Final Approach Runway Occupancy Signal (FAROS)

The Final Approach Runway Occupancy Signal (FAROS) is an advanced airfield safety capability that provides visual alerts to pilots on final approach when a runway is occupied. Rather than being a standalone light fixture, FAROS is an add-on control function for the existing Precision Approach Path Indicator (PAPI): when surface surveillance detects that the runway's protected zone is occupied, the FAROS controller commands the PAPI light units to flash instead of burning steady, giving the approaching pilot a clear, unambiguous "runway not clear" cue without requiring any additional cockpit equipment or waiting on an ATC radio call.

FAROS exists to close a specific, dangerous gap in runway safety: a pilot on short final has no reliable way to see for themselves whether the runway ahead is actually clear, since another aircraft, vehicle, or obstruction on the far end of a long runway may not be visible from the approach path, and radio communication about runway status can be delayed, missed, or misunderstood. By switching the PAPI — a light array the pilot is already looking at for glide-path guidance — into a flashing state exactly when the runway is occupied, the system gives a go-around decision cue that doesn't depend on air traffic control instruction or the pilot's own visual judgement of a runway they may not be able to fully see. Its purpose, in short, is to convert a hidden runway-occupancy hazard into a direct, self-evident visual warning at the single moment — final approach — when a wrong decision has the least time to be corrected.

FAROS systems are installed at busy airports with high traffic volumes and complex runway configurations where approach conflicts are a significant risk. They are typically used on:

• Precision approach runways (CAT II and III) where safety margins are critical
• Major international airports with intersecting runways and complex approach paths
• Runways with high traffic density where incursion risk is significant
• Airports implementing advanced safety management systems
• Runways with limited visibility operations where visual cues are essential

FAROS is retrofitted onto the runway's existing PAPI installation — typically a 4-box L-880 (or 2-box L-881) array sited beside the runway roughly 300 m (1,000 ft) from the threshold, on the same low, frangible-mounted ground stands used for standard PAPI. No new elevated light fixture or dedicated FAROS bar is required; the FAROS driver is added inline between the PAPI's own power/lamp-control unit and its light heads so it can switch the units into a flashing state without disturbing their aiming or glide-path calibration. The system is tied into the airport's surface surveillance source (ASDE-X, ASSC, or ADS-B-based tracking) via a FAROS controller that evaluates occupancy status for the defined activation zones and issues the flash command on detection of a conflict. The controller, its power supply, and its monitoring unit are usually housed in an equipment shelter near the PAPI site, and installation is coordinated closely with the surveillance system's own commissioning to validate accurate occupancy detection before activation is enabled.

The FAROS system operates through an integrated surveillance and alert mechanism:

• Surveillance: Surface surveillance data (ASDE-X, ASSC, or ADS-B-based tracking) tracks aircraft and vehicle positions on the airfield.
• Detection: The FAROS processing system evaluates whether a defined runway activation zone is occupied by an aircraft or vehicle.
• Activation: When an activation zone is occupied, the FAROS controller switches the runway's existing PAPI light units from steady-burning to a flashing state.
• Alert: The flashing PAPI gives the approaching pilot a clear, unambiguous indication that the runway is not clear for landing.
• Deactivation: When the activation zone is clear, the PAPI reverts to its normal steady glide-path indication and the pilot proceeds with the approach.

Unlike a series-circuit AGL fixture, FAROS is not commanded by the tower or an ALCMS brightness step — it is a self-contained, sensor-driven system that decides its own state in real time and acts on the PAPI it is attached to, rather than on a fixture of its own. Surface surveillance feeds (ASDE-X, ASSC, or ADS-B-based tracking) are continuously ingested by the FAROS occupancy processor, which evaluates whether the protected runway segment is occupied and, on detecting a conflict, drives the PAPI's existing light heads into a flashing mode independent of any ATC action. Because a missed occupied-runway call is far more dangerous than an unnecessary flash, the logic is fail-safe by design: any loss of confidence in the surveillance feed, a processor fault, or a communications dropout defaults the system to the conservative "occupied" (flashing) state rather than reverting silently to steady-burning, and the fault is separately reported to the ALCMS/tower for maintenance action.

FAROS does not introduce a new optical fixture: the light-emitting hardware remains the runway's existing PAPI light units (typically a sealed LED or halogen luminaire in a die-cast aluminium enclosure, anodised or powder-coated, with a red/white split beam behind a toughened glass lens), mounted low to the ground on frangible couplings beside the runway per standard PAPI siting. What FAROS adds is a driver/interface module — usually a card or unit built into or wired alongside the PAPI's power and control unit (PCU) — that can override the PAPI's normal steady output and switch the light units into a flashing pattern on command. This driver electronics package is potted or conformal-coated for moisture protection and connected via sealed, direct-burial-rated cable glands to a dedicated mains feed and UPS backup circuit, since the FAROS function must remain available even through a brief power interruption. A weatherproof junction box near the PAPI site houses the interface to the surface-surveillance/RWSL, A-SMGCS, or ALCMS control link, with corrosion-resistant fasteners and hardware used throughout given the fixture's exposed, low-lying siting near the runway environment.