Electronic Attack. EA Against Communication Signals

EA, which is also known as Electronic Countermeasures, involves actions intended to degrade the ability of an adversary to make use of the electromagnetic spectrum. It may be active or passive in nature.

EA Against Communication Signals. EA against communication signals can be carried out as deception operations or jamming.

Deception operations involve the transmission of signals to mislead the enemy intentionally. For example, after a ground formation has been redeployed for operations elsewhere, simulated radio traffic may be maintained to give the impression that the formation is still in its original location. Another technique involves the transmission of messages that contain misleading information in the expectation that the message content will be recovered and used by the adversary.

Deception operations must be carefully designed and organized to be convincing; the information provided should be consistent with other information that the intended recipient believes to be true. Large-scale deception operations that involve carefully coordinated activities can influence an adversary’s strategic planning with decisive effect. Several accounts of highly successful Allied deception operations in WW2 have been published (5, 28).

Jamming is intended to prevent an adversary from reliably receiving his communication signals by the transmission of signals that interfere with their reception. In the simplest form, a jammer consists of an antenna, power amplifier, and signal generator programmed to produce a signal with an appropriately chosen waveform. It is also possible to use a conventional transmitter or radio as an improvised jammer. Jamming systems are often deployed with an adjunct ES capability to ascertain the frequencies of signals worth jamming and to assess the effects of the jamming operation.

To be effective, jamming requires that the ratio of jammer and communication signal powers (J/S ratio) at the victim radio receiver be sufficient to degrade communication activity adequately. High-power transmitters may be used in combination with directional antennas and the judicious positioning of the jammer near the area where jamming coverage is desired.

Several distinct types of communication jamming techniques are as follows:

Narrowband Jamming. Individual communication signals can be attacked by transmitting an appropriately designed narrowband-jamming signal on the frequency used by the target signal. To determine whether the target signal is still being transmitted, the jamming may be periodically stopped and an ES capability used to check for the presence of the signal.

This method of attack has several advantages. First, the jamming range is maximized because the full jamming power is focused on a single signal. Second, the likelihood of interference with own side communication is minimized because only a small part of the radio spectrum is affected. If the jamming signal can be switched rapidly between frequencies, then a single transmitter may be able to jam two or more narrowband signals on a time shared basis.

A follower jammer is a special case of narrowband jammer used to jam a FH signal. The practical implementation of the concept is challenging; each hop transmission must be detected, its frequency measured by the ES functionality integrated with the jammer and, before more than a fraction of the hop is transmitted, the jamming transmitter must be tuned to the hop frequency (29).

One difficulty is that the jammer must discriminate reliably between the hops from the target transmitter and any other frequency hopping communication systems that may be operating in the environment. A more fundamental issue concerns the propagation delays associated with, first, the path from the transmitter to the jammer, and, second, the path from the jammer to the victim receiver. If the end result is that the overall delay, including the jammer response time, approaches the hop duration, then the effectiveness of the jamming will be degraded.

Barrage Jamming. A wideband jamming signal is used to degrade communication activities over a relatively wide range of frequencies. A high-power jammer may be needed to provide a useful range. A partial-band jammer is a variation on the barrage jammer concept. The aim is to jam a bandwidth that is sufficiently large enough to include a sufficient proportion of the hops transmitted by a FH radio to make it unusable. The idea is that, by not attempting to jam the full bandwidth used by the frequency hopping radio, the jammer power within the hop bandwidth can be kept higher and can provide an increase in the effective range of the jammer.

Many issues must be considered with respect to communication jamming:
1. Jamming often interferes with own side communication;
2. The value of information that is obtained by ES may be considered to be of greater military value than the effect of disrupting communication;
3. An adversary can infer the presence of enemy forces with EW capabilities from the observation of jamming signals and, if given time, may find ways of countering its effects.

Consequently, aside from some specialized applications, the decision to carry out communication jamming is usually made at a relatively high level and is closely coordinated with operational plans.

The deployment of communications jammers on aircraft provides several advantages. The jammer is mobile and can be positioned quickly to affect the desired area while minimizing the effect on friendly forces. Also, the required transmitter power can be reduced because, for a given range, the propagation losses are normally much lower than they would be for the signals from a ground based jammer. Recently, serious interest has been expressed in the idea of using low-power communications jammers on small unmanned air vehicles (UAVs) to provide localized jamming coverage in the direct support of small-unit operations (30).

EA Against Radar Signals. EA against radar signals is often concerned with degrading the performance of surveillance, target acquisition, and target tracking radars to protect platforms such as aircraft and surface ships. The value of these platforms and the potential effectiveness of radar-guided weapons has led to much emphasis being placed on EA.

Active EA techniques are used to create false targets or otherwise degrade the operation of the victim radar:

1. A noise jammer transmits wideband noise in the frequency ranges used by radar systems of potential concern, which makes it difficult for the radar to detect the target and get a range measurement;

2. A range gate pull-off jammer attempts to create a false target that seems to move away from the jammer platform. The jammer first creates a false target at the jammer platform by transmitting a pulse timed to coincide with the arrival of each pulse transmitted by the victim radar. The timing of successive pulses is gradually shifted so that the jammer pulses received by the victim radar correspond to a target that is moving away from the jammer platform.

The digital radio frequency memory (DRFM) improves the technique by storing and transmitting a replica of the radar-pulse waveform. This method makes it more difficult for the radar to discriminate against the jammer signal. Several practical problems are noted in the deployment of jammers. The operation of jammers used for the self-protection of platforms, such as aircraft, is usually restricted to the jamming of threat signals as required. This method minimizes several risks, which include the possibility of interference with other systems on the platform, and that the presence of the platform can be inferred by the detection and direction finding of signals transmitted by the jammer.

In this situation, an integrated ES capability for performing the detection, characterization, and assessment of threat signals is required to provide information needed for the control of the jammer. One way of sidestepping this issue is to deploy jammers on specialized platforms, and if possible to perform the jamming outside the defended air space. Other solutions include the towing of jammers behind the platform to be protected, or deploying jammers on UAVs.

Passive EA techniques attempt to degrade the effectiveness of enemy radars without transmitting signals. A widely used idea is to create false targets by dropping chaff (typically metal coated plastic strips) from aircraft to confuse tracking radars associated with antiaircraft defense systems. Chaff can also be dispersed via rockets or shells fired from platforms such as ships as a countermeasure to radar-guided missiles. Another approach is to tow decoys behind an aircraft or ship.

The use of passive EA to confuse the guidance systems of antiaircraft or antiship missiles is often combined with maneuvers designed to position the platform to minimize the likelihood that the missile-guidance system will reacquire its target or that the missile will fortuitously pass near its target. Another form of passive EA concerns the use of stealth techniques to reduce the reflected energy returned to a radar transmitter by a platform (i.e., reduce the apparent radar cross section of the platform). The effectiveness of this technique is increased if combined with active EA from other platforms.

Other forms of EA are also important. Radar systems can be destroyed by missiles designed to home in on the signals transmitted by the radar. Conventional military operations against deployed systems identified by EW sensors or other intelligence are also possible. Recently, the concept of using directed energy or electromagnetic pulse (EMP) to damage or disrupt the operation of electronic equipment has received attention.