Technology Explained

In all of these applications the stabilised thermal imaging telescope is boresighted with a laser rangefinder and thus can provide all three coordinates of a target- this allows a fire control computer to calculate a fire control solution for an unguided weapon or gun or to set up optimal release conditions for a guided weapon.

The laser however also doubles up as a target designator by transmitting an appropriate pulse code. A rapidly flashing spot of appropriately coded laser illumination on a target is used to guide semi-active laser homing weapons, which home in on the laser spot. Both conventional laser guided bombs such as the GBU-10/Mk-84 and GBU-12/Mk-82 used by the RAAF and laser guided missiles such as the AGM-65E Maverick or AGM-114A Hellfire use this technique, relying on a simple four quadrant detector (ie four detectors arranged in a maltese cross pattern) arrangement. The accuracy of these weapons is very high, with average miss distances of feet.

There is however a more insidious aspect to these systems. Nearly all laser rangefinders and designators employ 1. 066 micron near infra-red lasers which have both the power level and short wavelength required to damage the human retina. Partial and complete blindness may result from direct or indirect sighting of the beam from such a laser. The presence of laser rangefinders on almost all armoured vehicles, tanks and battlefield close air support aircraft and helicopters renders battlefields optically hazardous to the unprotected eye.

The Russians have taken a particular liking to the use of laser rangefinders as antipersonnel weapons, sweeping the battlefield ahead of their advancing armour to damage eyes and less robust optical fire control sensors. The USAF had investigated this application under the Compass Hammer programme, proposing an optical tracker capable of detecting gun muzzle flashes and directing a powerful blue green laser at the source to blind the gunner or fire control system. (A good reason not to join the artillery . . . Ed. ) A Westinghouse-designed Advanced Optical Countermeasures pod was successfully flight tested in the mid eighties with planned application to the B-52, however no information appears to be available on deployment plans. The countermeasure is the wearing of protective goggles, usually employing multiple interference filters for specific laser wavelengths, all the time by all personnel. From experience though this writer has found the use of such goggles impractical in the extreme. They are cumbersome and uncomfortable to wear, restrict head movement and greatly reduce natural vision in all but very bright sunlight. There would be a very great tendency for the troops not to wear them in many situations. Unfortunately, invisible and narrow laser beams offer no warning to the victim.

High Energy Laser Weapons

The High Energy Laser (HEL) is a weapon which is within a decade of deployment, if development is sustained. While often ridiculed as science fiction technology, the current generation of near and mid-infra-red chemical lasers has demonstrated usable power levels of hundreds of kilowatts and the ability to deliver power through beam folding optics to a beam director and target. The early eighties saw the demise of the USAF Airborne Laser Lab (ALL) programme (see TE Dec 1981) during which a carbon dioxide 10. 6 micron gas dynamic laser weapon on an NKC-135A airframe successfully engaged and shot down transonic drones.

Objectives met, the funding was stopped. Work is however currently being done on the US Navy’s land based Miracle deuterium fluoride 3. 8-micron chemical laser which coupled to a Sealite beam director has been involved in successful trials against supersonic Vandal missile targets. The USN sees the HEL Weapon System (HELWS) as a shipboard point defence weapon with the ability to kill aircraft and missiles in a high-density air/sea battle (interested readers are directed to USNI Proceedings Oct 1988). Coupled to Aegis battle management, optical fire control will be used.

High power tunable Free Electron Lasers are also under development for SDI applications (Lawrence Livermore labs and TRW), but these devices are so huge that they are unlikely to be practical in air defence applications.

Heatseeking and Imaging Weapon Guidance

Heatseeking and imaging guidance have been used in missiles for over two decades. Heatseeking guidance has progressed from simple rotating reticle tail aspect only capable missiles to jam-resistant flare rejecting all aspect weapons. The most recent variants of the established Sidewinder, the Mike and Romeo are described as substantially better than their predecessor, the Lima model which had a success·rate of 86% in the Falklands conflict. ·

Similarly heat-seeking SAMs have progressed considerably, with the current FIM-92C Stinger offering a dual colour rosette scanning seeker with a field programmable guidance processor. The Russians have also expended considerable effort in this area with the new SA-13 Gopher fitted with a cooled dual colour seeker. All aspect heatseekers are thus a real threat to all classes of aircraft at all altitudes and trivial countermeasures such as flares are largely ineffective with newer weapons. Capable jammers, warning systems and sufficient aerodynamic performance to sustain manoeuvre are thus necessary for survival.

Imaging missile guidance saw its first major application during the SE Asian conflict when the USAF demolished several communist targets with HOBOS TV guided bombs. Soon followed by the TV guided AGM-65A/B Maverick tank-killing missile, imaging guidance found a wide range of applications all requiring pinpoint accuracy. Imaging weapons will employ either conventional television or thermal imaging to provide an operator with a view of the target. An automatic tracker is then used (as described above) to ensure that the weapon tracks the selected target until impact. Simpler weapons such as Maverick are LockOn-Before-Launch where the launch platform flies to line-of-sight to the target, the operator then locks the missile on and launches it, more complex weapons such as AGM-84E SLAM and GBU-15 are Lock-On-After-Launch where a datalink carries the TV image to a remote launch aircraft. The latter family of weapons is about to expand with the deployment of optical fibre guided weapons which unwind a hair thin optical cable as they fly, this cable carries the TV image from a nose camera back to the launch platform which sends guidance messages back to the weapon via the same cable. Unjammable and electromagnetically silent, these standoff weapons (missiles, glidebombs) will be used against aircraft, helicopters and surface targets.

The Electro-Optical Defensive Suite

Defending an aircraft against optically guided weapons and fire control systems will involve detection, tactical flying and jamming where applicable. Detecting an inbound missile or tracking by a hostile fire control system is a primary concern here for obvious reasons, and may be accomplished by several means. Optically guided missiles are passive and their only detectable emission is the heat of their exhaust plumes. Two strategies are therefore used for detection and tracking. The first is that of a missile warning radar which scans the airspace about the aircraft looking for returns with the appropriate Doppler shift for an inbound missile. The second strategy is the use of an infrared detection set which is a tracker optimised for detecting and tracking missile exhaust plumes. An example of the former strategy is the ALQ-156, its strength lies in its ability to estimate the range of the threat, its weakness is that it is broadcasting the location of the aircraft it is protecting. An example of the latter Is the AAR-34 as fitted to the F/FB-111 or AAR-44 or 47 as fitted to C-130 aircraft. The latter strategy offers truly passive operation and should not compromise the position of the carrying aircraft.

Detecting tracking by a hostile optical fire control system can be more difficult as its platform need not be easily detected. Thermal imagers and IRS&T sets in passive (angular) tracking mode cannot be detected unless one resorts to something drastic, such as scanning airspace with a laser and looking for glint off the infrared filter covering the threat’s optics. Doing this may detect the enemy but will certainly compromise your location. However if the enemy is serious he will probably want to get range information for a fire control solution and will thus turn on his laser rangefinder thereby blowing his cover and intentions to a defending aircraft carrying a Laser Warning Receiver (LWR). Devices such as the Perkin Elmer AVR-2 Laser Warning Receiver are analogous to radar warning receivers in that they use a set of four suitable optical detectors and listen for the pulse trains associated with particular fire control systems or guided weapons, feeding warning information into existing aircraft/helo radar warning equipment.

Once the threat has been detected defensive measures can be taken. The objective of these is to cause the tracker in the threat system to break lock. If the threat is a rotating reticle tracker (AAM, SAM, IRS&T) dropping flares may seduce the tracker, just as rotating the tailpipe away from the threat may break lock. The most effective strategy is however jamming with a device such as the Northrop AAQ-4, AAQ-8, MIRTS, Loral ALQ-123, Matador or Sanders ALQ-144. These devices typically use an alkali metal vapour (eg cesium) lamp which is electrically or mechanically modulated to flash with a selectable code known to interfere with the tracker in the threat system.

A newer alternative may be the tunable free electron laser which could be tuned to the most sensitive colour of the threat tracker and jam it (as above). If the threat is an imaging tracker this may not be adequate and terrain masking (ie placing a hill between oneself and the enemy) may be the only choice. A possible countermeasure as suggested above would be the use of a laser to damage or dazzle the threat tracker, the deployment of any such system has not been publicised in the open literature. The modern battlefield environment is optically hostile and survival will eventually necessitate the carriage of laser warning systems, infra-red missile detection equipment, infra-red jammers and the wearing of laser protective goggles by aircrew and foot soldiers alike.

The threat of blinding by hostile and friendly laser equipment is very real on the battlefield of the nineties and should be considered carefully. Unlike situations such as nuclear, biological and chemical warfare which are arguably unlikely to be encountered, optical weapons are here to stay.