Mast-mounted sensors are electro-optical or radar sensors on a vertical mount that extends above the rotor hub of a military helicopter. Much like a submarine that only reveals its periscope, a helicopter can hover behind a concealing obstruction, and only allow the mast-mounted sensor to go above the protection of the obstruction and search for targets.
The mast-mounted assembly may contain a laser designator, laser rangefinder, or both to assist in guiding weapons to a target. Such weapons are usually fired from a land or air platform other the one doing the sensing, although an armed helicopter can raise only the mast to search for targets, pop up and fire a weapon, and drop back down. Typically, a helicopter doing such a pop-up will move rapidly to the side, to avoid counterfire.