What is SMGCS and A-SMGCS lighting?
How green centreline lights, red stop bars and yellow holding-position lights guide aircraft on the ground in low visibility — and how an advanced system switches them automatically along the route.
A Surface Movement Guidance and Control System (SMGCS) is the set of signs, markings, lights and procedures that guide aircraft and vehicles on the movement area and keep them apart, especially in low visibility. An Advanced SMGCS (A-SMGCS) adds surveillance, control, routing and guidance functions, so that a system, not only a controller, plans the route and switches the lights.
For the lighting, the key idea is guidance by segments. The green centreline is switched in sections, red stop bars hold traffic at runway holding positions, and with A-SMGCS the lit section moves with the aircraft: follow-the-greens. Taxiway centreline lights beyond a lit stop bar stay off (ICAO Annex 14 asks for at least 90 m).
The lights must be able to fall back to a safe state. If the surveillance, the routing or the link to the ALCMS fails, ATC takes over the switching and the SMGCS procedures apply.
| Function | SMGCS | A-SMGCS |
|---|---|---|
| Surveillance | Controller's view, pilot position reports, sometimes surface radar | Surface movement radar, multilateration and ADS-B on one display |
| Control | Procedures and controller judgement | Automatic detection and alerting of conflicts and incursions |
| Routing | Voice instructions, usually the standard route | Route planned by the system and shared with controller and pilot |
| Guidance | Signs, markings and lights switched from an ATC panel | Lights switched by the system along the route (follow-the-greens) |
| Lighting interface | Panel or ALCMS, by route, zone or circuit | ALCMS interface to the A-SMGCS, segment by segment, with status feedback |
| ICAO manual | Doc 9476 | Doc 9830 |
| Light | Colour and role | With A-SMGCS |
|---|---|---|
| Taxiway centreline | Green; continuous guidance along the route | Segments ahead of the aircraft lit, others off |
| Stop bars | Red, across the taxiway; a lit bar means stop | Extinguished only on ATC clearance, by the controller or by the system under controller authority; held on for conflicts |
| Intermediate holding position lights | Yellow; mark a hold short of another taxiway | May follow conflict alerts, as the aerodrome decides |
| Runway guard lights | Yellow, flashing; warn at a runway holding position | Not changed; operate by the aerodrome's rules |
| Exit taxiway centreline | Alternate green and yellow out to the edge of the ILS critical/sensitive area or the inner transitional surface (whichever is farther from the runway), then green | Not changed; fixed pattern |
| Edge lights, signs, markings | Fixed reference that stays in place | Not changed |
Frequently Asked Questions
Q1What is SMGCS?
A Surface Movement Guidance and Control System is the whole set of aids and procedures that guide and control aircraft and vehicles on the movement area. Guidance means signs, markings and lights that show pilots where to go and where to stop. Control means the procedures and ATC actions that keep traffic apart. It is described in ICAO Doc 9476 and matters most in low visibility.
Q2How is A-SMGCS different?
An Advanced SMGCS adds automation to the same job. Surveillance shows all traffic on one display, control detects and alerts to conflicts, routing plans a route for each movement, and guidance switches the lights along that route. It is described in ICAO Doc 9830. The lights are the same types as in a basic SMGCS, but the system, rather than a panel operator, decides which segments are on.
Q3Which lights give the guidance?
Green taxiway centreline lights show the route, red stop bars hold traffic at runway holding positions, and yellow intermediate holding position lights or clearance bars mark other points on the taxiway. Runway guard lights add a warning at runway holding positions, and exit taxiway centreline lights alternate green and yellow out to the edge of the ILS critical/sensitive area or the inner transitional surface, whichever is farther from the runway. Taxiway edge lights, signs and markings stay in place as the fixed reference.
Q4What is 'follow-the-greens'?
It is a guidance method where only the centreline segments ahead of the aircraft are lit, so the pilot follows the green lights to the destination, and the lights go out behind. The segments end at a lit stop bar, and centreline lights beyond a lit stop bar stay off, for at least 90 m under ICAO Annex 14. It works only if the taxiway centreline can be switched by segment, as in the selective switching entry.
Q5How does A-SMGCS connect to the airfield lighting?
The A-SMGCS server takes the traffic picture and the planned route, and sends switching commands to the airfield lighting control and monitoring system through a defined interface. The ALCMS switches the segments and stop bars and reports their status back. The interface must be able to address each segment, so the circuit design and lamp control have to support it.
Q6What happens if A-SMGCS or the interface fails?
The lighting must fall back to a safe state that the aerodrome has defined, with ATC taking over the manual switching of routes and stop bars and the SMGCS procedures applying. The fallback, the alarm and the handover should be tested at commissioning and after every change. Stop bars should not be left off because of a lost link.
Q7Is it safe to work on?
Changes to routes, segment definitions, interlocks or the interface alter what pilots see in the worst conditions, so they need ATC and authority agreement, a permit-to-work and a documented return-to-service test. Plan work outside low-visibility periods and tell ATC before any segment is taken out of service.
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