Stationary beacon
A stationary beacon was 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 a course. 

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

In 2375,  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. ()

See also
 Homing beacon
 Subspace beacon

de:Signalbake
Communications technology