Threshold Coordinates and Precision Approaches
Runway threshold coordinates are foundation data for:
- ILS-coupled GNSS approaches —the glide path reference is computed from the Localizer and Glide Slope antenna positions, referenced to WGS-84
- RNP AR procedures —Required Navigation Performance approaches use ICAO-published threshold coordinates directly in the vertical path calculation
- GBAS (Ground-Based Augmentation System) —precision approach data is referenced to the threshold datum point with cm-level accuracy requirements
ICAO Accuracy Requirements
| Data Element | ICAO Required Accuracy |
|---|---|
| Runway threshold (Cat I) | ±0.5 m horizontal |
| Runway threshold (Cat II/III) | ±0.25 m horizontal |
| Navaid coordinates | ±3 m horizontal |
| Waypoints | ±20 m horizontal |
What a 3-Meter Threshold Error Can Cause
A 3-meter horizontal error at the runway threshold shifts the entire approach procedure geometry. For Cat I ILS approaches:
- Decision height bias: the pilot's GNSS-computed height at the decision point is relative to the published threshold —a 3m horizontal error shifts the virtual threshold and the computed glide path
- Obstacle clearance surface: approach obstacle clearance surfaces are anchored at the published threshold —a displaced threshold changes what terrain and obstacles may require additional clearance or recharting
- RNP AR angular offset: terminal segment geometry errors can cause flight technical error additions that push approaches outside required containment boundaries
Common Threshold Coordinate Error Sources
- Using local national datum without proper WGS-84 transformation (e.g., JGD2000 vs JGD2011 in Japan)
- Using an older WGS-84 realization that differs from the broadcast GPS reference
- Grid-to-ground conversion errors in aerodrome survey fieldwork