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In HARM's way

Several show debutantes at Le Bourget indicated that defeating air- defence radars remains high on air-force agendas.

The USA is working on rocket-ramjet-powered dual-mode-seeker missile designs to provide a successor to the Texas Instruments AGM-88 High Speed Anti-Radiation Missile (HARM) in the defence suppression role. Two potential replacement designs were displayed in mock-up form at the Paris air show.

The eventual follow-on to the HARM is being designed for internal carriage on the US Air Force's Lockheed Martin/Boeing F-22 Raptor, the US Air Force, Navy and Marine Corps Joint Strike Fighter and the Northrop Grumman B-2 bomber, as well as for conventional carriage on the likes of the McDonnell Douglas F-18.

The USN is funding a dual-mode-seeker programme with Science and Advanced Technologies (SAT) under the Small Business Innovative Research programme. SAT is working with ramjet design house Atlantic Research on its Advanced Anti-Radiation Guided Missile (AARGM). This design builds on the basic AGM-88 airframe, but replaces the solid rocket with a ramjet sustainer. The AGM-88's passive-radar seeker would be replaced by a millimetre-wave unit, coupled with a passive seeker.

Opting for a dual-mode design provides a counter to the possibility of an air-defence radar being shut down during the missile's flight time. The active sensor would then be used to try and re-acquire the target. The ramjet sustainer would provide the weapon with a higher average velocity and much greater fly-out range.

SAT is in competition with Texas Instruments (TI), which is working on its Advanced Strike Weapon (ASW). The ASW is also envisioned as using a dual-mode seeker, as well as rocket-ramjet propulsion. TI is also considering an Atlantic Research-designed powerplant. Performance estimates project a range of more than 185km (100nm) with flight speeds of about Mach 4. The ASW could form the core of a family of air-launched supersonic weapons.

Talks are under way on an AARGM flight-demonstrator proof-of-concept programme - although a ground-launched approach could be adopted to save on the cost of the tests.

Also at the show for the first time, and displaying considerably more leisurely acceleration characteristics, was Israel Aircraft Industries' Harpy anti-radiation unmanned air vehicle. As with the AARGM and ASW, the Harpy has a passive anti-radiation homing seeker - but this is where the similarity ends.

While the AARGM and ASW would cruise at beyond M3, the Harpy is distinctly subsonic, with a pusher propeller mounted at the rear of the delta-wing platform. It is also designed to be ground launched - whereas the AARGM and the ASW are intended as air-launched weapons.

Even this, however, is not the fundamental conceptual difference - which is one of cost. The present generation of anti-radiation missiles are expensive. Neither the AARGM nor the ASW will be cheaper than present ARMs.

The price of the weapon becomes even less attractive when this cost is combined with the combat utilisation of ARMs simply to suppress enemy air-defence radars. The probability of a kill with a HARM-class weapon is also variable, with the missile's performance being seriously degraded against a radar which ceases to emit during the missile's flight time to target.

In addressing the cost-per-kill equation, missile designers are proposing to adopt dual-mode seekers - in the case of the AARGM, coupling broadband passive anti-radiation-homing seeker technology with millimetre-wave seekers.

A dual-mode seeker would allow the weapon to be used against a broader range of targets than can the present generation of ARMs. TI, for example, is proposing the ASW as the basis of a family of high-speed stand-off missiles. By comparison, the Harpy class of air vehicle is less expensive than the proposed AARGM or ASW.

The Harpy, however, is potentially only the beginning of suppression of enemy air defences (SEAD)requirements being met by winged-airframe, rather than missile, designs. Unmanned-air-vehicle and uninhabited-combat-air-vehicle (UCAV) designers are looking to the SEAD as a role which emerging technologies can meet.

Future air-force planners will certainly have more options from which to choose in planning SEAD missions. Cost-per-kill trade-offs and hard- versus soft-kill issues will feature increasingly in the planning of SEAD missions.

While the ARM will not be replaced by the UAV in the SEA Drole - it will give mission planners greater choice in the type of weapon to be deployed. The ARM will remain the chosen weapon of self-defence for crewed aircraft.