Which type of guidance uses external illumination to track the target?

Semi-active guidance uses external illumination to track a target. In this type of guidance, a target is illuminated by an external source, such as a laser or radar, and the missile or vehicle relies on the reflected signals from that illumination to home in on the target. This method requires that the target be illuminated while the weapon is en route, allowing it to make corrections based on the returned signals.

This guidance system contrasts with others, such as active guidance, where the missile carries its own guidance system and actively seeks out targets without the need for external illumination. Self-guided systems typically employ onboard sensors to navigate and guide themselves, while autonomous guidance involves pre-programmed flight paths that do not rely on real-time tracking or external inputs. In semi-active guidance, however, the reliance on an external illumination source distinguishes it as the correct choice.