Airport Runway Threshold Coordinates: Accuracy and Risk

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

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