Isegoria

Most MANPADS fire heat-seeking missiles, but the British Starstreak does not, as I mentioned when they started shipping them to Ukraine:

In contrast, the Starstreak uses laser-beam-riding guidance, in which the operator fires the missile as soon as a target is detected in the optically stabilized sight. Line-of-sight is then maintained throughout the engagement process. The aiming unit projects two laser beams onto the target, with sensors on the missile calculating the relative positions until impact. The intensity of these laser beams is low enough that, the manufacturer claims, the targeted aircraft won’t be able to detect them.

Overall, this guidance method is more accurate than traditional laser guidance, in which the target is ‘painted’ with a single beam. The twin-laser approach is more resistant to maneuvering targets that could otherwise break the laser lock. At the same time, unlike infrared-guided MANPADS, the Starstreak cannot be spoofed by flares or other heat sources. Unlike most air defense missiles, it’s effectively immune to countermeasures, including the latest L-370 Vitebsk (exported as the President-S) directional infrared countermeasures (DIRCM) found on many Russian Aerospace Forces helicopters.

Another advantage is that smaller targets can be engaged (as long as the operator can see them through the sight), including those with infrared signatures that might be insufficient for a heat-seeking missile to track.

Its laser-beam-riding guidance evolved from earlier radar beam-riding guidance:

Beam riding is based on a signal that is pointed towards the target. The signal does not have to be powerful, as it is not necessary to use it for tracking as well. The main use of this kind of system is to destroy airplanes or tanks. First, an aiming station (possibly mounted on a vehicle) in the launching area directs a narrow radar or laser beam at the enemy aircraft or tank. Then, the missile is launched and at some point after launch is “gathered” by the radar or laser beam when it flies into it. From this stage onwards, the missile attempts to keep itself inside the beam, while the aiming station keeps the beam pointing at the target. The missile, controlled by a computer inside it, “rides” the beam to the target.

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By placing receiver antennas on the rear of the missile, the onboard electronics can compare the strength of the signal from different points on the missile body and use this to create a control signal to steer it back into the center of the beam. When used with conical scanning, the comparison can use several sets of paired antennas, typically two pairs, to keep itself centered in both axes. This system has the advantage of offloading the tracking to the ground radar; as long as the radar can keep itself accurately pointed at the target, the missile will keep itself along the same line using very simple electronics.

The inherent disadvantage of the radar beam riding system is that the beam spreads as it travels outward from the broadcaster (see inverse square law). As the missile flies towards the target, it, therefore, becomes increasingly inaccurate.

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Another issue is the guidance path of the missile is essentially a straight line to the target. This is useful for missiles with a great speed advantage over their target, or where flight times are short, but for long-range engagements against high-performance targets the missile will need to “lead” the target in order to arrive with enough energy to do terminal manoeuvres.

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Beam riding guidance became more popular again in the 1980s and 90s with the introduction of low-cost and highly portable laser designators. A laser beam can be made much narrower than a radar beam while not increasing the size of the broadcaster.

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