The external navigational interferometer was a navigational system. Aboard the Galaxy-class starships, there was a left and right system. 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 left and right external navigational interferometers were off-line. (TNG: "Conundrum" )