Weapon lock

Weapon lock is a system that alerts the user of a guided projectile weapon that automatic tracking of a target vehicle has been acquired, and that the fired round will follow the target to the best of its ability.

Lock alarm
On many vehicles, a complementary alarm system alerts its occupants that such a weapon is being aimed at the vehicle, giving attentive users enough time to prepare evasive maneuvers or launch countermeasures such as smoke, IR flares, or active defense. The system uses a visual indicator and an audible klaxon. The alarm can also detect rangefinders used by other vehicles for their main weapons (a tank's main gun or rocket packs on various other vehicles).

The lock alarm does not actually warn against the projectiles used, but rather that an enemy is aiming a launcher towards it. There is also no indication of where the offending launcher is located. Anti-vehicle infantry can thus spoof vehicle operators by merely aiming at them (even with empty AV weapons), causing them to waste countermeasures or even leave the area to avoid conflict. Skillful users can also aim off-axis until their munitions are close to the target, or fire unguided to prematurely end the alarm.