Chaffs

The two primary chaff employment tactics are force screening and self-protection. Different chaff dispensing techniques are used for each employment tactics. Force screening tactics include area saturation and corridor operation:

• The objective of area saturation operation is to present multiple false targets in a specific area in order to saturate radar systems and confuse the enemy air defence. Area saturation can be accomplished by fighter aircraft or drones equipped with chaff pods exploiting random chaff dispensing techniques. The chaff dispenser is set to release random bursts of chaff along the ingress and egress route of the attack package.
• Corridor chaff is a support tactics in which chaff dispensed from an aircraft at a steady rate over a long period is used to form a corridor that conceals the following aircraft. A series of partially overlapping chaff clouds are released by dedicated planes to form chaff corridors through which attacking planes will pass hided from the radar of the victim system.
  In some scenarios, the use of corridor chaff can be massive. The rate at which the chaff is released is selected in order to create a continuous corridor for the attacking aircraft.

Figure 1: Corridor Chaff

Self-protection tactics include the reactive employment of chaff to negate a potential lethal engagement. The goal of the self-protection chaff is to create a corner reflector deception by having the missile seeker angle tracking transferred from the target to the chaff.

The chaff can be dispensed in packets both ahead and behind the plane. The effectiveness of the chaff technique can be increased if, prior and in coordination with the chaff deployment, the defending aircraft uses some ECM techniques that induce the victim seeker to transfer range and/or velocity tracking from the aircraft to the chaff:

• Range gate pull-off (RGPO)
• Velocity gate pull-off (VGPO)
• Coordinated range / velocity gate pull-off (CRVGPO)

Cooperative target manoeuvres that introduce changes in the update rate of angular tracking or reductions in radial Doppler can be executed at the same time as the ejection of the chaff to favour the effectiveness of this tactics.
A further cooperative on-board/off-board technique is JAFF (Jammer illuminated chAFF) that has the goal to force the victim seeker to detect, chase and drive on the jamming signal reflected by a newly emitted chaff cloud rather than on the target’s true echo.
The JAFF tactic consists of an aircraft that dispenses chaff into one or more bundles and simultaneously illuminates it with thejamming signal. This technique is also known as CHILL (CHaff ILLumination) and is a combined off-board (CHAFF) on-board (on-board-jammer) technique.

As a practical example, a chaff cloud was launched to obtain angle deception.
Aircraft radars often use a continuous-wave Doppler mode to reject chaff decoys from actual aircraft targets (as chaff are merely floating in the wind, compared to a fast-moving aircraft). Therefore, the countermeasure could be not effective.
Illuminating the chaff cloud with the on-board jamming using VGPO, can give to the chaff cloud a credible Doppler, and both Velocity deception and Angular deception are obtained and the combined technique would be effective at defeating the radar.

When the chaff is used in self-protection (airborne) applications the dispensers must be quick-reaction devices that eject relatively small quantities of chaff in controlled bursts. This is commonly achieved by using cartridge fitted with pyrotechnic squibs, where a programmable control unit fires electrically the squibs. Self-protection chaff cartridges typically contain 100 g to 150 g of chaff carried in modules of 30 cartridges. At least two modules are normally carried. Alternatively, mechanical dispensers can be used, where individual packs are ejected in short burst from an assembly of long tubular magazines.

Chaff is used also for self-protection of naval ships against radar-guided anti-ship missiles. The principles of chaff protection for ships are generally the same as for aircraft, except that the large RCS of ships makes the timing and implementation of the naval mission more difficult. For naval use, chaff is most commonly ejected from rocket, shell or mortar systems.

In most navies, there are three main modes of use of chaff at sea:

• Distraction decoys that are dispensed by rockets and shells at ranges up to 2 Km from the vessel, and a pattern of several rockets fired in different directions is used to provide alternate targets to missiles when they are still at some distance from the vessel.
  The decoys must last for several minutes, and if the threat is still present, more decoys are periodically spread.
• Dump mode that is used closer to the vessel (e.g. 1 Km away), where active EA jamming is used to deny range information to the seeker and in conjunction with chaff fired from a rocket, to lure the attacking missile away.
• Centroid mode: the chaff cloud is dispensed at a range of about 100 m to 400 m. A large echoing are must be realized within a few seconds of firing the chaff near the ship. The ship then moves quickly out of, and away from, the chaff echo and the missile is lured away, thus avoiding a direct hit.