PAUL LEWIS / WASHINGTON DC

The Lockheed Martin Patriot Advanced Capability (PAC-3) missile at the vanguard of US anti-Scud defences now deployed in the Middle East was born out of the last Gulf War in 1991, during which the earlier Raytheon Patriot system proved only partially successful against Iraq's Al-Hussein missiles. PAC-3 is a hit-to-kill weapon specially designed to intercept tactical ballistic missiles (TBM) and is the first in a series of layered systems planned to be developed by the US Missile Defence Agency.

"It's revolutionary in that it is truly a hit-to-kill interceptor that detects targets, determines the aimpoint and then manoeuvres the entire missile on to the warhead, be it a longbody Scud, a deployed re-entry vehicle, cruise missile or even a manned aircraft or helicopter," says Clayton Daughtry, Lockheed Martin Missiles and Fire Control business development manager. "Most air-defence missile use a proximity fuze and warhead, but PAC-3 uses nothing but kinetic energy."

While high-explosive warheads are effective against relatively large, slow- moving targets such as aircraft, a TBM closing at a rate of 2km/s makes a proximity detonation a hit and miss affair. The challenge of defeating an incoming missile is made all the more difficult by submunition payloads, some of which can survive fragmentation warheads.

Even more daunting are chemical or biological payloads that need to be intercepted at a higher altitude and require more energy to destroy completely.

Devoid of a warhead, PAC-3 is only a quarter the size of PAC-2 and with a launch weight of just 320kg (700lb) it flies higher and faster. PAC-3 harnesses the existing improved Patriot ground radar, communications and launcher, modified to accommodate 16 missiles in place of four containerised PAC-2 rounds. After launch and initial fly-out, PAC-3 is highly autonomous and uses a C-band uplink/downlink for periodic target trajectory updates only. "All commands are computed on board the missile. We only initialise the missile with the type of target we're going after, the predicted intercept point and vector of the incoming target. It computes the intercept point," says Daughtry.

PAC-3 use an onboard Ku-band seeker to acquire and home in on the target. It is equipped with an aft aerodynamic manoeuvring system, augmented in the homing and endgame phase of flight by five radio banks of thrusters for rapid responses.

During development testing between 1997 and 2001, PAC-3 destroyed nine out of 10 targets, though subsequent operational testing was less successful because of a series of system anomalies. These are being addressed by follow-on testing, but in the meantime the US army is pressing ahead with the deployment of the first of a planned eventual buy of 2,200 missiles.

Source: Flight International