Shoot the messenger--with electrons; as recent military campaigns in Afghanistan, Yugoslavia and Iraq have shown, the enemy's command and control facilities are an early target for attack. Front-line forces unable to receive orders and commanders unable to contact the units they command face inevitable defeat. Communications jamming is an important tool in creating this electronic `fog of war'. (Electronic Warfare).

Like many US defence assets, the Grumman EA-6B Prowler electronic-warfare aircraft was committed to action in the war against al-Qaeda and the Taliban in the late autumn of 2001, but soon found itself without a job. Afghanistan had little in the way of advanced air defences, and the limited number of radar and Sam sites were high on the US targeting list. By 9 October, General Richard B Myers could announce that, "essentially we have air superiority over Afghanistan".

With most of Afghanistan's air defences destroyed, Prowler crews soon found a new target for the Northrop Grumman AN/ALQ-99 jamming suite carried by their aircraft. Although this had been developed to jam enemy radars operating over a wide range of frequencies thought to extend from 30 MHz to 18 GHz or more, its band 1 and 2 capabilities (reported to cover from 30 MHz to 1 GHz to cope with Russian VHF and UHF radars) proved effective when used against communications systems operating at VHF and UHF frequencies.

The aircraft already had a jammer specifically designed for use against communications systems. Developed by Sanders (now BAE Systems North America), the AN/USQ-113 was upgraded in the late 1990s, being fitted with new receivers, power amplifiers and transmitters which extended the frequency range of the system. A new signal recognition subsystem allows aircrew to analyse enemy signals, while a new Windows-based operator display makes the system easier to use.

According to a report in Aviation Week & Space Technology, the ALQ-99 could deliver more power than the AN/USQ-113, while its extensive frequency coverage allowed the EA-6B to split the communications jamming task with the EC-130H Compass Call jamming aircraft. EA-6Bs were flying missions lasting up to six or seven hours, during which they took up positions which allowed them to jam directional communications links.

Modus Operandi 

The primary task of a military communications jamming system is to disrupt an enemy's voice communication and datalinks. Given wide enough frequency coverage, secondary targets can include civil radiotelephony services, citizen-band radios, the radio systems used by paramilitary or police services and some FM radio stations. During the Cold War era, many communist governments used ground-based jammers to prevent their population receiving Western radio broadcasts.

Jamming of a communications link is achieved by transmitting a signal at the same frequency as that used by the enemy communications transmitter, and powerful enough to upset the operation of the communications receiver by submerging the wanted signal in some form of interference signal.

Jamming power is limited, so the effects of the jamming are restricted to a geographic area where the jamming signal will be strong enough to prevent reception of radio signals. If a large area is to be jammed, the task may have to be shared by multiple jamming systems.

As with radar jamming, the simplest jamming technique involves transmitting a noise signal. More sophisticated techniques used for communications jamming include noise-modulated FM, noise bursts, CW tones (spot jamming), and swept signals (swept-spot jamming). One form of deception jamming uses previously recorded signals.

The effective radiated power (ERP) of the jammer depends on output power and antenna gain, and should be high if maximal jamming efficiency is required. Since the jammer will probably be some distance from the receiver or receivers it is trying to jam, jammer effectiveness can be maximised by keeping this distance to a minimum, and by minimizing path loss (attenuation of the jamming signal as it passes along the route to the target receiver).

Spot jamming makes the most effective use of a jammer's power by concentrating the radiated energy on a narrow band centred on the enemy transmission. This requires prior knowledge of the frequencies that the enemy will use, or the availability of an automatic system or an operator to monitor the range of frequencies covered by known enemy systems and identify the transmissions which must be jammed.

Barrage jamming covers a preset range of frequencies, but dilutes the energy of the jammer over a large number of potential channels. It affects enemy and friendly signals alike, so must be used with care.

Another technique that allows the full power of a jammer to be used against several targets is time-division multiplex jamming. The jammer switches rapidly from one frequency to another, exposing each of the target channels to a brief but regular burst of jamming. To the layman, it might seem that under such an attack, each enemy channel would be able to communicate for most of the time, but in practice the circuitry within a receiver being jammed takes a finite amount of time to recover from the effects of the jamming. During a presentation to the technical press in 1998, DaimlerChrysler (part of Eads) likened the effect to that of trying to have a conversation with a blacksmith who is hammering a piece of iron. Before the ear has recovered from the noise of one hammer blow, it will be exposed to the next.

To assess the effectiveness of jamming, many systems use a technique known as `look-through'. This involves briefly interrupting the jamming, then using a receiver to monitor the channel under attack. If the enemy transmitter has ceased transmitting or has shifted to an alternate frequency, the jammer can be retuned or even reassigned to another target.

A frequency-hopping radio system jumps from one channel to another, transmitting a short burst of signal at each frequency for a set period of time known as the dwell time. The hopping sequence must be agreed between the transmitter and the receiver(s), and all participants in the network must be synchronised. Slow-hopping radios change their frequency at a rate of 50 to 500 hops per second, while fast-hopping systems can use rates of 1000 hops per second or more.

If the jammer does not have enough power to jam the entire band over which the target is hopping, it needs to be able to rapidly relocate the transmission after each hop, re-tune to the new frequency, then transmit on that frequency for long enough to create enough error bits to make the signal unusable. According to a recent paper on the vulnerability of cell-phone systems, roughly 20 per cent of the bits should be in error in order to disrupt a very error-resistant signal. In the case of a slow hopper working at a rate of 100 hops per second, the system dwells on each frequency for approximately 10 ms, so the jammer must locate the signal, re-tune, and raise its output power to the necessary level in less than 8 ms.

Jammer designers are rising to the challenge of creating `hop-along' systems able to scan the entire band in search of the new frequency of a fast-hopping radio, and start to jam again. In his 1999 book Electronic Warfare in the Information Age, D. Curtis Schleher says that a hop rate of 10,000 hops per second is needed to counter sophisticated fast-follower military jammers.

Any sort of jamming campaign must be carefully co-ordinated with friendly users of the RF spectrum, and with friendly comint (communications intelligence gathering) units. The need to deny an enemy use of tactical or strategic communications must be weighed against the intelligence information which can be obtained by eavesdropping on such communications, comint also serves the role of locating the various stations which made up the targeted network.

The Variety

Jammers can be land-based, fitted to warships, or carried by aircraft or drones. The text that follows will describe some examples of equipment currently in service or under development, to illustrate the range of hardware and capabilities.

The most common configurations for land-based communications jamming systems are shelter and vehicle installations. Being transportable or mobile, these can be moved to an operating location as close as possible to the network or networks to be jammed.

Several Nato nations operate the Thales International Rhino transportable HF-band detector-jammer. This covers 1.5 to 30 MHz, and has an output power of 1 kW. Rhino is a spot jammer that uses time-sharing to provide a multi-channel capability. It can be deployed against frequency agile, burst or frequency-hopping communications networks, and will cope with hop rates of up to a few tens of hops per second. The system can operate in a local mode under the control of an operator, or in unmanned remote mode in which one or more units operate under the control of a central control facility. The system uses 12 metre whip and V-slope antennas, so can handle both ground and sky wave transmissions.

DaimlerChrysler Aerospace (now Eads) has developed a series of communications jammers. At least two Nato nations use the Hummel, a 20 to 80 MHz multi-channel system able to simultaneously jam up to ten channels. An integrated ESM system detects and identifies target emitters, and selects suitable jamming modulations. Germany uses a vehicle-mounted variant, which is reported to be the subject of an upgrade programme, while Spanish systems are shelter-mounted.

Work on the German upgrade started in 1999, and adds a wideband direction-finding antenna system located on a twelve-metre erectable mast. Other changes include an emitter classification and identification function, and the addition of a Kess mission-planning tool devised by SEL.

More recent systems from the same company cover a wider range of frequencies, and can jam VHF and UHF threats, including satellite communications, cellular phones, drone datalinks and the GPS and Glonass satellite navigation systems. The SGS 2300 H, V and U series are shelter-mounted systems for use against HF, VHF and UHF voice and datalinks. All have a high output power (typically around 1 kW), and incorporate an automated surveillance sub-system. They are capable of rapid frequency-hopping to deal with modern communications links.

Tadiran Electronic Systems Tactical Automatic Communications Jamming System (Tacjs) was designed for mobile use, either within an armoured personnel carrier or in a Shelter mounted on a high-mobility wheeled vehicle. This is a power-managed 2 to 1000 MHz system which can be deployed in stand-alone mode, or netted to a central control facility. Power output depends on frequency, and ranges from 0.5 to 2 kW.

Smaller systems are available in man-portable form, so can be carried close to the enemy networks they are intended to disrupt. The Pacjam portable countermeasures system which BAE Systems North America has supplied to an unidentified American Government agency is made up of three units (a receiver/transmitter unit, battery pack and antenna) and weighs around 30 kg. The jamming power from such small systems is limited, but the Pacjam produces around 100 W at frequencies ranging from 100 to 500 MHz. It can transmit intermittent jamming for up to three hours.

The Bulgarian company Kintex devised its R-047 Shturets 20 to 100 MHz VHF barrage jammer in a modular form. Individual sub-units such as the transmitter, VHF transceiver, control panel and directional or non-directional antenna can be combined to create several operational configurations. In the field, it can be remotely controlled via the VHF transceiver or using a wire link and a control panel.

Expendable jammers are even smaller and lighter. After being deployed, they can be activated and left to operate independently. An example of this low-cost approach is the Thales Communications BLB 20, a 4.3 kg unit able to generate 20 W of jamming power at 20 to 110 MHz for up to three hours. Since the unit must operate autonomously, it must be programmed by means of a data-fill device.

Communications jamming payloads can also be fitted into artillery rounds. Russia's Splav State Research and Production Enterprise offers 1.5 to 120 MHz jammer payloads for the 122 mm BM-21 multiple-launch rocket system. This system was developed in co-operation with a Bulgarian manufacturer, almost certainly Kintex. The latter also offers its own range of artillery-launched jammers, such as the Lilia series of 1.5 to 120 MHz jammers that can be delivered by 122 mm BM-21 rockets or 152 mm guns. Kintex products also include the 20 to 100 MHz Starshel 122 barrage jammer for 122 mm guns, and a similar Starshel 152 for 152 mm guns, the D-20 and ML-20 towed howitzers. All of these artillery-delivered systems have a transmitting time of one hour or more.

At Sea

Warships can also be equipped to disrupt enemy communications. The electronic warfare suite on the three F3000S frigates for the Royal Saudi Naval Forces (RSNF) includes a Thales Altesse communications-band ESM system and the TRC 281 communications jammer, and Eads is reported to be considering the use of technology from its SGS 2000 for naval applications. Altesse can also be linked to the Thales RRC 274 HF/VF/UHF-band digital communication jammer.

In some cases, a jamming capability is offered as an optional extra for a comint system. Indra DTD's shipboard SCR-390(V) is an example of this approach, consisting of a 30 MHz to 1 GHz surveillance system which can be linked to an optional subsystem able to generate noise jamming or to transmit `spoof' traffic.

Designers are creating systems that are suitable for fixed-site, ground-vehicle, shipboard or airborne use. This is the approach that Elta Electronics took with its EL/K-7000MT series of jammers. Fast broadband receivers are used to intercept enemy signals and to `look through' the jamming to judge its effectiveness, while the jamming facilities can employ a range of operating modes, optimising the jamming output to match specific targets. Frequency range is 1 MHz to 1 GHz, but can be extended if required. Output power is from 200 W to 1 kW or higher.

The UST-107 Command and Control Warfare system being developed by Rockwell Collins is another multi-platform system, and covers frequencies from 500 MHz to 2 GHz. As its title suggests, this is not just a jamming system. It consists of 95V-1 surveillance receivers, 95E-1 jamming exciters, and RF power amplifiers, all under the control of a computer.

It can operate in communications mode (acting as a transceiver for voice and data), in ES/Surveillance mode (detecting and if necessary recording signals from enemy emitters, and in reactive jamming mode) with its output power-managed to focus on the highest-priority targets, then switching to lower-priority once the most important targets have been forced to shut down.

A multi-platform capability is also planned for the Tarax portable VHF communications jammer that TRL Technology has developed for Sweden's FMV defence procurement agency. It is suitable for vehicle use, but could be adapted as a drone payload. Tarax can jam up to five bands simultaneously, and each band can be up to 10 MHz in width.

Mounting the jammer in an aircraft, helicopter or drone, has two advantages. Firstly, the jammer may be able to get closer to the target system than would be possible with a ground-based jammer. Secondly, the jammer may achieve a direct line-of-sight transmission path to the target, while the signal of the ground-based enemy transmitter may experience significant levels of path loss.

The US Air Force's numerically-small fleet of EC-130H Compass Call communications jamming aircraft have been heavily used in Afghanistan, and in other recent conflicts which have involved US forces Recognisable by the presence of a large vertical aerial above the fuselage, and two underwing antennas, the EH-130H entered service in 1982 with the 41st Electronic Combat Squadron.

Since then, it has been regularly upgraded to improve its jamming performance, and its ability to cope with multiple threats. The current operational standard is believed to be the Block 30, but this is giving way to the Block 35 and Block 40 standards. Conversion of around six aircraft to the Block 35 standard was due to begin in fiscal year 2000. This upgrade will replace existing analogue systems with digital systems and add the Tactical Radio Countermeasures Systems (Tracs) and a digital compressor.

South Africa operates several types of communications-jamming aircraft. The Boeing 707 SOCJ (Stand-Off Communications Jammer), the Douglas C-47TP and the EW version of the Denel Oryx transport helicopter are all equipped with the Grinaker Systems Technologies (GST) GSY 1501 jamming system.

Helicopters are also used as jamming platforms. Best-known example is probably the US Army's Sikorsky EH-60A Quick Fix II aircraft. Created by Tracor Aerospace in the late 1980s by modifying a batch of UH-60A transport helicopters, these carry the TRW AN/ALQ-151(V)2, a direction-finding, intercept and countermeasures jamming suite which uses four dipole antennas mounted on the fuselage of the helicopter, plus a deployable whip antenna. Designed to cope with AM, FM, CW, and SSB threats operating at 2 to 80 MHz and with bandwidths of 8, 30 or 50 kHz, it has a power output of 500 W. Under present plans, the EH60A is due to be withdrawn from service by the end of 2005.

Although a number of EW-dedicated aircraft were developed by the former Soviet Union, the only types known to have a communications jamming role are EW variants of the Mi-8 and Mi-17TPB helicopters. At least nine EW versions of the Mi-8 have been reported, along with at least four Mi-17 models, but it is not clear which of these are equipped for communications jamming as opposed to the comint, elint and anti-radar roles.

The Taran airborne EW suite developed by the Spanish company Indra DTD is unusual in having a low band designed to jam enemy tactical communications, plus a 0.5 to 2 GHz high band for use against radio navigation aids and Identification Friend-or-Foe (IFF) systems.

While many types of drone are offered in EW variants, the models mentioned below are those for which a communications jamming role has been reported.

Drones

The US military has shown significant interest in communications jamming payloads for drones; some of the 500 or more BAI Aerosystems BQM-147A Exdrones delivered have been configured as communications jammers. The TRW/IAI RQ-5A Hunter is primarily used for reconnaissance, but a communications jammer was one of the alternative payloads tested by the US Army.

As part of its planned Prophet 20 MHz to 2 GHz sigint system, the US Army proposes to mount comint and jamming subsystems into a drone such as the AAI Tactical drone (Tdrone). A drone-compatible Northrop Grumman jammer has already been demonstrated, while BAE Systems was asked to provide a similar payload for trials aboard a Hunter drone.

One of the first practical results of the proposed Franco-German Bussard EW programme was the MBDA Dragon concept validation testbed. Flown for the first time in 1995, this used a Meggitt Spectre drone modified by TTL France. The mission payload was a Thomson-CSF (now Thales) Bacarat jamming system whose twin antennas were mounted on the port side of fuselage and deployed to the vertical location above and below the nose during flight. Having been operationally tested, Dragon could be manufactured should a requirement emerge. The definitive Bussard is expected to enter service around 2010.

The Meggitt Spectre airframe is also used by the Sagem Crecerelle (Kestrel) drone. Originally developed for surveillance and target acquisition duties, this entered service in the mid-1990s. In June 1999 the DGA ordered a batch of EW Crecerelles. These will be equipped with the Thales Bred VHF/UHF tactical communications jammer.

Originally developed to meet a German-American requirement, then a German national requirement, the Eads Dornier Dar drone fell victim to budget cuts in 1992. It is still being offered as a multirole drone, whose payloads could include a high or low power VHF jammer.

Like the STN Atlas Taifun attack drone on which it is based, the same company's Mticke (Mosquito) is still in development. It will carry an Eads jammer designed to counter radio networks operating at 20 to 500 MHz.

The concept of drone-based communications jamming systems is also attracting interest in Russia, where Yakovlev is reported to be developing a dedicated version of its Pchela drone. Another system designed Moshkar has been reported; this uses GPS navigation and up to 32 can be controlled simultaneously by a single ground-control station.

Bulgaria operates a jamming variant of the Yastreb-2MB aerial target. The payload consists of an AJ-045A jammer said to be able to jam radio receivers up to 10 km from the drone.