Fighting an invisible threat: air warfare involves both visible and invisible battlefields and the latter becomes more demanding every year—having extended from combat aircraft to support aircraft and now down to the helicopters and even civil aviation

Armada International, Dec, 2005 by E.R. Hooton

In the early days of air defence missile warfare, the weapon was remotely guided to its target by a ground-based or an airborne radar. Later, the missiles had the ability to guide themselves autonomously or semi autonomously with onboard sensors. This threat continues to exist, but it is essentially one for conventional or semi-conventional operations rather than asymmetrical ones, for these are extremely sophisticated weapons that require considerable maintenance support.

But while the threat from radar-guided weapons may have grown fainter it is not one that can be ignored. An aircraft's first line of defence remains the radar-warning receiver (RWR), which is now a common fit on both fixed- and rotary-wing combat aircraft as well as on the support aircraft that may have to operate within a combat zone.

The warning receiver uses either crystal video or instantaneous frequency measurement receivers to detect radio-frequency waves in the ranges employed for weapon guidance such as the I/J-band (1 to 18 GHz). The very first generation merely alerted the crew of the presence of the enemy within their vicinity and were then followed by systems that not only provided an audio warning (the famous 'Sam song' of the Indochina conflict) but also an indication of the direction of the threat.

Most receivers now feature digitally-controlled processing systems and a library of signal signatures that provides the pilot not only with an alert and an indication of the threat but also the latter's nature. It does this by comparing the incoming signal with an extensive threat library, usually one prepared by the user, that contains the parameters such as wavelength, pulse repetition frequency and pulse width of both hostile and friendly radars.

One major problem is that in areas such as Western Europe, but now also many areas of the Third World, the wide variety of emitters--including radios and wireless communications systems as well as air traffic control radars--create a dense electronic environment. This may confuse or even overwhelm the radar detector, even worse, create numerous false alarms that could encourage the crew to disable it. The systems must consequently be sensitive enough to detect the threat even in such an environment and sufficiently robust to operate in that very same environment, a performance that is akin to detecting a whisper in a football crowd.

A typical modern radar warning receiver is the widely used BAE Systems North America/Northrop Grumman Electronic Systems' AN/APR-39A family, a digital version of the analogue AN/APR-39 that weighs between 7 and 16 kg. The sensor is used mostly in helicopters such as the AH-1 Cobra and the AH-64 Apache, the UH-60 Blackhawk and the UH-1 Iroquois as well as in transport aircraft such as the C-130 Hercules. This system consists of a digital signal processor, two H/M-band (6 to 100 MHz) receivers providing continuous coverage, pairs of spiral antenna/detectors, a single C/D-band (0.5 to 2 GHz) blade antenna providing correlated data on the status of detected emitters, a radar signal indicator and a detecting set control unit. The signal processor incorporates a main system control microprocessor, a video processor, display and articulated audio units, a multiplexer, a C/D-band receiver, a power supply and a user data module that programmes the sensor.

The versatility of modern receivers may be gauged not only from the APR-39 but also from the 30-kg BAE Systems Australia ALR-2002. This consists of four quadrant receivers, a low band receiver, a data processor, track and interface processor and azimuth display indicator. Its dual receiver architecture is claimed to provide nearly 100 per cent probability of intercept for all emitter types and densities. It is part of the F/A-18 Hornet upgrade programme and equips both the S-70A-9 Blackhawk and CH-47D Chinook helicopters.

Asymetric Warfare Threat

A growing threat in the era of asymmetric warfare is the missile with electro-optical sensors, also known as a heat seeker. Indeed Northrop Grumman has noted that 80 per cent of aircraft shot down between 1958 and 1992 fell to these types of missile and only 20 per cent to radar-guided threats. During Operation 'Desert Storm' in 1991 alone heat-seeking missiles accounted for 75 per cent of American aircraft losses.

Particularly affected are helicopters and transports, because many of these fall victim to man-portable missiles. These are easy to conceal and fire and the weapons themselves require relatively little servicing which make them a useful tool for insurgents, indeed the recent downing of a Royal Air Force Hercules in Iraq is likely to have been to one of these weapons. What makes them even more significant is that terrorists have shown themselves willing to use them against civil aircraft with some 30 shot down in the past 40 years at the cost of 1000 lives.

These missiles home in on the infrared emissions from the aircraft. The passive nature of the missile seeker meant it was initially extremely difficult to detect but sensors equivalent to the RWR are now available. A typical one is the CMC Electronics AN/AAR-44 infrared missile warning receiver used by the US Air Forces Special Operations Command as well as the Belgian Air Force. CMC, by the way, officially became L-3 Cincinnati Electronics in October 2005.