Penetration aids
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Chevaline Penetration Aid Carrier (PAC) on display at the Bristol Aero Collection [1] at RAF Kemble The warheads lay in recesses on the left and right of the bus, which are not obvious in this image. The silver tubes near the top and bottom (pointing fore and aft), some wrapped in cork sheet, contained the decoys. The decoy tubes were capped and were opened by the solenoid operated latches visible, before the decoys, numbering 27 in total, were rocket-ejected.

A penetration aid is a device or tactic used to increase an intercontinental ballistic missile (ICBM) warhead's chances of penetrating a target's defenses. These can consist of both physical devices carried within the ICBM, as well as tactics that accompany its launch, and may include:

• chaff: Chaff wires may be deployed over a large area of space, creating a large, radar-reflecting object that will obscure incoming warheads from defensive radar.
• decoys: Decoys consist of mylar balloons that can be inflated in space and are designed to have the same radar characteristics as the warhead. Because the warhead and the decoy balloons may be at different temperatures the warhead and the balloons may both be surrounded by heated shrouds that put them all at the same temperature. This defeats attempts to discriminate between decoys and warheads on the basis of temperature, which can confuse an enemy's missile defense systems.
• Incidental or deliberate fragmentation of the final-stage rocket booster can cloud the enemy's radar by projecting a radar cross-section much larger than the actual missile.^[1]
• radar jammers: These are transmitters that can be deployed on the decoys and the warhead to jam the frequencies used by defensive radars.
• nuclear radar blackout: A nuclear device may be deliberately exploded in space by the attacker in order to provide a radar blackout that will allow subsequent warheads to pass through the enemy's defenses undetected.

References

1. ^ Bethe, H. "Countermeasures to ABM systems" in "ABM: An evaluation of the decision to deploy an anti-ballistic missile system", edited by Abram Chayes and Jerome Weisner. Published by Macdonald (London) in 1969.