The location of the field geometry sensor in the nacelles of the Enterprise-B
The field geometry sensor was a type of sensor used on the Excelsior-class starships in the late 23rd century. These sensors, located in the warp nacelles, were used for detecting and measuring the warp field generated by the starship.
In 2293, the location of these sensors were labeled in the USS Enterprise-B master systems display on the ship's bridge. (Star Trek Generations)