From Halopedia, the Halo wiki
Guidance locks, also known as guidance packages, are missile components designed to guide missiles to their intended targets. The lock is projected through the launcher, be it a shoulder-mounted missile launcher, or a capital ship.
Most UNSC missile systems possess guidance locks, including the M19 missile, Anaconda surface-to-air missiles, the Lancet micro-missile, Argent V missiles, the ordnance of the LAU-65D/SGM-151, and ordnance launched from starships such as Shiva-class nuclear missile and Archer missiles. Some mine-type explosives such as Moray space mines also possess guidance locks.
Some Covenant weapons also possess a sort of built-in automatic guidance lock, such as the Type-33 fuel rod gun and the Type-33 Needler. Strikecraft such as the type-31 Morsam-pattern Seraph, carry four Class-3 Fuel Rod Cannons, which possess a guidance lock. Smaller Covenant ships, such as the Covenant stealth ship, sometimes use electronic countermeasures to counter guidance locks, as was the case in the Battle of Sigma Octanus IV. However, larger Covenant warships usually use point defense lasers to outright destroy incoming missiles instead.
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