SBAS Explained: WAAS, EGNOS, MSAS and GAGAN

Introduction

Satellite-Based Augmentation Systems - universally known as SBAS - represent the first widely deployed form of GNSS augmentation. They are free to use, require no ground infrastructure at the receiver end, and extend the accuracy and integrity of basic GNSS from the typical 3 to 5 metres down to approximately 1 metre. More importantly, they provide the integrity monitoring that aviation regulators require for instrument approach procedures.

The Four Major SBAS Systems

Additional systems are operational or under development in Russia (SDCM), China (SNAS), South Korea (KASS), and Australia (SouthPAN).

How SBAS Works

An SBAS network consists of three segments working together:

1. Wide Area Reference Stations (WRS): A network of precisely surveyed ground stations - approximately 40 stations across North America for WAAS, 40+ across Europe for EGNOS - continuously track all visible GNSS satellites. Because the locations are exactly known, any deviation between measured and expected pseudorange reveals errors in the satellite signals.
2. Wide Area Master Station (WMS): Processing centres receive data from all WRS stations, separate the various error sources (clock, ephemeris, ionosphere), and compute corrections along with integrity bounds called User Differential Range Errors (UDREs) and Grid Ionospheric Vertical Errors (GIVEs).
3. Geostationary Uplink Stations (GUS): Computed corrections are uplinked to geostationary satellites, which rebroadcast them on the GPS L1 frequency (1575.42 MHz) - the same frequency band as GPS satellites, using the same CDMA modulation. Standard GNSS receivers can decode SBAS corrections without hardware modification.

Geostationary Relay - Strengths and Limitations

Using geostationary satellites as relay stations is elegant: every GPS receiver already has an L1 antenna and SBAS-capable firmware. The downside is geometric. GEO satellites sit at 35,786 km altitude, fixed over the equator. This means:

• At high latitudes (above approximately 70 degrees north), GEO satellites are very low on the horizon and may be obscured.
• The GEO satellites themselves can be used as additional ranging sources, but their contribution to DOP is limited due to their fixed position.
• The ionospheric grid model works best in the mid-latitude regions where the reference network is densest.

Accuracy and Integrity Performance

Under normal conditions SBAS delivers:

• Horizontal accuracy: Typically 0.5 to 1.0 m (95th percentile)
• Vertical accuracy: Typically 1.0 to 1.5 m (95th percentile)
• Time to First Fix: No convergence required - corrections are applied immediately
• Integrity alarm time: 6 seconds or less for WAAS/EGNOS Safety of Life service

Aviation Applications - LPV Approaches

The primary motivation for SBAS investment was aviation safety. EGNOS and WAAS support Localiser Performance with Vertical guidance (LPV) approaches - a form of instrument approach procedure that uses GNSS with SBAS integrity to guide aircraft to a Decision Height of just 200 feet (61 m) above the runway. LPV approaches are operationally equivalent to an ILS CAT-I approach but require no ground-based radio equipment at the airport. As of 2024, over 4,000 LPV approach procedures are published in the United States alone, extending precision approach capability to thousands of runways that could never economically justify an ILS installation.

Non-Aviation Uses

While aviation drove SBAS development, the free and open signal is valuable across many sectors:

• Maritime: Harbour and coastal navigation where 1-metre accuracy and integrity alerts improve collision avoidance.
• Rail: Train detection and track occupancy monitoring in EGNOS Rail applications.
• Agriculture: Tractor guidance using EGNOS for field operations where 1-metre accuracy suffices.
• General surveying: Rapid mapping and GIS data collection where sub-metre accuracy is adequate.

Summary

SBAS systems such as WAAS and EGNOS transformed GNSS from a metric-level positioning tool into a reliable, integrity-monitored system suitable for safety-of-life applications. Their key strength is universal accessibility - any GNSS receiver with SBAS capability can use these corrections at no cost. For applications needing better than 1-metre accuracy or centimetre-level positioning, more sophisticated augmentation methods are required, as covered in the following lessons.