All-spectrum awareness: if helicopters are to fly and fight, their crews need to be aware of all potential threats--hostile radar, laser emitters and incoming missiles and projectiles. EW system houses have developed a range of warning systems priced at a level which rotary-wing operators should be able to afford. (Electronic Warfare).

There is probably no air force whose flying schools are immune from the legend of the mother who, on hearing that her son had qualified as a military pilot, admonished him to "always fly low and slow". This is hardly good advice in terms of fixed-wing flight, and in combat "low and slow" is a recipe for being shot down.

Unlike their fast-jet contemporaries, helicopter pilots have no choice but to fly low and slow, a flight regime which exposes them to anti-aircraft weapons, many of which are radar-guided or infrared homing. The development of laser beam-riding missiles and even specialised anti-helicopter rounds fired from tank guns means that yet another part of the spectrum needs to be monitored for possible threats.

Threat-warning systems fall into four main classes:

* Radar warning receivers (RWR)

* Laser warning receivers (LWR)

* Hostile fire indicators

* Missile approach warners (Maw).

Such sensors can be used simply as alarm devices, allowing the crew to begin self-protection manoeuvres, or the manually controlled release of expendables. Most RWRs have the ability to automatically control the release of chaff and flares, while there is a growing trend for them to be teamed with LWRs, Maws, active jammers and dispensers for expendables in order to create complete defensive aid suites (Das).

RWRs are wideband receivers covering anything from as low as 100 MHz to 20 and now even 40 GHz. These are normally of the crystal video or superheterodyne type. Crystal video is the older and simpler technology, while superheterodyne receivers provide higher sensitivity but at a higher acquisition cost. Detected signals are passed to a processor which measures parameters such as frequency, modulation type or pulse repletion frequency and compares these with the values associated with friendly and hostile emitters contained in a database. Details of threat signals are presented to the aircrew via a cockpit-mounted display, which gives an indication of their nature, bearing and priority.

The Raytheon AN/APR-39 is one of the most widely deployed helicopter RWS. At least ten specific versions are reported to exist--the AN/APR-39A, -39A(V)1, -39A(VE), -39B, -39B(V), -39B(V)2, -39(V), -39(V)1, -39(V)2 and -39(XE2). Most were developed and manufactured by E-Systems (now part of Raytheon), but some are from Northrop Grumman or BAE Systems North America. Not all of these are intended for helicopter use, but the variants listed above are all either in service on helicopters or are suitable for this role.

When a specific threat appears which cannot be handled by a standard RWR, the solution can be to install a second specialised receiver. The US Army and US Marine Corps took this route by selecting the Sanders (now BAE Systems North America) AN/APR-44(V) lightweight RWR in (V)1, (V)2 and (V)3 variants, which handle different frequency bands associated with unidentified but specific radar signals. The US Army uses the AN/APR-44(V) on the MH-47E, MH-60K, OH-58D and some UH-60A helicopters, and the system is also on some US Marine Corps aircraft.

Northrop Grumman's AN/ALR-66(V) family of RWRs and ESM systems includes several used for helicopter applications. US Navy Reserve SH-2G Lamps 1 helicopters are reported to be fitted with the 2 to 20 GHz ALR-66A(V)1, the ALR-66(V)2 has been identified as in use on Greek Navy S-70B-6 and Taiwanese S-70C(M)-1 helicopters, while Japanese Maritime Self-Defence Force SH-60J helicopters are reported to be equipped with a version of the ALR-66(VE).

The GEC Marconi Sky Guardian 200 RWR has been widely sold for fixed and rotary-wing applications. Helicopters fitted with the system include the Lynx (British Army Air Corps), Chinook (Royal Air Force) and Puma (Royal Air Force). Frequency coverage is three bands between 2 to 20 GHz, but 0.5 to 2 GHz and 20 to 40 GHz options are available.

The Sky Guardian 2000 from the same company has been selected to form part of the Helicopter Integrated Defensive Aids Suite for the British Army Air Corp's Apache AH Mk 1 attack helicopter and for use on the Royal Air Force's Merlin HC Mk 3 battlefield-support helicopters. It covers 2 to 20 GHz and has a threat library with more than 4000 entries. Like the Sky Guardian 200, it incorporates display persistence and will continue to warn the aircrew of briefly detected threats.

The Thales Airborne Systems Sherloc is an airborne crystal video RWR suitable for use in fighter and attack aircraft or helicopters. It weighs 13 kg, and covers from D to J-band (1 to 20 GHz), but can be supplied in variants whose coverage includes 0.5 to 1 GHz or 20 to 40 GHz, should this be required by the customer, and which have an extended threat library. The Sherloc H was deployed on Super Puma helicopters in 1988, and in Sherloc F form on the Eurocopter Panther in 1995. The F version is also offered for use on the Cougar.

In 1995, the Dassault Electronique (since part of Thales Airborne Systems) EWR-99 Fruit was selected for installation on French Army Aviation SA341/342 Gazelle, SA 330 Puma and AS 532 Cougar helicopters, and the system was used in combat during Operation Allied Force in 1999. It has also been selected by PZL-Swidnik to equip the W-3 Sokol multirole helicopter.

Specifically designed for helicopter use, the EWR-99 can detect pulse, pulse Doppler and Continuous Wave (CW) threats, and can be used to control countermeasures dispensers. It can be interfaced with a missile warning system and is part of Thales' SPS-H V1 defensive suite, which also incorporates an MWS-20 Maw and a countermeasures dispensing system.

Thales Airborne Systems also offers the wide-band TDS-H, a user-programmable RWR that uses instantaneous frequency measuring technology, and is reposted to be in widespread use on French Army helicopters, including the Gazelle, Puma and Super Puma types. This RWR also forms part of the company's SPS-H V2 self-protection suite. This is similar to the V1 version but uses the TDS-H rather than the EWR-99.

The Thales Airborne Systems Threat Warning Equipment (TWE) is a combined radar and laser warning system which forms part of the Electronic Warfare System (EWS) selected for use on all versions of the Eurocopter Tiger battlefield helicopter. On the NH90 tactical transport helicopter, it is teamed with the Milds AN/AAR-60 passive missile warner and a counter-measures dispensing system. Frequency coverage of the TWE is 2 to 40 GHz and laser bands I and II.

Russia's Mil Mi-24 attack helicopter is fitted with the SPO-15 Beryoza. The same receiver is probably carried by the Kamov Ka-50 (Hokum-A) close support helicopter and the Mi-28 `Havoc'. The latter also has an LWR and two fixed IR sensors which could be some form of Maw.

The BAE Systems Australia ALR-2002 is being developed under Australia's ambitious Echidna programme whose goal is to develop a common self-protection EW suite for use on Australian Army S-70A-9 Black Hawk and CH-47D Chinook helicopters, and on Royal Australian Air Force C-130J-30 Hercules transports, and F/A-18 and F-111C/G strike aircraft. The RWR variant intended for helicopter applications is reported to be the ALR-2002D.

Developed by the Chilean company DTS, the DM/A-104 RWR is a 0.7 to 1.3 GHz and 2 to 18 GHz receiver intended for helicopter and fixed-wing use, but the current status of this programme is uncertain.

The Avitronics RWS-50 RWR uses four antenna heads and covers 2 to 18 GHz, with optional extensions of down to 0.7 GHz and up to 40 GHz. It can operate as a stand-alone receiver or be linked to laser and missile approach warning sensors and to countermeasures.

The electronic warfare sensors on the Agusta A129 Mangusta are the Elettronica ELT/156-05 or ELT/ 156(V)2 RWR--both reported to be of the crystal-video type, but of unknown frequency coverage--plus a BAE Italia Ralm-101 laser warning receiver. Both types of RWR provide visual threat warnings on a cockpit display and audio warnings via the aircraft's intercom system.

GEC-Marconi (now BAE Systems Avionics) supplied the 2 to 18 GHz Aware-3 RWR for retrofitting to British Army Gazelle and Lynx helicopters of the British Army Air Corps, and for service on the SH-14D Lynx helicopters of the Royal Netherlands Navy. It consists of four planar spiral antennas, two dual-channel crystal video receivers, an Instantaneous Frequency Measurement (IFM) receiver and a signal processor, and can detect and identify pulse, pulse-Doppler, Continuous Wave (CW) and interrupted CW threats.

Elisra has developed the SPS-20(V) receiver to detect pulse radars operating at 0.7 to 1.3 GHz or 2 to 8 GHz, and the Elisra SRS-25 6.5 to 18 GHz super-heterodyne receiver which detects CW, high pulse-repetition-frequency (PRF), low power radars. This equipment is normally teamed with the company's LWS-20V laser-warning receiver to form the SPS-65(V) integrated electronic warfare system.

The best-known laser warning receiver is probably the Raytheon Electronic Systems (formerly Hughes Danbury) AN/AVR-2A(V). Designed to interface with the AN/APR-39(V) and/A(V) series RWRs, this consists of four sensor units and an interface unit comparator. Production of this unit has run into four figures, and the system is known to be in service on the AH-1F, AH-1W, AH-64, HH-60H, MH-47E, MH-60K, OH-58D and UH-1N helicopters. It could be adopted for the SH-60B, SH-60F and RAH-66.

Originally developed as a multirole sensor suitable for use on aircraft, armoured fighting vehicles and ships, the BAE Systems Series 1220 laser warning system can provide visual and aural warning of hostile laser emissions from sources up to several kilometres away. In the rotary-wing field, its application would probably be medium-sized transport helicopters. Frequency coverage of the basic system is 0.35 to 1.1 [micro] m, but 1 to 1.8 [micro] m and 8 to 11 [micro] m have been offered as options. The system has been sold to several customers in Europe and Asia, but no specific applications have been reported.

The company's Series 1223 LWR was selected to form part of the Helicopter Integrated Defensive Aids System (Hidas) of the WAH-64 Apache. It covers the 0.4 to 1.7 [micro] m region of the spectrum used by laser designators, rangefinders and trackers, and the 0.8 to 1.1 [micro] m region associated with laser beam-riding missiles.

Elisra has enjoyed significant sales success with its LWS-20 laser warning system. Versions are known to be in service on or on order for SH-2G(A) helicopters (Royal Australian Navy), CH-146 Griffon helicopters (Canada) and CH-53G (German Army). On the Canadian and German aircraft it forms part of the SPS-65(V) integrated self-protection system. This includes the Elisra SPS-20(V) and Elisra RWRs described earlier. The latest known version is the LWS-20V-2, which uses four sensor, heads, a Laser Warning Analyser, a system control panel and a pilot's display. The latter shows threats in alphanumeric and symbolic form, indicating threat type, azimuth and potential lethality. Audio warnings are also provided.

Avitronics has developed three sensor systems for its Airborne Laser Warning System (ALWS), a system designed to detect, identify and locate laser rangefinders, laser designators and laser-based missile guidance systems. The LWS-200 covers 0.6 to 1.8 [micro] m, the LWS-300 0.5 to 1.8 [micro] m, while the LWS-400 is a dual-band device covering 0.5 to 1.8 and 2 to 12 [micro] m. These sensors are teamed with a common pattern of laser warning controller, which is linked to radar warning or ESM receivers, or with other hardware making up an integrated EW suite. The LWS-300 and 400 are form and fit interchangeable, while all three sensor types use the same electrical interface, making system changes or upgrades relative easy.

The Spanish Ministry of Defence is believed to have procured the Indra DTD EN/ALR-310 RWR for use on helicopters. A lightweight crystal-video system covering from 0.7 to 1.5 GHz, and the E/J-band in two channels, it is designed to detect pulse and CW radar threats. It can also be used on fighter and strike aircraft.

Helicopters can also be fitted with sensors intended to detect hostile projectiles or missiles. Britain, Canada, Italy and several other countries have procured the MS Instruments Hostile Fire Indicator (Hofin). This lightweight system weighs less than six kg, and uses sensors mounted beneath the helicopter to detect the shock wave from passing projectiles and convert the pressure change into an electrical signal. The Hofin provides an audio warning signal to the aircrew, and a display unit mounted on the helicopter's instrument panel shows the general direction from which the rounds are coming.

Missile approach warners can be active or passive. The former can directly measure the range of an incoming threat, but, in doing so, emit RF energy which could betray the aircraft.

The Sanders AN/ALQ-156(V) missile warning system is a pulse-Doppler radar sensor developed to trigger countermeasures able to protect an aircraft from heat-seeking and radar-guided missiles. It consists of between two and four antennas, a radar transceiver and a control indicator, and is compatible with the widely used AN/ALE-39, -40, -47 and M-130 chaff and flare dispensers. The ALQ-156(V)1 version used on US Army CH-47D helicopters has two antennas, while the ALQ-156(V)2 used on the EH-60A has four. The ALQ-156(V)2/(V)3T variant was initially developed for use on Greek Army CH-47Ds.

Originally developed by Dassault Electronique, the Thales Airborne Systems MWS-20 active pulse-Doppler Maws is reported to be in service on the French Army's Horizon radar-surveillance helicopter and the French Air Force's Cougar combat search and rescue helicopters. It consists of a transceiver/processor unit, four antennas and a cockpit display/control unit. Total system weight is 10 kg. Being a radar-based system it can be used in all weathers, detecting missiles and measuring their range, direction of approach and time to impact.

Elta's EL/M-2160 is a solid-state, pulse-Doppler Maw suitable for use on fighter and transport aircraft and helicopters. Initial applications seem to have been fixed-wing, but the system is now believed to be operational on the Bell 212/412, Mil Mi-17 and other helicopters. It can be used as a stand-alone system, warning the crew of incoming threats and giving a predicted time to impact, or can be linked to a countermeasures dispensing system or a more comprehensive form of integrated self-protection system.

Some Mil Mi-24 and Kamov KA-29 helicopters have been fitted with the Lip active-radar missile approach warner. This, probably a pulse-Doppler device, is also used on Sukhoi Su-25 strike aircraft.

The Alliant Integrated Defense AN/AAR-47(V) is one of the most widely-deployed passive missile warning systems, and has been manufactured by Alliant and by Sanders (now BAE Systems North America--Information and Electronic Warfare Systems). It consists of a Computer Processor (CP), a Control Indicator Unit (CIU) and four to six Optical Sensor Converter (OSC) units. Mounted on the skin of the aircraft in a pattern designed to give all-round coverage, the OSCs detect the motor plume from enemy missiles and pass this data to the CP for processing. The CIU displays the bearing of the highest priority threat to the aircrew, while the CP triggers the appropriate countermeasures. Three variants are known--the AAR-47(V) baseline model, the upgraded AAR-47(V)1 and the AAR-47(V)2--a (V)1 with an additional laser-warning capability.

Northrop Grumman's AN/AAR-54(V) Passive Missile Approach Warning System (Pmaws) is a UV-band device which has been extensively tested in several hundred live-fire demonstrations. It has been used on Dutch AS 532U2 and CH-47D helicopters and fixed-wing transports, but its most important future role may be in cueing the Northrop Grumman AN/AAQ-24(V) Directed Infra-Red CounterMeasures (Dircm) IR jamming system.

The AN/ALQ-212(V) Advanced Threat Infra-Red Countermeasures/Common Missile Warning System to be fitted to the AH-64D Longbow Apache consists of the BAE Systems AN/AAR-57 tri-service Common Missile Warning System (CMWS), a laser jamming-power source, two steerable jamming heads, a jam-head control unit, an AN/ALE-47 sequencer for expendable countermeasures and two improved countermeasures dispensers.

During tests carried out on March 11 through 13 2002 at White Sands Missile Range, New Mexico, an AN/AAR-57 mounted on a QF-4 drone version of the Phantom successfully detected eight man-portable surface-to-air missiles fired towards the aircraft from various distances and angles.

The Eads/LFK AAR-60 Missile Launch Detection System (Milds) is based on an imaging sensor device designed to detect the ultraviolet radiation emitted from the exhaust plume of incoming missiles. The system can determine the approach angle of missile threats, prioritise them, warn the pilot and automatically initiate appropriate countermeasures. A complete system can use up to six sensors, with one acting as master unit and the others as slaves. It uses high spatial resolution and advanced temporal processing to provide a high probability of detection coupled with a minimal risk of false alarms. The system is qualified for use on the NH90 naval/transport helicopter and the Franco-German Tiger attack helicopter.

Elisra, based its Passive Airborne Warning System (Paws) on infrared technology and, according to the manufacturer, it can detect and track one or more threats even in cluttered environments, determine if the missiles threaten the aircraft and provide an estimated time to intercept. Good spatial coverage, high angular resolution and long detection range are all claimed for the system, which is in service on Israel Defence Forces UH-60 Yanshuf transport helicopters and CH-53 Yas'ur 2000 assault helicopters. The Paws can be used as a stand-alone warning system or be integrated with radar and laser warning receivers to form an all-threat warning system. It is also being offered teamed with the Northrop Grumman's Dircm system.

Rafael's Guitar 350 is a passive missile warner designed to detect electro-optical emissions from a threat missile's motor, detecting surface-to-air and air-to-air missiles with what the company describes as an `extremely low' false alarm rate. The Guitar, suitable for fixed and rotary-wing applications, provides audio and visual alarms, giving the crew between four and six seconds of warning time, and can automatically control an infrared countermeasures system.

The Avitronics Maw-200 is an ultraviolet missile approach warner consisting of a system controller and four sensor heads positioned to give 360-degree coverage. It is being marketed as part of the company's Multi-Sensor Warning System (MSWS) for fixed and rotary-wing applications. The latter system also includes an RWS-50 RWR and LWS-400 laser warning receivers.

One proposed upgrade of the Mi-35M3 helicopter would install the Mak-UFM, a Russian missile warner originally developed for the Su-24M strike aircraft. This is a passive system, but it is not known whether it operates in the ultraviolet or infrared band.

For an RWR to be militarily useful, it must be reliable, producing the minimum possible number of false alarms. Unfortunately, the APR-39 has had its share of highly publicised problems. This was illustrated when the US Army deployed a force of AH-64 Apaches to the Balkans to support Operation Allied Force against Yugoslavia, but failed to make combat use of these expensive assets.

At a hearing by the House Armed Services Subcommittee on Military Readiness held soon afterwards, Brigadier General Richard Cody, US Army commander of the Task Force Hawk Apache mission in Albania described how the AH-64 pilots had lacked faith in the self-protection EW systems fitted to their helicopters, particularly the aircraft's AN/APR-39 radar warning receiver.

He explained that it was "very scary for pilots to think that someone is tracking them when they are not, to think that someone is engaging them when they are not, or to get a missile alert. That is actually what our pilots were getting on every mission. We had the same problem with the APR-39 radar warning system used in the Gulf War, and we fixed some of it with the new version of the APR-39 radar warning system.

"We tried everything with the help of the experts to fix the problems, but we still got them ... I don't see right now in the near term how we are going to fix that problem.... Basically, the system picks up RF, radio frequency stuff; and you will be sitting on an airport, and it tells you something is tracking you, and there is nothing there."

In practice, Nato was able to complete its mission without the assistance of the Apaches, but the whole affair reminds us of the folk tale of the little boy who cried "wolf". As he learned the hard way too, many false warnings result in the genuine warning being ignored. It's a lesson which the designers of threat warning systems must always keep in mind.