Airfield Ground Lighting (AGL) systems are designed to support aircraft operations under different visibility conditions. These conditions are defined by precision approach categories (CAT-I, CAT-II, and CAT-III), supported by Instrument Landing System (ILS) facilities of matching performance.
For professionals working in airport lighting systems, runway lighting design, and AGL project execution, understanding these categories is essential. Each category defines how critical the AGL system becomes for pilot guidance, especially during approach, landing, and taxiing operations.
Higher AGL category means higher dependency on lighting, higher reliability requirements, and higher operational safety. The progression from CAT-I to CAT-III is not simply an increase in intensity — it's increasing dependence on precise visual guidance.
What Defines CAT-I, CAT-II, CAT-III?
The classification is based on Decision Height (DH) and Runway Visual Range (RVR):
| Category | Decision Height (DH) | Runway Visual Range (RVR) |
|---|---|---|
| CAT-I | ≥ 200 ft | ≥ 550 m |
| CAT-II | < 200 ft and ≥ 100 ft | ≥ 300 m |
| CAT-III | < 100 ft or no DH | < 300 m (IIIA ≥ 175 m; IIIB 50–175 m; IIIC no limit) |
CAT-III is further divided into IIIA, IIIB, and IIIC (by RVR as shown), and is widely associated with low-visibility airport operations, particularly in fog-prone regions such as North India. As visibility decreases, requirements for precision, reliability, monitoring, and redundancy all increase. Actual minima for a given operation also depend on the runway, aircraft, crew and operator approvals.
AGL Systems for CAT-I
CAT-I is the basic precision approach category used at many standard airports. A typical CAT-I lighting system includes:
- Runway edge lights
- Threshold and runway end lights
- Precision approach CAT I lighting system, where practicable (FAA equivalents include MALSR or ALSF-1)
- Taxiway lighting system
CAT-I systems generally provide standard intensity levels, ICAO photometric compliance for the fittings installed, and limited redundancy — suitable for normal night operations and medium-visibility conditions.
At this level, AGL provides essential visual guidance while operating with comparatively lower system complexity and redundancy than CAT-II and CAT-III installations.
AGL Systems for CAT-II
CAT-II requires higher precision and is commonly used at busy international airports. Important additions include:
- Runway centreline lights — required for CAT II/III
- Touchdown Zone (TDZ) lights
- CAT II/III approach lighting system (FAA equivalent: ALSF-2)
- Tighter brightness control and monitoring through the Constant Current Regulator (CCR)
CAT-II systems add required elements, tighter intensity matching and aiming, and higher serviceability and monitoring expectations.
At this level, AGL becomes a primary visual reference system for pilots — alignment accuracy, photometric performance, brightness control, and reliability all grow in importance.
AGL Systems for CAT-III
CAT-III systems are designed for very low-visibility operations, including dense fog conditions. Critical features include:
- Runway centreline lighting system (as for CAT II)
- Touchdown Zone (TDZ) lighting (as for CAT II)
- CAT II/III approach lighting system (FAA: ALSF-2)
- Stop bars at runway-holding positions (Annex 14 ties these to low-RVR operations)
- Taxiway centreline lights for low-RVR taxi guidance
- Redundant CCRs and interleaved circuits
- Secondary power meeting Annex 14 switch-over times (as short as 1 second for key CAT II/III elements), typically via UPS and generators
CAT-III systems rely on strict ICAO compliance, tight tolerances on intensity matching and aiming, and a high-integrity design with redundancy — critical for CAT-III-capable airports such as Delhi IGI and other major hubs.
At CAT-III level, the lighting system must provide highly reliable and precise visual guidance when pilots have very limited external visual references.
Role of AGL Photometry in All Categories
AGL photometry plays a key role in ensuring compliance and safety across all categories. Important considerations include:
- Precise beam alignment for correct pilot perception
- Exact intensity as per ICAO Annex 14
- Uniform light distribution across runway and taxiway
- Glare control at high intensity levels
For AGL manufacturers, testing laboratories, and airport authorities, photometric validation is normally required, by the project specification and the regulator, before commissioning.
Correct intensity alone is not sufficient. Beam direction, distribution, uniformity, and glare control must also be considered to ensure effective visual guidance.
Reliability & Redundancy Requirements
System reliability increases with category level:
This increasing level of reliability can include dual power sources, multiple CCR systems, continuous monitoring and control systems, and fast fault detection and isolation. Annex 14 Chapter 10 also sets minimum serviceability levels for lights (for example 95% for key CAT II/III elements and 85% for CAT I elements, with no adjacent lights out), which are the regulatory floor rather than a design target.
For CAT-III operations in particular, the AGL system must be designed so failures can be identified and managed quickly while maintaining the required operational capability.
Operational Importance for Airports
A higher AGL category directly improves airport operational performance. Key benefits include:
Airports with CAT-III capability can operate efficiently even during dense fog and adverse weather — making advanced AGL infrastructure key to operational continuity in severe low-visibility regions. AGL is only one of the enablers: CAT II/III operations also depend on ILS performance, aircraft and crew approvals, and the aerodrome’s Low Visibility Procedures.
Key Takeaways
- Categories are defined by Decision Height and Runway Visual Range — CAT-I ≥ 200 ft / ≥ 550 m, down to CAT-III < 100 ft / < 300 m
- CAT-II/III require centreline and TDZ lighting and a CAT II/III approach lighting system; stop bars, taxiway centreline lights and tighter reliability follow low-RVR operations
- Photometric validation — intensity, beam direction, uniformity, and glare control — is normally required before commissioning at every category
- Reliability requirements progress from basic circuit reliability (CAT-I) to full redundancy and fail-safe operation (CAT-III)
- Higher-category AGL infrastructure enables Low Visibility Operations and protects airport capacity during fog and adverse weather
| Parameter | CAT-I | CAT-II | CAT-III |
|---|---|---|---|
| Visibility environment | Normal to medium visibility | Reduced visibility | Very low visibility |
| Runway centreline lighting | Not required (recommended in some cases) | Required | Required |
| TDZ lighting | Not required | Required | Required |
| Approach lighting | CAT I approach lighting (where practicable) | CAT II/III approach lighting | CAT II/III approach lighting |
| Brightness control | Multi-step | Multi-step, tighter control | Multi-step, monitored switching |
| Redundancy | Limited | Enhanced | Full/redundant |
| Monitoring | Basic | Improved | Continuous/advanced |
| Operational role | Standard precision approach | Higher-precision operations | Low Visibility Operations |
Increasing Dependence on Precise Guidance
CAT-I → CAT-II → CAT-III — reliability and redundancy scale with reduced visibility
Conclusion
CAT-I, CAT-II, and CAT-III AGL systems represent increasing levels of precision, performance, and safety in modern aviation infrastructure. From basic runway guidance to highly dependent low-visibility operations, AGL systems play a critical role in safe aircraft movement and airport efficiency.
Designing and executing these systems requires a deep understanding of ICAO standards, FAA specifications, AGL photometry, system reliability engineering, electrical infrastructure, and installation and commissioning requirements.
In practical airport projects, these requirements directly influence lighting configuration, electrical architecture, photometric validation, commissioning, maintenance, and operational readiness.
Higher AGL category means higher dependency on lighting, higher reliability requirements, and higher operational safety.
Frequently Asked Questions
How are CAT-I, CAT-II and CAT-III defined?
They are defined by Decision Height (DH) and Runway Visual Range (RVR). CAT-I is DH of 200 ft or more with RVR of 550 m or more, CAT-II is DH below 200 ft and at least 100 ft with RVR of at least 300 m, and CAT-III is DH below 100 ft or no DH with RVR below 300 m, further divided into IIIA, IIIB and IIIC. Actual minima also depend on the runway, aircraft, crew and operator approvals.
What AGL is typical for a CAT-I runway?
A typical CAT-I system includes runway edge lights, threshold and runway end lights, a precision approach CAT I lighting system where practicable and taxiway lighting. It generally provides standard intensity levels, ICAO photometric compliance for installed fittings and limited redundancy.
What does CAT-II add to the lighting system?
CAT-II adds runway centreline lights, touchdown zone (TDZ) lights, a CAT II/III approach lighting system and tighter brightness control and monitoring through the CCR. It also brings tighter intensity matching and aiming and higher serviceability and monitoring expectations, making AGL a primary visual reference for pilots.
What is special about CAT-III AGL systems?
CAT-III systems support very low-visibility operations and include centreline and TDZ lighting, a CAT II/III approach lighting system, stop bars at runway-holding positions and taxiway centreline lights for low-RVR taxi guidance. They also use redundant CCRs and interleaved circuits, and secondary power meeting Annex 14 switch-over times, as short as 1 second for key CAT II/III elements.
Why does AGL photometry matter at every category?
Photometry ensures precise beam alignment, exact intensity as per ICAO Annex 14, uniform light distribution and glare control at high intensity levels. Photometric validation is normally required by the project specification and the regulator before commissioning, and correct intensity alone is not sufficient.
How do reliability and redundancy requirements change with category?
They progress from basic circuit reliability at CAT-I, to improved monitoring and control at CAT-II, to full redundancy and fail-safe operation at CAT-III. Annex 14 Chapter 10 also sets minimum serviceability levels, for example 95% for key CAT II/III elements and 85% for CAT I elements with no adjacent lights out, which are the regulatory floor and not a design target.
Is AGL the only thing needed for CAT-II and CAT-III operations?
No. AGL is only one of the enablers. CAT II/III operations also depend on ILS performance, aircraft and crew approvals and the aerodrome's Low Visibility Procedures. A higher AGL category supports Low Visibility Operations, reduces delays and diversions and helps protect airport capacity during fog and adverse weather.