Graham Warwick/WASHINGTON DC
In October, at the White Sands Missile Test Range in New Mexico, a US Air Force Boeing F-15 fired two air-to-air missiles at a stationary cable car - and the car survived.
The tests marked the first use of directed infrared countermeasures (DIRCM) against air-to-air missiles. In the first shot, the missile was launched at 7km (4 miles) range, detected and jammed so that it hit the ground more than 5km short of its target. The second shot, also at 7km, ended with the missile hitting the ground just 20m (60ft) from the first.
"It was very impressive," says Steve Kelley, manager of Lockheed Martin Sanders' Tactical Aircraft Directed Infrared Countermeasures (TADIRCM) programme. A prototype laser-based TADIRCM was used to jam the air-to-air shots and two surface-to-air missiles.
Also mounted under the cable car for the White Sands tests was a laser-based DIRCM developed by Northrop Grumman. This system successfully jammed four surface-to-air missiles launched at ranges of up to 4.7km.
The tests were part of a US Navy programme to demonstrate directed infrared countermeasures systems suitable for installation on fighters. Current DIRCMs, in production or under development for helicopters and transport aircraft, use pointer/trackers, or "jam heads", which are too large for combat aircraft.
Sanders' TADIRCM is a derivative of the Advanced Threat Infrared Countermeasures (ATIRCM) system it is developing for US Army helicopters. The Agile Eye system, designed for installation on the USN's Boeing F/A-18, uses a faceted "micro" jam-head that is both stealthy and has low drag.
Northrop Grumman's Wanda system is a derivative of the AAQ-24 Nemesis DIRCM in production for the UK Ministry of Defence, to equip helicopters and fixed-wing aircraft. It is also being supplied to the USAF for special-operations Lockheed Martin MC-130s and AC-130 gunships. The Wanda has a miniaturised turret.
The key to both systems is that they are all-laser, which allows the optics in the jam head to be scaled down substantially. All-laser DIRCMs represent the latest weapon in the "tit-for-tat" war between aircraft and missiles.
Since their introduction in the early 1950s, infrared-guided missiles have remained the most effective anti-aircraft weapons available, responsible for 80% of US combat losses in Operation Desert Storm in 1991. The standard defence is to use decoy flares, but later missiles incorporate flare countermeasures.
Infrared jammers were introduced in the 1970s. These "hot-brick" devices comprise a heat source with a fixed modulation that is effective against the simple seekers of early infrared-guided missiles.
Later missiles with improved seekers are capable of countering these low-power jammers. The solution is to direct more jamming energy on to the seeker. This requires sensors to detect the missile and cue the jammer and gimballed optics towards the missile.
A problem facing DIRCM designers is that they must jam the earliest infrared-guided missiles as well as the latest. While early seekers operate in the near-IR band, homing on to hot engine parts, later seekers operate in the mid-IR band, detecting the engine exhaust plume. To be effective against the range of weapons now in service, a DIRCM must operate in three IR bands, known as I, II and IV.
The first DIRCM to enter production, Northrop Grumman's AAQ-24, uses a high-intensity xenon-lamp jamming source. This reportedly operates across the three IR bands. Sanders' ALQ-212 ATIRCM, under development, uses a xenon lamp to cover bands I and II and a carbon-dioxide laser to provide increased jamming power in band IV.
"The lamp provides enough power the defeat threats in bands I and II," says Kelley, but the laser "puts out a lot more power" in band IV because of its narrower beam. While lamp-based jamming causes guidance gain degradation, which forces the missile to miss its target, Kelley says that laser jamming can quickly achieve optical breaklock, causing the seeker "to look completely away for the aircraft."
The ATIRCM is the first DIRCM to use a laser. Development problems have delayed the programme, with a production decision due in 2001-2. Kelley says it proved difficult to point the laser, so it has a separate optical path with a feedback loop to keep the beam on target.
A typical helicopter installation will have two jam heads fed by a single laser source. The ATIRCM is cued by Sanders' Common Missile Warning System (CMWS). This is an ultra-violet (UV) sensor which detects the missile's exhaust plume and tells the ATIRCM's fine-pointing sensors where to look.
While the US Army plans to install the ATIRCM/CMWS suite on a range of helicopters, the USAF and USN will use the stand-alone AAR-57 CMWS on combat aircraft to cue the deployment of flares. Northrop Grumman's AAQ-24 DIRCM is cued by its AAR-54 UV missile warning system.
Development of a DIRCM suitable for combat aircraft requires several changes. These include miniaturising the jam head and upgrading the missile warning system.
The USN's TADIRCM was developed under an option in the original ATIRCM contract. The prototype combines the ATIRCM's band IV laser with the Agile Eye micro jam-head and a two-colour IR missile-warning sensor, which reduces the false-alarm rate by detecting the unique characteristics of a missile plume's intensity in two IR bands.
Testing the system
Last June, the system was flight tested in a USN Lockheed P-3. The aircraft was flown over a ground-based target at 1,500ft and distances of 1-4km, and demonstrated that the IR missile warning system could detect and identify the simulated threat and cue the jam head, which acquired and lased the target.
This led to the live-fire tests at White Sands in October, where the TADIRCM was mounted on a stationary cable car alongside Northrop Grumman's Wanda all-laser DIRCM. Both systems were required to detect, declare and track 20 missile launches and each was allowed to jam four shots.
Kelley says telemetry from the "advanced foreign" air-to-air missiles jammed by Sanders' system shows that the seekers "broke lock very quickly. We could see the [jamming waveform] in the guidance system". The design goal is to generate a jamming signal in the seeker that is 1,000 times stronger than the IR signal from an F/A-18 in afterburner.
Northrop Grumman says its system "outperformed expectations" in the White Sands live-fire tests. The system combines the Wanda miniaturised jamming-head with the Viper multi-band laser and the company's Multi-Imaging Multi-Spectral two-colour IR missile warning sensor. The solid-state mid-IR laser is being developed to upgrade the lamp-based AAQ-24.
The next step in the USN's TADIRCM technology demonstration is due in May-July this year, when the Sanders prototype will be installed in a pod on a McDonnell Douglas QF-4 drone. Initial tests will repeat the P-3 flights, but then surface-to-air and air-to-air missiles will be fired at the QF-4 to determine the TADIRCM's effectiveness.
Kelley says that the USN plans to begin a TADIRCM development programme in 2002, leading to production four to five years later. Sanders is proposing a two-phased programme. As a first step, installed AAR-57 CMWS would be upgraded with two-colour IR sensors. Then the jam-heads would be fitted - probably two per fighter, he says.
The USAF, meanwhile, is focusing its DIRCM efforts on protecting transport aircraft. Under the Large Aircraft Infrared Countermeasures (LAIRCM) programme, the USAF plans to demonstrate a slightly different approach to jamming IR missiles.
DIRCMs are open-loop systems. Once locked on to a target, they repeatedly cycle through a series of predetermined jam codes designed to defeat a range of missile types. The USAF believes that closed-loop jamming could lead to faster breaklock, allowing the DIRCM to handle more near-simultaneous threats.
Closed-loop jamming requires a more powerful and accurate laser that can bounce a beam back from the missile, allowing the DIRCM to measure the seeker's spin frequency and determine the best jamming waveform.
A demonstration is planned for 2001, with Lockheed Martin supplying the solid-state multiband laser and the two-colour IR missile warning sensor. If successful, a development programme could begin soon after, with the Boeing C-17 airlifter as the principle candidate for the LAIRCM.
Some experts doubt the need for closed-loop jamming, given the demonstrated effectiveness of open-loop DIRCMs. Also, Kelley believes, the technique will not help defeat the latest missiles, which use staring focal-plane array sensors. Missiles with imaging infrared (IIR) seekers, to enter service over the next few years, represent a challenge for DIRCMs. Sanders is looking at potential countermeasures under a USArmy contract. Most involve increasing the laser's power, to blind the seeker by scattering and reflection within the sensor, or to damage the detector array. "We are still waiting for a real [IIR] missile to be fielded, but we think we have solutions," says Kelley.
Source: Flight International
