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Aviation History
1973
1973 - 0144.PDF
90 FLIGHT International, 18 January 1973 Guiding anti-ship missiles BUj Avionics EXOCET, THE FRENCH SHIP-TO-SHIP missile, which has also been bought by the Royal Navy, is guided by inertial navigation for the greater part of its flight, before switching on its Electronique Marcel Dassault radar, which locates and locks onto the target. The radar is not switched on until the missile is near the end of its flight in order to give the minimum possible reaction time for the target's jamming system to operate. The initial position of the target is determined by the launch-ship's radar and thus its relative bearing and range are both reasonably accurate. Exocet's maximum range is 20 n.m., although it will probably be more effective at shorter ranges, say about 15 n.m. Under these condi tions its time of flight is unlikely to be more than 2min, so the "aim off" to allow for the target's movement during flight is very small. For example, a ship steaming at 30kt would have moved only a mile from its original position. It follows that the missile's radar would not need to search a very wide, arc, but the actual width needed depends very much on the separation between missile and target, and increases as the missile approaches the target. It has long been a dream of missile builders and users to devise a ship- based weapon which could be fired well outside the radar range of the target, thus creating greater surprise, i.e. an OTH (over-the-horizon) missile. With present techniques, it would have to be fired on a target position obtained from an aircraft, or another ship. Such position information may not be as accurate, relative to the launching ship, as that obtained by her own radar, so the OTH missile's radar has to be capable of searching a much wider arc. The electronic and mechanical arrangements then become more difficult. The joint French (Matra) and Italian (Oto Melara) missile Otomat will shortly be coming into service in the Italian Navy. It has a maximum effective range of over 40 n.m. Its Thomson-CSF radar is capable of searching an arc which includes a ship crossing at up to 40kt at what ever the range. Russian designers have gone for OTH surface missiles from the earliest days. Their standard ship missile, the Shaddock, has a range variously estimated as between 300 and 450 n.m. The exact form of guidance employed is not clear, but one possibility is that the aircraft illuminates the target with its radar and the missile homes on the reflected radar waves. Thus the missile would only be fitted with a directional radar receiver; a process long employed for anti-aircraft missiles and known as active-homing or beam-riding. Western specialists are not enthusi astic about mid-course guidance by aircraft because, apart from the possi bilities of jamming the radar, the air craft calls attention to itself and is very vulnerable to attack by missiles or fighters. That is not to say that Nato has discarded aircraft-guidance techniques altogether. An alternative method of guiding an OTH missile of limited range, say 50 n.m., might be for the launching ship to fly a helicopter fitted with a detection radar. The heli copter remains in the vicinity of the ship, but acquires the target on its radar at high altitude. The missile then homes on the reflected radar energy, using a semi-active homing system. A variation of this theme would be for the helicopter to pass the posi tion of the target to the ship for insertion into the missile's inertial- navigation system. Provided the heli copter remains close to the ship, and its position can be accurately deter mined, the location of the target can be determined with sufficient accuracy for the missile to use. The US Navy is developing the Harpoon missile, which is understood to have an over-the-horizon perform ance and which can be launched by ships or aircraft. A ship-launched OTH missile which does not require radar guidance from either ship or aircraft thus has an obvious advantage. Prior illumination by radar is an effective early-warning to the target. Also, radars can be jammed however they are carried or used. Infra-red homing equipment is much more difficult to jam. It is passive in nature, simply picking up informa tion without having to transmit signals for it. If the initial position of the target can be determined with out the target being aware of the action, an IR-guided missile can be launched with a good chance that its flight will be undetected. There are, of course, ways of foxing IR-homing missiles by presenting them with other, more "attractive" targets, such as flares, which radiate more intensely. The flare may be fired from a gun to a position well clear of the ship, or it can be dropped over the side. In both cases the critical factor is the time taken to deploy the flare after the missile has been detected. Such techniques appear quite realistic, if some way in the future. The target's position might be obtained from the bearing of its radar transmissions, or from monitoring the noise of its passage through the water, using hydrophones. The missile could then be fired by a ship well below the horizon, or even better, from a submarine under water. An infra-red homing device would need to have a wide arc of search, or to be sufficiently sensitive to pick up the target's IR emission at long range, thus reducing the width of its search pattern. Television guidance Television is another possible passive guidance system and there are two forms. In both cases a television camera is mounted in the nose of the missile, but in the first method its pictures are transmitted back to the controlling ship or aircraft, and an operator steers the missile to hit, also sending his commands by radio. In the second case the television camera measures the angle between its axis and that of the missile, and uses this error signal to keep the missile's flight-path directed towards the target. The first method is used by the air-launched Anglo-French, anti-ship Martel missile, but it suffers from the disadvantage that the radio trans missions can be intercepted and jammed. The second method, used by the USAF's air-ground Maverick, would certainly give no warning to the target, but there is always the possibility of the missile's television looking at the wrong target. At present operator-guided tele vision missiles use microwave fre quencies for data-transmission and guidance, implying line-of-sight ranges only. This is satisfactory for air- launched missiles where the aircraft can increase its height, and thus its horizon, at will. For surface-launched missiles this form of guidance limits the missile's range once again to that of the ship's horizon. No television- guided missile has yet been developed for use by ships and if an OTH weapon was required it would cer tainly have to be of the self-guided type. Ship-to-ship missiles are still in their infancy. The only navies to have them in operational service are those of Russia, Israel (with its Gabriel missile) and Greece, which has just fitted Exocet in some patrol boats. ex.
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