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Aviation History
1977
1977 - 2430.PDF
FLIGHT International, 13 August 1977 491 Lasers pierce the battlefield gloom OVER the last few months many RAF Harriers have sprouted bottle noses, and new equipment has appeared behind Jaguar chisel-nose windows. The reason for these changes is the introduction of the Ferranti laser ranger and marked- target receiver (LRMTS). Most combat aircraft have nose radar, and the introduction of some thing as radically different as a laser seems at first sight inexplicable. Lasers have some characteristics which make them more suitable for jobs formerly carried out by radar, however, and their growing grip on target tracking and ranging is unlikely to relax in the future. Laser equipment radiates at a single wavelength and in a coherent beam (i.e., all the light waves are in phase). The laser beam is also very narrow and intense. It can be aimed accu rately, and is easy to point and steer even when the target is approached at the low grazing angles needed for low-level attacks. These attributes allow the laser to compare impres sively well with the best radar sys tems, and the fact that laser beams are difficult to detect and jam gives them the advantage. Their accurate ranging and target-seeking perform ance against ground targets has long been sought by designers, and lasers seem destined to become a common element in nav-attack equipment. Laser technology is relatively new —the first practical source was built as recently as 1960—and the physics involved is well into the advanced sections of scientific textbooks. The source used in the RAF's Ferranti system is a neodymium-doped yttrium aluminium garnet (YAG) crystal pro ducing a beam of light at 1 • 064,» wavelength, in the near infra-red por tion of the electromagnetic spectrum. The British ground forces are equipped with target-marking systems (otherwise known as designators, a term originally used in America and now gaining popularity in Britain) operated by forward air controllers. The ground equipment is a small portable source which illuminates the target with laser pulses, usually from a range of up to five miles. The air craft seeker detects the target by col lecting reflected laser energy, and uses its own laser to measure range once directed at the target. Range is derived by measuring the time taken for pulses of laser energy to travel out to the target and back to the aircraft, just as with conven tional radar. But a laser's very pre dictable and sharp-edged pulses result in ranging accuracy unsurpassed by any equivalent system. The equip- n n n u u u _J i Avionic as those described for Jaguar and Harrier. If there is no ground control ler to identify targets, the airborne lasers in all of these systems provide accurate ranging data during visual attacks. The UK's approach to laser-based weapon systems has been funda mentally different to that adopted elsewhere, particularly in the US. Most American laser systems are mounted in detachable pods, a trend which may be followed in Britain. This approach allows a few expensive systems to be shared between a large number of aircraft, improving cost- effectiveness. Against such savings have to be weighed drag penalties and the loss of at least one pylon as a weapon carrier. The UK has not When not in use the Harrier laser is pro tected by "eyelids" which shield the optics from dust thrown up during vertical take off and landing. Con ventional aircraft can have the system mounted behind trans parent panels in the nose or on a retrac table tray under the fuselage ment used in Harrier and Jaguar emits about ten pulses per second, a compromise which provides suffi ciently rapid updates without demand ing so much power that complex cool ing is needed. During a typical close-support attack a Harrier or Jaguar navigates to the neighbourhood of the target using its inertial navigation system, switching on the laser to search for designator illumination when close to the target. Laser ranging perform ance is affected by weather, but under normal conditions the target would probably be acquired at 5-10 n.m. Once the target is acquired by the reflected- radiation receiver, the laser ranger begins to provide accurate range in formation. Weapons can then be re leased with a degree of precision un matched by non-laser ranging devices. High position-fixing accuracy, essen tial to the success of low-level single- pass attacks, is almost guaranteed by LRMTS. Similar equipment is fitted to Tor nado. En route navigation is handled by an inertial system with radar up dating and, possibly, radar-commanded terrain-following. The laser is fitted on to a retractable tray under the forward fuselage. Tornado positioning and ranging procedures are the same yet developed a laser-guided weapon, buying instead the American Pave Spike/Paveway system (Pave Spike is the designator pod, Paveway the weapon) for retrofitting to some RAF aircraft. Meeting a special require ment, Pave Spike/Paveway is not seen as a competitor for UK systems. For designation and ranging the laser is usually more compact and cheaper than radar, and certainly more accurate. So if radar's other attributes (e.g. mapping) are un wanted, the laser wins handsomely. Lasers are usually very reliable and currently invulnerable to jamming, although target-seeker decoys are a possibility. A laser - countermeasures market has not yet appeared, but this could be an area of intense activity in the future. One change likely to affect future UK laser development is the move towards carbon-dioxide gas lasers, a field in which British teams are very active. These lasers emit radiation in the 10-6,« (far infra-red) wavelength, well clear of the visible-light spectrum and harmless to the eye. Further more, carbon-dioxide lasers are less affected by smoke, a common battle field hazard, and can be readily inte grated with thermal-imaging (infra red) devices.
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