See but survive: air-defence radar: surveillance radars are the primary sensors of any modern air-defence system, but are also its most vulnerable component.

See but survive: air-defence radar: surveillance radars are the primary sensors of any modern air-defence system, but are also its most vulnerable component. Recent air campaigns against Iraq and Yugoslavia have shown how radars can be forced to shut down or be destroyed, but have lessons for the defender. A combination of new technologies such as active-array radar and damage-resistant networks, the use of decoys and careful use of emission-control techniques should make future radars harder to locate and destroy. (Electronic Warfare)

Earlier this year, Armada International visited the Golan Heights in northern Israel. From here we had a good view of the demilitarised zone, which lies between the front-line Israeli positions and those occupied by the Syrian army. In the far distance, but concealed by haze, was the Syrian capital Damascus. It is a city which, a few weeks earlier, had been effectively stripped of its air defences when the Israeli Air Force destroyed a Syrian radar station in response to a cross-border attack by Hezbollah.

The radar that had been attacked was a carefully selected target, explained a representative of the Israeli Ministry of Defence. It was a major node of the Damascus air defences, and its destruction had exposed the Syrian capital to the threat of Israeli air attack for the week or so it took to replace the lost facility.

Such a destruction of air-defence radars is not a new development. For more than 50 years, early-warning radars have been singled out for attack.

The biggest problem in ensuring the survivability of surveillance radars is that they are big and thus hard to conceal. Their size is dictated by the task they must perform and the laws of physics.

Keeping an Eye Out

Detecting a target at long range involves illuminating the distant target with a signal that is strong enough to produce a detectable echo. The more powerful the transmitted pulse and the more sensitive the radar receiver, the greater the range at which a target of a given radar cross-section can be detected.

For more than a decade after the Second World War, the frequency and power requirements of radars steadily increased. By around 1960, radars were exploiting the full range of frequencies available for practical use, and their peak and average power had reached levels about as high as those used today. Although techniques such as pulse compression will extract as much power as is possible from the transmitter hardware, in the long run there are unlikely to be major increases in output power. The power available from a travelling wave tube (TWT) -- the most common power source in modern radars -- is limited by practical factors such as a the need for high-voltage power suppliers, and liquid cooling.

At the receiver, the returned signal must co-exist with various forms of noise, and the receiver must be able to detect the echo from the noise background. Some of this noise is generated by the receiver itself, but this internally-generated noise tends to be swamped by the noise caused by unwanted signals arriving at the antenna -- for example, clutter and jamming. Modern signal and data-processing techniques can help detect and reject noise signals. Since these tend to be random, while the true echo signal will be repeated, an automatic plot extractor can accept only those signals which repeat a pre-determined number of times. As with transmitter power, no dramatic breakthroughs in receiver sensitivity are likely in the near future.

To extract the maximum range performance from a given level of transmitter and receiver technology requires the use of the largest practical size of antenna. The larger the antenna, the higher its gain. An antenna with higher gain is better at focussing the transmitter energy into a narrow beam, maximising the power delivered at the target. It will also be more efficient in capturing the faint echo returned by the distant target.

The narrower beam also pays dividends in the form of greater angular resolution. The narrower beam will be more effective at determining the exact bearing and elevation of the target, and better able to discriminate between multiple targets in a distant formation. A narrower beam will also reduce the amount of unwanted noise entering the receiver.

The gain of an antenna -- and thus the width of the main beam -- is directly related to the antenna dimensions. However, as any radar textbook will show, calculations of antenna gain are based on the antenna size expressed in terms of wavelength. Since wavelength is inversely proportional to frequency, the use of the highest-practical frequency will minimise the antenna size.

The key word in this instance is `practical' -- the Earth's atmosphere forces compromises on the radar designer. The main problem is due to the absorption of radar energy by water vapour in the atmosphere. This is negligible at frequencies below 1 GHz but then rises with frequency, and there is a point of very high attenuation at around 22.24 GHz which is caused by resonance.

The two most common ranges of the spectrum used by long-range surveillance radars are 23 cm in the lower region of D-band, and 3 GHz on the boundary between the E and F-bands. These regions correspond to wavelengths of 23 cm and 10 cm respectively.

Russian radar designers have traditionally favoured even lower frequencies such as C-band (0.5 to 1 GHz), B-band (250 to 500 MHz) or even A-band (100 to 250 MHz). Similar low frequencies are also used in some Chinese surveillance radars. The resulting antennas are often massive.

Tactical surveillance radars and the surveillance sets used by surface-to-air missile systems have more modest range requirements, so can use higher frequencies and thus much smaller aerials, but have practical limits in transmitter and receiver technology, and the need to avoid absorption of the radar energy by atmospheric water vapour make it unlikely that the antenna of future long-range surveillance radars will be significantly smaller than today's equivalent models. What is likely to shrink is the size of the radar's electronics.

Just Any Old Shape Won't Do

The traditional form of antenna used by microwave radars is a circular or rectangular reflector illuminated by one or more feed elements. In most large surveillance radars, this has given way to an array antenna, which consists of a large number of individual radiating elements. These may be dipoles, wave-guide slots or horns. The array applies the proper phase relationship to the signal from each element to make the wave front of the radiated signal flat, thus forming a beam.

The relative phase between the original individual elements determines the position of the beam. By giving each radiating element in the array its own phase shifter, the beam can be rotated or steered without moving the antenna. Known as passive electronic scanning, this has the disadvantage of creating a significant loss of power within the phase shifters.

In an active-array radar the antenna is made up of a large number of small transmit/receive (T/R) modules. Within each module, the transmit and receive components are mounted physically close to the radiating elements, an arrangement that virtually eliminates microwave losses. A typical antenna will be made up of several thousand T/R units, each providing about ten watts of RF power. The radar designer can choose waveforms that are most appropriate to the requirements of the beam position, allowing for a clutter, target size and range. Waveforms can be varied on a pulse-by-pulse or burst-by-burst basis.

An active-array radar consists of an antenna and a shelter, or other housing, to hold the processing equipment and power supplies. The RF power is generated in the antenna, eliminating the bulky and heavy transmitter equipment used by earlier-generation radars.

A technique known as digital adaptive beam-forming allows the radiation pattern of the antenna to be varied as required, matching it to the target and tactical circumstances, and dramatically reducing vulnerability to jamming. It overcomes the disadvantages of traditional sidelobe-cancellation schemes and allows for several jammers to be countered at the same time with greater levels of suppression. It also allows one radar to fulfil several tasks, tailoring its beam and scan pattern as required. BAE Systems believes that a single E/F-band radar with an active-array radar measuring two to four metres can provide all the functions needed by a medium-range Sam system.

"Military radar users have already formed preferences for active array radar -- a technology that is expected to dominate military radar procurement within the next five to seven years", says Katrina Herrick, an analyst at Frost & Sullivan. "At present, however, the major problem preventing market expansion for active array radar is its developmental status and subsequent high cost."

Whatever their form, surveillance radars can expect to be hunted down. Since the Vietnam War, the biggest threat to radar sites has been the anti-radiation missile (Arm). Fitted with a passive seeker head designed to detect radar signals, this forces the target radar to shut down or run the risk of being destroyed.

Although the seeker of an anti-radiation missile is of relatively small diameter, and thus limits the size of the seeker antenna that can be carried, in practice this is not a problem. The Arm is tasked with detecting the powerful signal generated by the target's radar transmitter. Although the main beam of the radar antenna is unlikely to be directed towards the .incoming missile, the seeker of a modern Arm such as the AGM-88C is sensitive enough to home on the smaller levels of energy contained in the sidelobes and backlobes of an emitter.

Whether generated by an active-array radar or by a more traditional array, low or ultra-low sidelobes will help make the radar less visible to the seeker of an anti-radiation missile. Lockheed Martin has been offering its ultra-low sidelobe antenna (Ulsa) in designs such as the AN/TPS-75. According to the company, this reduces sidelobe emission by more than 50 per cent and considerably reduces the radar's vulnerability to anti-radiation missiles.

The American air strikes against Iraqi Soviet-produced Tall King and the French Volex early-warning radars on 16 February 2001 used Jsow guided bombs. Although its effectiveness was partly blunted by aiming errors, it showed that certain submunition-dispensers such as the Jsow could be an effective future weapon for use against radar sites whose geographic location is known. In some cases, the submunition pattern from the Jsow was wide enough that rounds which missed their planned aim points were still able to damage radars.

How Not to Be Seen

The oldest technique to preventing attacks on surveillance radars is strict emission control. This can involve operating the radar only for restricted periods, radiating only in specific sectors and shutting down if an Arm is launched against it. However, all military forces are good at this; indiscriminate use of radars by Iraq resulted in the destruction of many in the first few days of the 1991 Gulf War. Once the Iraqis realised that there early-warning and GCI radars were being singled out for destruction, along with Sam sites, they were less likely to transmit, and tried to camouflage or move systems wherever possible.

Yugoslavia proved a tougher opponent during operation `Allied Force' in 1999. The JRV i PVO [Yugoslav Air and Protective Defence Force] and VJ [Yugoslav Army] maintained a high standard of emission control during the 72 hours preceding hostilities, and maintained this once Nato bombing had begun.

The problems which this created for the Nato forces are highlighted by two reports published in early 2001. In Airborne Electronic Warfare: Issues for the 107th Congress, the US Congressional Research Service noted that although the sensors on the F-16CJ aircraft are optimised to find and attack radiating radars, "like the Harm, the F-16CJ has difficulty finding and targeting the radar if the adversary is careful to limit its emissions."

In its January 2001 report Electronic Warfare -- Comprehensive strategy needed for suppressing enemy air defenses the General Accounting Office of the US Congress reported that both sides had faced problems. "During Operation `Allied Force in 1999' ... US suppression aircraft faced Yugoslavia's integrated air defense system and experienced significant difficulty in trying to destroy it, as Yugoslav forces often engaged in elaborate efforts to protect their air defense assets. These protective efforts allowed Yugoslav forces to thwart many attacks, but they also reduced Yugoslav opportunities to track and engage US and coalition aircraft because their air defense assets could not be utilized and protected simultaneously."

If an air-defence system is to practice good emission control yet remain combat effective, the individual radars and control centres need to be networked, and wherever possible the radars should be supplemented by other networked sensors such as passive trackers.

That networking will inevitably become a target, so must be damage-resistant. In 1991, Iraq had a very centralised air-defence system based on Soviet doctrine. It proved highly vulnerable to attack.

The Answer -- Fibre Optics 

The lesson was learned; by the time of the Allied air strikes against early-warning radars and command facilities on 16 February 2001, Iraq was deploying a more damage-resistant network based in part on optical fibre links intended to allow the co-operative tracking of air targets. According to the US, this was being installed with the help of Chinese technicians.

The fibre-optic upgrades were intended to allow Iraqi air-defence systems to use increased cueing of surface-to-air missiles (Sam) by remote radars, and to allow Sam engagements using a technique identified by US Navy sources as `shared-time irradiating'. This technique would allow radars out side the no-fly zones to pass a fire-control solution to Sam batteries operating under the no-fly zone. Individual radars would only track the target for a brief period of time in order to minimise their vulnerability to Arms, but a composite fire-control solution can be developed by linking together track files from several radars, giving positional information accurate enough to allow Sams to be launched. This technique is reported to have been developed by Yugoslavia and was used against Nato aircraft in 1999.

In its January 2001 report Electronic Warfare -- Comprehensive strategy needed for suppressing enemy air defenses the General Accounting Office noted that, "many nations, including some hostile to US interests such as Iraq and North Korea, operate what is referred to as integrated air defense systems. These systems use various means to track and target aircraft, including modern telecommunications equipment and computers to create networks of early warning radar, missile system radar, and passive detection systems that pick up aircraft communications or heat from aircraft engines. Integrated networks provide air defense operators with the ability to track and target aircraft even if individual radar elements of the network are jammed or destroyed."

Mobility allows a radar to be moved to where it is needed in the light of current tactical circumstances, either to cover a new sector, or to replace a system which has been destroyed in combat, but the sheer size of a long-range surveillance radar means that set-up and tear-down time can be measured in hours. Although advertised as mobile, the largest models of surveillance radar are essentially transportable -- they can be moved from one location to another, but the process requires time and manpower. Once deployed, they may rarely be moved to a new location.

In the latest generation of long-range surveillance radars, more emphasis is now being placed on mobility. Many manufacturers now claim set-up times of around 30 minutes. This allows radars to move from one operating location to another more easily than in the past. A series of pre-surveyed operating locations, perhaps `pre-wired' to a network, would allow the defender to play a `shell game' with radar assets. Assuming that an opponent possesses a useful elint (electronic intelligence) capability, that location of any repositioned radar will become obvious as soon as the equipment begins transmitting and, inevitably, will be added to the list of targets being drawn up by the other side. The trick will be to move before the new location can be accurately determined and an air strike despatched.

Another method of countering an anti-radiation missile attack is to lure the missile with a decoy able to produce signals that resemble those of the radar being protected. The false signal will have to match the frequency, pulse-repetition frequency and other parameters of the radar. The better the technology in the Arm, the more realistic the decoy must be. It is not necessary to reproduce the power level of the radar's main beam; a lower level will be enough to look like a sidelobe.

Very little has been published on the subject of anti-anti-radiation missile decoys. ITT Gilfillan manufactures the AN/TLQ-32 Arm-D, a miniature radar transmitter able to emulate frequency-agile radars. Three would normally be deployed to protect a single radar such as the Northrop Grumman AN/TPS75. It is not clear what measures have been has taken to avoid destruction of the decoy, but the company says that an individual decoy can be transported by two people and deployed within fifteen minutes.

The US company Malibu Research has developed a low-cost anti-radiation missile decoy antenna which consists of a passive reflector, several of which will be deployed at a safe distance from the radar they are protecting. The system operates in C-band. When an incoming missile is detected, the radar is switched off and a decoy reflector is illuminated by a remote feed to provide a target for the missile.

The reflector is designed to withstand the missile blast by using a hardened steel frame and nylon string to support its array of dipole elements. The frame is staked to the ground and can survive a blast from any direction. Most of the shock from the blast travels through the aperture as it falls to the ground. The reflector then springs back to its operating position. This antenna system has successfully survived extensive blast testing, says the company.

Yugoslavia is known to have fielded some decoys intended to lure anti-radiation missiles; a British officer who spent six months in Kosovo recently voiced a theory that the Yugoslav Army may have used microwave ovens looted from Albanian homes to create improvised decoys operating in E-band. Although the US military analysis company Stratfor believes that the ovens could have been modified to increase their RF signature, it's hard to see how a modern Arm could be persuaded to home into onto an emitter that did not have the signal characteristics of a threat radar.

Inevitably the attacker will try to find a way to attack and destroy the best radar networks that the defender can deploy. But it won't be easy, and developing an effective solution will take time. The GAO report earlier this year warned that a 1998 Pentagon study found that the US services "had not adequately adapted to the evolution of enemy air defenses from fixed, stand-alone radar systems that could be easily suppressed to integrated air defenses incorporating modern telecommunications links, passive sensors and other sophisticated means of avoiding suppression". It also found that "maintaining an effective suppression capability will require the development of innovative and non-traditional solutions cutting across individual service and functional (e.g. suppression, reconnaissance, and command and control) lines."

RELATED ARTICLE: In short.

* "Surveillance radars are high on the target list of any attacker"

* "Their size is partly dictated by the laws of physics"

* "Active-array technology will reduce the radar's size and make it more flexible"

* "Mobility, emission control and networking will make air-defence radar networks less vulnerable."