Introduction
Aviation has the most demanding requirements for GNSS: safety-critical integrity, continuity, and accuracy. Yet pilots trust GNSS to land in fog that would have closed airports just decades ago.
The Aviation Requirements
| Requirement | What It Means |
|---|---|
| Accuracy | Position within required limits |
| Integrity | Must warn within 6 seconds if system is unusable |
| Continuity | No unexpected interruptions during an operation |
| Availability | System operational when and where needed |
Phases of Flight
- En route: Following airways; accuracy: 1–2 nautical miles; GPS sufficient with RAIM
- Terminal area: Approaching airport; accuracy: 0.3–1 nautical mile; GPS sufficient
- Non-precision approach: Guidance to runway without vertical guidance (LNAV); 0.3 nm accuracy
- APV (Approach with Vertical guidance): LPV approaches; 200–250 foot decision heights; requires SBAS (WAAS/EGNOS)
- Precision approach: Category I/II/III; down to zero visibility; requires GBAS or ILS
SBAS: The Game Changer
WAAS (US) and EGNOS (Europe) enabled LPV approaches at 35,000+ airports in the US alone, with 200–250 foot minimums equivalent to Category I ILS, but available at airports with no ground infrastructure. The critical benefit is integrity: SBAS warns pilots within 6 seconds if the system fails.
GBAS: Ground-Based Augmentation
For Category I, II, and III approaches, GBAS uses local ground stations at the airport to broadcast corrections via VHF, achieving centimetre accuracy with full integrity monitoring. Advantages over ILS: one system serves all runways, curved approaches are possible, and maintenance costs are lower. Currently operational at several airports including Newark, Sydney, and Bremen.
RAIM: Receiver Autonomous Integrity Monitoring
Before SBAS, aircraft used RAIM, the receiver checks consistency of measurements. If one satellite is bad, the others won't agree. RAIM requires 5 satellites for detection and 6 for exclusion. It warns but cannot correct. A limitation: it only works when enough healthy satellites are visible.
Aviation GNSS Equipment
TSO-C145/146 certifies aviation GPS receivers with RAIM and SBAS capability. TSO-C196 is the next generation with higher performance. Class 1–3 equipment covers different integrity levels, with Class 3 required for approaches.
The Future: A-PNT
Alternative Positioning, Navigation, and Timing acknowledges that GNSS is vulnerable to jamming. Backups include DME ranging, VOR, inertial navigation, and eLORAN (reviving legacy technology). Future multi-sensor systems combine all available sources for integrity through diversity.
Vital Points
- Aviation requires integrity, not just accuracy
- SBAS (WAAS/EGNOS) enables LPV approaches, landing in low visibility
- GBAS will eventually replace ILS at major airports
- RAIM provides integrity monitoring without ground infrastructure
- GNSS is primary means of navigation for most flights today
- Backups still needed, no single point of failure in safety-critical systems