How it Works

Operational Concept & Status Light Description

Runway Status Lights increases pilots and vehicle operator’s situational awareness by directly providing runway occupancy status through the autonomous illumination of in-pavement lights on runway and taxiways. Runway Status Lights consists of three light types:

Runway Entrance Lights are placed at runway/taxiway intersections and are visible to pilots taxiing toward runways and warn pilots it is unsafe to enter or cross a runway because it is or soon will be occupied by high-speed traffic such as an aircraft taking off or landing RELs
Takeoff Hold Lights are placed on runways at departure positions and visible to pilots in position for takeoff and warn pilots it is unsafe to takeoff because the runway ahead is occupied by another aircraft of vehicle THLs
Runway Intersection Lights are placed on runways approaching a runway/runway intersection and warn pilots in a takeoff or landing roll that the intersection ahead is unsafe to enter or cross because a conflict exists RILs

Combined Operation of RWSL

Runway Status Lights depend on the Runway Entrance Lights, Takeoff Hold Lights, Runway Intersection Lights, and FAROS to maximize effectiveness in runway incursion and accident prevention. The concept of operations relies on the ability to warn at least one of the aircraft or vehicles in the conflicting scenario and in some cases, provide redundant warnings to everyone involved for increased safety.


The Runway Status Lights system uses fused surveillance data processed through intricate state and safety logic to command in-pavement lights to illuminate on the airport surface providing safety information to pilots and vehicle operators autonomously the RWSL Block Diagram to the right illustrates the sequence in which this occurs:

Surveillance Input

RWSL CombinedThe Runway Status Lights system utilizes existing airport surveillance technology in conjunction with highly advanced data fusion techniques and state logic to automatically drive the status lights on the airport surface. RWSL has knowledge of the location of each aircraft and vehicle on the airport surface and arriving or departing the airport based on information provided by three diverse surveillance sources 1) Primary radar returns from the Airport Surface Detection Equipment (ASDE), 2) Time difference of arrival multilateration utilizing interrogation and replies from transponder-equipped aircraft and vehicles, and 3) the terminal radars used for air traffic control, Airport Surveillance Radars.

Surveillance Data Fusion

The technical challenge that exists is fusing the plethora of data that is provided from diverse sensors such as the ASR, ASDE and multilateration. The major fusion technical challenges are to avoid multiple tracks for the same aircraft and to cope with the bias between positional data from the ASDE (which senses radar centroid) and multilateration (which senses beacon antenna location). Additionally, fusion must also deal with the practical problems associated with turned off, malfunctioning, miswired, or non compliant transponders and the fact that the multilateration system can output position estimated for both desired transponder replies and unsolicited replies generated by the other systems that use it. Fusion techniques to deal with these problems are still being developed. As a consequence, the Runways Status Lights system utilizes additional fusion techniques beyond what is accomplished prior to receipt of all the surveillance reports of interest. RWSL fusion uses all input sources available to create “clean” system tracks that are finally processed through the state machine and light logic. RWSL fusion allows for system tracks to be generated from single or multiple surveillance sources to address the issue of malfunctions or non-existence of sensors or equipment. An example of this inherent flexibility: service vehicles that operate on the runways and taxiways are not always equipped with transponders that respond to multilateration interrogations or aircraft which are required to be equipped do not always have it turned on or functioning properly, in these cases RWSL can still use the primary radar returns solitarily to create a RWSL system track that will be processed through the light logic. RWSL can also use multilateration data single-handedly without the primary radar returns in cases of sensor outages or maintenance. This is an extremely valuable feature that greatly increases RWSL’s flexibility in providing a safety benefit in variable situations.

RWSL Light Logic

RWSL safety logic process accepts the fused surveillance, determines the operational state of the track (e.g., stopped, taxiing, landing, or departing), predicts likely future behavior based on the current state, and determines when and which lights should be illuminated. Location of traffic and their dynamics states drive the decision-making process for light illuminations. A simplified example of how RWSL safety logic determines when RELs should turn on and off along a runway during a departure operation is provided in the table below:

Example RELs Safety Logic During a Departure Operation

Aircraft Behavior
Stopped on the runway awaiting takeoff clearance
Begins departure
Becomes a high-speed operation (> 25 kts)
taxiway intersections
Rotates and begins climbing
DEP (departure)
DEP (departure)
AIR (airborne)
OFF at nothreat locations,

Field Lighting System

Lighting System DiagramUsing output data provided from the surveillance, fusion, and safety logic processes, the Light Control Computer (LCC) will then communicate with the Field Lighting System (FLS) to activate and de-activate lights installed on and around the equipped runways. Illumination of these lights will indicate to pilot or vehicle operators the status of the runway or runway/runway intersections.