The Federation navigational beacon sensor was a component in a navigational system. Aboard the Galaxy-class starships, there were at least two of these sensors.
In 2368, after the computer systems of the USS Enterprise-D were disrupted by a energy wave from a Satarran starship, Geordi La Forge accessed the basic system directories of the Engineering Department. According to the department, the Federation navigational beacon sensors were off-line. (TNG: "Conundrum")