which is why chaff on its own has long ceased to be enough:
- Doppler gives it away immediately (the main weakness). Fibres have practically no mass: the cloud slows down to wind speed within a second. A pulse-Doppler radar or modern seeker (Pulse-Doppler radar and Moving Target Indicator) sees one target at 250 m/s and one at 10 m/s — and rejects the cloud automatically. Velocity discrimination has largely dethroned chaff as a break-lock means against modern systems (hence the active decoys of Expendable Active Decoy).
- Bloom time. The cloud needs half a second to a few seconds to unfold to its full RCS. Against a seeker at short range that can be just too late; timing (controlled by RWR/MAWS, EW: Electronic Support Measures) is everything.
- Wind and separation. The cloud drifts with the wind; the platform does not. The tracker only has to wait until target and cloud separate in range/angle — good trackers "coast" through the chaff (Introduction).
- Limited duration and coverage. Airborne chaff sinks and thins out; against long engagements or repeated lock-ons many cartridges are needed, and the supply is finite.
- Self-interference (fratricide). Chaff also disrupts friendly and allied radars and can actually mark corridors; large-scale use requires deconfliction with the friendly air picture and air traffic.
- Polarisation and resolution tricks. High range resolution (Range resolution) sees that the cloud has a diffuse, smeared-out profile where a real target is compact; polarimetry (Polarisation of the electromagnetic wave) recognises the chaotic polarisation behaviour of a cloud of fibres.
The modern role of chaff is therefore one layer in a layered package — at the same time as manoeuvre, jamming and/or active decoys, to load the threat on several axes at once. As cheap expendable ammunition that gives every seeker at least something to choose for a moment, chaff remains indispensable.