It works by providing a visual signal to aircraft on final approach to land that the runway ahead is occupied by another aircraft or a vehicle. This is done by adapting the Visual Approach Slope Indicator Systems or Visual Approach Slope Indicator Systems system to alter from steady lights to flashing mode whilst the identified hazard remains. Externally, the PAPI or VASI system is unaltered and continues to function normally in its primary role as an angle of approach awareness indicator whether or not a FAROS input has temporarily caused the flashing mode to activate.
The input signal to the FAROS visual signal is provided automatically by the embedded inductive loops which are installed at all runway entry and exit points at certain airports for various operational uses and which are able to detect transiting traffic by the disturbance of the loop magnetic field which it causes. The Final Approach Runway Occupancy Signal (FAROS) system is provided to enhance pilot awareness only. It does not substitute for, or interfere with, existing ATC authority or flight crew procedures, and activation does not affect the validity of an existing ATC Landing Clearance. In many cases, it may be activated on short finals as another aircraft departs from the same runway or an aircraft or vehicle cross it in accordance with their ATC clearances.