Mast Mounted Sight

On military helicopters, a mast mounted sight (MMS) is analogous to a periscope on a submarine: it is a way to provide a line-of-sight for sensors, without the rest of the vehicle being visible to enemy sensors. In this case, the "mast" is the central driveshaft for the helicopter's rotary wing (i.e., rotor).

For a MMS implementation, a hollow extension goes above the top of the driveshaft, capped with a roughly spherical sensor housing. The housing may be flattened, so it is more of a thick "flying saucer", or if more spherical, is called a "disco ball". In use, the pilot hovers the helicopter behind an barrier to observation, such as a building or hill, and then slowly rises until the sensors in the MMS have a view beyond the barrier.

Typical sensors in the ball include:
 * Millimeter wave radar

Most new helicopters use the technique; see, for example, AH-64 Apache and OH-58 Kiowa Warrior.