A stationary beacon dropped by USS Voyager into chaotic space
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 they 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-D sent out a stationary beacon in order to determine those factors. (TNG: "Where Silence Has Lease")
In 2375, USS Voyager dropped a beacon every 200,000 kilometers while stranded in chaotic space. This was a futile attempt to maintain a straight course, as it turned out the ship had actually traveled full circle in three hours due to the spatial distortion of the realm. (VOY: "The Fight")