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Light fighter, big punch

The British Aerospace Sea Harrier Blue Vixen/AIM-120 upgrade is proving its worth with the Royal Navy.

Douglas Barrie/YEOVILTON

WHEN the Royal Navy's British Aerospace Sea Harriers were armed with Raytheon AIM-9L Sidewinder missiles, engagement with an enemy was compared to having a "knife fight in a phone box". Upgraded to carry the Hughes AIM-120 Advanced Medium Range Air-to-Air Missile (AMRAAM), the Sea Harriers can now be used to engage targets, at beyond visual range (BVR).

The Sea Harrier F/A2s' recent excursions to the North Sea air-combat-manoeuvring instrumentation range, and to the US Purple Star amphibious-warfare exercise, have shown their potential as BVR platforms. Cdr Richard Dawkins, Commander Air for the RN's Fleet Air Arm, says that "-the F/A2 is the best air-defence aircraft in Europe".

The RN says that the F/A2 was seen to be the more capable when it was pitted in BVR engagements against Royal Air Force Panavia Tornado F3s fitted with semi-active BAe Skyflash air-to-air missiles (AAMs). Their ability to carry out multiple-target engagements with the active-radar-guided AIM-120, instead of being limited to a single-target-engagement capability with the Skyflash, undoubtedly proved to be an advantage. In terms of detection range, the F/A2 radar is reputed to have repeatedly detected the F3 first, also a bonus in a BVR engagement.

The F/A2s were first operationally deployed with the AIM-120B in 1995. A total of ten test firings were carried out from Eglin AFB, Florida, and the first in-service firing will be carried out this year. It is not only the use of the AIM-120 which has turned the RN's Sea Harrier from a Sidewinder-only within-visual-range-capable aircraft to an all-weather BVR-engagement aircraft. At the heart of its radically improved combat capability is its GEC-Marconi Blue Vixen multi-mode pulse-Doppler radar.

The AMRAAM/Blue Vixen provides the vehicle for a shift in operational philosophy. In the traditional fleet-defender role, the AIM-9L-equipped FRS1 would be used defensively. The new radar/active BVR missile allows the aircraft to be used offensively. The aircraft effectively becomes a "hunter" rather than a "stopper".

DESIGN LINEAGE

The Blue Vixen is a pulse-Doppler, rather than a pulse radar, offering considerably increased detection performance and improved situational awareness. It replaces the Sea Harrier FRS1's Blue Fox radar, marking a generational leap in radar design, coupled with a similar increase in capability. In May, the last of the RN's FRS1s was returned to BAe Dunsfold for conversion. The RN has 18 new-build F/A2s on order, with 28 FRS1s being upgraded to F/A2 standard.

The Blue Vixen's lineage goes back to the Ferranti Blue Falcon, a medium-pulse-repetition-frequency (PRF) radar project launched in 1980. The Blue Falcon was a private-venture programme aimed at producing a next-generation fighter radar, particularly targeted at light fighter aircraft, including the Sea Harrier. The idea of fitting this radar on to the Sea Harrier appeared to be doomed by then-defence minister John Nott's expenditure cuts. The Falklands/Malvinas War in 1982, however, changed all this. The conflict demonstrated that there was a need to carry out all-weather air-intercept engagements, at BVR ranges, against aircraft armed with anti-ship missiles such as the Aerospatiale AM39 Exocet, and it underlined the need for maintaining and deploying organic carrier-borne air power .

The UK Ministry of Defence invited offers for a new radar for its Sea Harrier in 1983, with the then-independent Ferranti emerging as the winner. Its proposal was to use the Blue Falcon as a technology-demonstrator programme for the Blue Vixen. Although it was initially designed as an air-cooled radar, a shift to liquid-cooled technology was required by design issues beyond Ferranti's control. The company also opted for a low-duty ratio-wave form for the radar, which effectively means that the radar's antenna transmitting time is comparatively small in comparison to the amount of time which it spends in the receiving mode. High-, medium- and low-PRF wave forms can be used, allowing for a variety of scan patterns to be generated.

The first radar was delivered in 1991, with the main production batch of 56 radars being completed in the second quarter of 1996. Deliveries of a second production batch of eight radars will begin in 1997.

One of the key objectives driving the Blue Vixen project was to automate as much of the radar's operation as possible. As one RN weapons instructor admits, "-there is the great pilot concept of fiddling. What you need to do is to point the aircraft in the right direction and leave the radar to it."

As Hawkins points out, there is also "the optimisation process in a single-seat aircraft, compared to a two-seat aircraft". In the F3, for instance, the pilot can concentrate on flying the aircraft while the weapons-systems officer (in the back seat and known as "talking baggage") provides the best engagement-geometry.

The Blue Vixen is set up for air-to-air engagements, with the choice of mode being selected automatically, depending on the environment. The pilot is not required to designate a track; this is done automatically as soon as there is a target detection. Targets are also prioritised by the radar. Underlying the radar architecture is the aim of having a minimum of pilot intervention, of keeping the "pilot's head out of the cockpit".

Squeezing such a capable package into the Sea Harrier's airframe was a challenge to the manufacturer. As one commentator says: "The Sea Harrier is basically a Rolls-Royce Pegasus power plant with a round bit at the front to reduce drag, a pilot stuck on top and a tail at the back." Space is at a premium.

The FRS1s' diminutive Blue Fox radome has been superseded by the bulbous F/A2 cover to provide additional volume. The radar-data processor was placed in the rear of the airframe to address centre-of-gravity issues, replacing ballast in the FRS1.

HOSTILE ENVIRONMENT

Placing the processor in the rear raised the issue of the hostile electromagnetic (EM) environment provided by EM spikes from the engine. Rather than use traditional wiring to link the radar-data processor with the front end, it was decided to opt for a fibre-optic cable. This would not only provide the high-speed data capacity required, but would also address the EM-interference issues.

One unforeseen, niggling, issue is the need to keep the fibre-optic connectors clean. When disconnected, the cables have to be covered by small caps. It has taken time to educate ground crews to remember this always.

The F/A2's first operational deployment in Bosnia came, as Hawkins recognises, "at a delicate time". FRS1s were being returned for the upgrade, limiting the number of aircraft available for service, and front-line aircrew had no experience of the F/A2. Despite this, the aircraft was successfully deployed in theatre in 1995 with 801 Sqn, based on the HMS Invincible.

Using the high-PRF look down mode, the Blue Vixen was used by F/A2 operators to provide a gap-filling capability during the Bosnian operation. Data from the Blue Vixen radar were used to cover areas where the US Air Force's Boeing E-3A Airborne Warning and Control System (AWACS) aircraft encountered radar-performance problems. The F/A2's radar proved capable of picking up slow-flying helicopters at low level.

Radar data from the Blue Vixen are normally displayed on the right-hand multi-function display (MFD), with the left-hand MFD being used for navigation data. This can also be used as a tactical display, putting a radar "elevation slice" on screen by using the display's soft keys.

The radar display offers a 150km (80nm) range scale in 40km blocks. Tracks can be established at distances of considerably more than 85km. Another capability enshrined in the Staff Requirement was that the radar/missile package should provide the capability to engage sea-skimming anti-ship missiles. The radar's ability to detect targets with low radar cross-sections has already been established.

Despite the relative immaturity of the radar, its mean time between failures (MTBF) is already about the same as that of the Blue Fox, a few problems having been ironed out in the past 12 months. The MTBF is certain to climb to well beyond that of the Blue Fox.

The F/A2's BVR air-combat capability will be further enhanced when it is fitted with the joint tactical-information-display system (JTIDS). This will allow the aircraft to receive threat and targeting data from other JTIDS-equipped aircraft, whether they are the RAF's E-3D AWACS, other F/A2s or the upgraded Sea King airborne-early-warning helicopter. The Sea King could be the next to benefit from the Blue Vixen, as a derivative of the radar is being offered to meet the RN's requirement for a replacement for the Searchwater surveillance radar.

THE NEXT CHALLENGE

As far as the F/A2 is concerned, the radar offers greater possibilities than just that of a radically enhanced air-combat capability. Doppler-beam-sharpening for ground mapping is one area of potential development, although there is no official requirement yet.

The next challenge facing F/A2 crews is a deployment to the air-combat range at Decimomannu in Sardinia, where the opposition will include the Italian navy's McDonnell Douglas AV-8B Plus fitted with the Hughes APG-65 multi-mode radar.

The Fleet Air Arm is looking forward to further proving how big a punch it has packed into such a small platform.

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