Stationary beacon

A stationary beacon or simply beacon is a type of Starfleet space buoy used to establish a fixed point in space. The beacons had two flashing lights and sent out a continuous "ping" signal which could be tracked in order to confirm a ship's speed and to establish a reference point for maintaining course.

In 2365, while trapped in Nagilum's void, the USS Enterprise (NCC-1701-D) sent out a stationary beacon in order to determine the speed of the ship. 

In 2375, USS Voyager dropped a beacon every 200,000 kilometers while stranded in chaotic space in a futile attempt to maintain a straight course. It turned out the ship had actually traveled full circle in three hours due to the spatial distortion of the realm.