Stationary beacon

From Memory Alpha, the free Star Trek reference.

A stationary beacon is a Starfleet device that is used to establish a fixed point in space. The beacon sends out a "ping" signal which can be tracked in order to confirm a ship's speed and to establish a reference point. 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")