With command guidance the missile itself sees nothing at all. The fire control system uses its own sensors to track two objects — the target and its own missile — computes the steering solution on the ground (or the ship), and sends steering commands to the missile via a datalink (radio) or even a wire (older anti-tank missiles).
Advantages: the missile is cheap and simple (no seeker), all the intelligence sits safely on the platform and can be upgraded, and the guidance is immune to countermeasures aimed at a seeker (flares, chaff in the terminal phase). Disadvantages: the accuracy of all measurements degrades with the distance from the platform — at long range the angular errors become large in metres (Angular resolution) — and the datalink is an element that can be jammed and located. Moreover, the platform stays tied down and transmitting for the whole flight.
Command to line-of-sight
Command to Line-Of-Sight (CLOS) is the most common form: the system continuously keeps the missile on the line of sight between platform and target. The tracker follows the target; a second measurement (radar, TV or an IR beacon on the missile's tail) follows the missile; the computer steers the missile back onto the line of sight as soon as it deviates from it.
- MCLOS (Manual CLOS): the gunner steers both missile and aim point with a joystick — the earliest anti-tank missiles; heavily dependent on training.
- SACLOS (Semi-Automatic CLOS): the gunner only has to keep the sight on the target; the system automatically measures the missile position and corrects — most classic wire-guided anti-tank weapons and short-range SAMs.
- ACLOS (Automatic CLOS): tracker and missile tracker are both automatic (radar/EO) — typical of gun/missile CIWS and short-range air defence.
Characteristic: the missile flies the whole way on the (rotating) line of sight — kinematically the same drawback as beam riding, and effectively limited to short/medium range. The difference from beam riding: with CLOS the corrections are computed on the platform and sent up; with beam riding the missile measures its position in the beam itself.
Command off line-of-sight
With Command Off Line-Of-Sight (COLOS) the missile does not have to stay on the platform–target line of sight: the system tracks target and missile independently (possibly with different sensors in different directions) and steers the missile along a computed, more favourable path — for example with a lead point, an altitude profile above the fog, or a flanking move.
This makes possible:
- Intercept on a collision course instead of a chase: less energy loss, greater range against fast targets.
- Engagements outside the line of sight of the launcher (the tracker can be somewhere other than the launcher).
- Guidance beyond the range where a seeker is needed: many longer-range SAMs use command guidance as the midcourse method and only switch to homing in the terminal phase (Phases of the missile intercept, Homing guidance).
The price: even heavier demands on sensors and datalink (two precise tracks plus an uplink), and the same fundamental vulnerability — the datalink and the tracking radars can be jammed, and accuracy keeps decreasing with range. That is why "command midcourse + homing terminal" has become the dominant recipe: command guidance where it is good (the long middle section), homing where precision is needed (the last stretch).