Semi-automatic command to line of sight (SACLOS) is a method of missile guidance. In SACLOS, the operator has to continually point a sighting device at the target while the missile is in flight. Electronics in the sighting device and/or the missile then guide it to the target.
SACLOS devices commonly work using one of two methods: either wire and radio-guided, or beam-riding.
Wire and radio-guided SACLOS
With wire and radio-guided SACLOS, the sighting device can calculate the angular difference in direction from the missile position to the target location. It can then give electronic instructions to the missile that correct its flight path so it is flying along a straight line from the sighting device to the target. Most of antitank SACLOS systems such as Milan and TOW use a strobe or flare (visible, infrared (IR) or ultraviolet (UV) light) in the tail of the missile with an appropriate sensor on the firing post, to track the missile's flight path. The launching station incorporates a tracking camera with two lenses. A wide field of view lens that locates and "gathers" the missile near the center of the gunners line of sight immediately after launch, and a narrow view lens with automatic zoom that accomplishes the fine tracking adjustments. In most configurations, the narrow field camera utilizes electronics that translate the brightest spot in the view - the flare or strobe of the missile - into an electrical impulse. This impulse changes as the missile leaves the center of the field of view, and the electronics automatically apply a correction instruction in the opposite direction of the change to re-center the missile.
These instructions are delivered either by a radio link or a wire. Radio links have the disadvantage of being jammable, whereas wire links have the disadvantage of being limited to the length of the wire and being breakable (e.g., not very good for penetrating/attacking targets in vegetated areas such as forests) and can not be fired over bodies of water due to potential shorting of the wires.
- Wire-guided: MILAN, Swingfire, AT-4 Spigot, AT-5 Spandrel, AT-7 Saxhorn, BGM-71 TOW, Bumbar, ENTAC, SS.11, Vickers VIGILANT
- Radio-guided: ASM-N-2 Bat, 9K33 'Osa' (SA-8 'Gecko'), Javelin (surface-to-air missile)
With beam-riding SACLOS, the sighting device emits a directional signal directed toward the target. A detector in the tail of the missile looks for the signal. Electronics in the missile then keep it centered in the beam.
It differs notably from the SARH (semi-active radar homing) and SALH (semi-active laser homing) in which target is illuminated by a powerful emitter and a sensor on the head of missile direct it to the target.
Radar was the most common form of SACLOS signals in early systems, because, in the anti-aircraft role the target is typically already being "illuminated" by a radar signal. However, a beam-riding missile flies directly at the target, which is often inefficient for a high-speed target like an aircraft. For this reason, most anti-aircraft missiles follow their own route to intercept the target, and do not "ride" the beam. A more modern use of beam-riding uses laser signals because they are compact, insensitive to distance, and are difficult to detect and jam.
This was also one of the main advantages over concurrent SALH systems: regarding detection laser riding beam emitter is typically a low powered device and does not need to be pointed immediately to the target, and because the missile sensor "looks" backward to it, the whole system is impervious to most jamming devices. Another advantage in antitank applications is that the backward-looking guidance system does not interfere with the process of jet formation of HEAT charges, thus maximizing weapon's effectiveness. However, such systems don't allow for a top-attack mode, or target illumination from a different source than the launcher itself, so choice between the two operating modes may vary between operators. The main disadvantage of both SACLOS guidance systems in an anti-tank role is that working on angular differences evaluation, it doesn't allow any notable separation between guidance system and missile launch post opposite to former MCLOS ones, thus allowing updated version of such anti-tank weapons (notably AT-3 Malyutka) to still remain in service in some countries.
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