SACLOS is an acronym for Semi-Automatic Command to Line of Sight, a second-generation method of missile
Though a missile may be any thrown or launched object, it colloquially almost always refers to a self-propelled guided weapon system.-Etymology:The word missile comes from the Latin verb mittere, meaning "to send"...

 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
Infrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...

 (IR) or ultraviolet
Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...

 (UV) light) in the tail of the missile with an appropriate sensor on the firing post, to track the missile's flight path.

These instructions are delivered either by a radio link or a wire. Radio links have the disadvantage of being jammable
Radio jamming
Radio jamming is the transmission of radio signals that disrupt communications by decreasing the signal to noise ratio. Unintentional jamming occurs when an operator transmits on a busy frequency without first checking whether it is in use, or without being able to hear stations using the frequency...

, 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).
  • Wire-guided: MILAN
    MILAN " is French and German for "kite bird") is a European anti-tank guided missile. Design of the MILAN started in 1962. It was ready for trials in 1971, and was accepted for service in 1972. It is a wire guided SACLOS missile, which means the sight of the launch unit has to be aimed at the...

    , Swingfire
    Swingfire was a British wire-guided anti-tank missile developed in the 1960s and produced from 1966 until 1993.-Development:Swingfire was developed by Fairey Engineering Ltd and the British Aircraft Corporation. It replaced the Vickers Vigilant missile in British service...

     AT-4 Spigot
    AT-4 Spigot
    The 9K111 Fagot is a SACLOS wire-guided anti-tank missile system of the Soviet Union. "9M111" is the GRAU designation of the missile. Its NATO reporting name is AT-4 Spigot.-Development:...

     AT-5 Spandrel
    AT-5 Spandrel
    The 9M113 Konkurs SACLOS wire-guided Anti-tank missile of the Soviet Union. "9M113" is the GRAU designation of the missile. Its NATO reporting name is AT-5 Spandrel.-Development:...

     AT-7 Saxhorn
    AT-7 Saxhorn
    The AT-7 Saxhorn is the NATO reporting name for the 9K115 Metis man-portable SACLOS wire-guided anti-tank missile system of the Soviet Union.-Development:The missile was developed by the Tula KBP...

    In the composites industry, a tow is an untwisted bundle of continuous filaments, and it refers to man-made fibres, particularly carbon fibres ....

  • Radio-guided: ASM-N-2 Bat, 9K33 'Osa' (SA-8 'Gecko'), Javelin (surface-to-air missile)

Beam-riding SACLOS

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 is an object-detection system which uses radio waves to determine the range, altitude, direction, or speed of objects. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, weather formations, and terrain. The radar dish or antenna transmits pulses of radio...

 was the most common form of SACLOS signals in early systems, because, in the anti-aircraft role the target is typically being hit by a radar signal anyway. 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 the target, and do not "ride" the beam.

A more modern use of beam-riding uses laser
A laser is a device that emits light through a process of optical amplification based on the stimulated emission of photons. The term "laser" originated as an acronym for Light Amplification by Stimulated Emission of Radiation...

 signals as they are compact, insensitive to distance and they are very hard both to detect and jam.

That was also one of the main advantages over concurrent SALH systems: regarding detection laser riding beam emitter is typically a low powered device and doesn't need to be pointed immediately to the target and as the missile sensor look backward to it, the whole system is impervious to most of jamming
Jamming may mean:* Interfering with communications or surveillance:** Radio jamming** Radar jamming and deception** Mobile phone jammer** E-mail jamming* Jamming , cheered show-offs during social dancing...

Another advantage in antitank applications is that the backward -looking guidance system doesn't interfere with the process of jet formation of HEAT
In physics and thermodynamics, heat is energy transferred from one body, region, or thermodynamic system to another due to thermal contact or thermal radiation when the systems are at different temperatures. It is often described as one of the fundamental processes of energy transfer between...

 charges, thus maximizing weapon's effectiveness.

However such systems don't seems to allow top-attack mode nor target illumination from a different source than the launcher itself, so choice between the two operating modes may vary between operators.

Main problem to both SACLOS guidance systems in anti-tank role is however that working on angular differences evaluation, it doesn't allow any notable separation between guidance system and missile launch post opposite to former MCLOS
MCLOS is a first-generation method for guiding guided missiles.With an MCLOS missile, the operator must track the missile and the target simultaneously and guide the missile to the target. Typically the missile is steered with a joystick, and its path is observed through a periscope-type...

 ones, thus allowing updated version of such anti-tank weapons (notably AT-3 Malyutka) to still remain in service in some countries.

  • Kornet, Starstreak
    Starstreak missile
    Starstreak is a British short range surface-to-air missile manufactured by Thales Air Defence , in Belfast. It is also known as Starstreak HVM where HVM stands for "High Velocity Missile". After launch the missile accelerates to approximately Mach 3.5, at which point it launches three laser beam...

    , Ingwe
    Ingwe (missile)
    The Ingwe is a modern South African multi-role laser guided anti-tank guided missile manufactured by Denel Dynamics .The missile was designed to be employed in various roles, either by infantry...

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