Introduction
Did you know there are free satellite systems that improve GNSS accuracy across entire continents? They're called SBAS, and if you have a modern receiver, you're probably using them without even knowing it.
What Is SBAS?
SBAS stands for Satellite-Based Augmentation System. It is a network of ground stations and geostationary satellites that broadcast correction data to improve GNSS accuracy, integrity, and availability.
Major SBAS Systems
| System | Region | Coverage | Geostationary Satellites |
|---|---|---|---|
| WAAS | North America | USA, Canada, Mexico | 3 |
| EGNOS | Europe | EU plus expanding | 3 |
| MSAS | Japan | Japan, Asia-Pacific | 2 |
| GAGAN | India | India, surrounding | 2 |
| SDCM | Russia | Russia, CIS | 2 |
| BDSBAS | China | Asia-Pacific (expanding) | 2+ |
| SouthPAN | Australia/NZ | Australia, New Zealand | 2 |
How SBAS Works
- Reference stations, ~30–40 widely spaced ground stations continuously monitor all GNSS satellites
- Master station, Collects data from all reference stations, computes corrections for each satellite, generates integrity messages
- Uplink station, Sends corrections to geostationary satellites
- Geostationary satellites, Broadcast corrections on GPS L1 frequency, covering the entire region; also act as additional ranging sources
- Your receiver, Applies corrections to improve accuracy and receives integrity warnings
What SBAS Provides
Accuracy improvement
- Without SBAS: 3–5 metres
- With SBAS: 1–2 metres
- Best in centre of coverage, degrades at edges
Integrity
The system warns within 6 seconds if satellites are unhealthy, critical for aviation safety. This allows GPS to be used as the primary means of navigation for aircraft.
Availability
Additional geostationary satellites to track, providing better geometry and availability.
SBAS for Aviation
SBAS was designed primarily for aviation, enabling LPV (Localizer Performance with Vertical guidance) approaches with 200–250 foot decision heights. This is equivalent to Category I ILS and is now available at thousands of airports without any ground infrastructure.
SBAS for Everyone Else
- Hikers: 1–2 metre accuracy from your handheld GPS
- Farmers: Free improvement for basic guidance
- Mariners: Consistent accuracy at sea
- Surveyors: Useful as backup, but higher accuracy usually needed
- Developers: Many receivers output SBAS corrections via NMEA
Limitations
- Coverage boundaries: Accuracy degrades at coverage edges; not available in polar regions
- Ionosphere limitations: During severe storms, accuracy degrades
- Single-frequency only: SBAS corrects L1 only, dual-frequency receivers can do better independently
Vital Points
- SBAS provides free accuracy improvement (1–2 metres) over entire continents
- Essential for aviation, integrity is as important as accuracy
- Built into most modern receivers, you're probably already using it
- Different regional systems (WAAS, EGNOS, etc.) all work similarly
- Not a replacement for RTK but excellent for general use