Introduction
Standalone GNSS receivers - whether a smartphone or a survey-grade unit - compute position using only the signals received directly from satellites. This works remarkably well in many situations, but it has limits. Atmospheric delays, satellite clock imperfections, and orbital errors all degrade accuracy. GNSS augmentation systems exist to correct these errors, providing better accuracy, integrity, availability, and continuity than any standalone receiver can achieve.
Why Augmentation Is Needed
A GNSS receiver measures pseudoranges - the approximate distance to each satellite based on signal travel time. These measurements are corrupted by several error sources:
- Satellite clock errors: Even with atomic clocks, residual timing errors translate directly into range errors of 1 to 2 metres.
- Ephemeris errors: The satellite orbit broadcast in the navigation message differs slightly from the true orbit, adding errors of up to 1 metre.
- Ionospheric delay: The ionosphere slows GNSS signals by amounts that vary with solar activity, time of day, and location - typically 2 to 10 metres on L1.
- Tropospheric delay: Temperature, pressure, and humidity bend and slow signals passing through the troposphere, adding approximately 2 to 3 metres of delay near the horizon.
Augmentation systems measure these errors at known locations and broadcast corrections to users, dramatically reducing their impact.
OSR vs SSR: Two Approaches to Corrections
Modern augmentation systems use one of two fundamental approaches to broadcast corrections:
| Approach | Full Name | What Is Sent | Examples |
|---|---|---|---|
| OSR | Observation Space Representation | A single combined correction per satellite, valid near the reference station | Traditional DGNSS, SBAS, single-base RTK |
| SSR | State Space Representation | Individual corrections for orbit, clock, ionosphere, and troposphere - usable anywhere | PPP, PPP-RTK, Galileo HAS, Trimble RTX |
OSR corrections degrade with distance from the reference station because the combined correction is only perfectly valid at the point where it was measured. SSR corrections model each error source separately, allowing them to be applied globally and enabling faster convergence when combined with carrier-phase positioning.
Global vs Local vs Regional Systems
Augmentation systems are also categorised by their geographic scope:
- Global systems: PPP services such as Galileo HAS, Trimble RTX, and TerraStar broadcast corrections worldwide via satellite or internet. Convergence time is typically 5 to 20 minutes but no nearby infrastructure is required.
- Regional/continental systems: SBAS networks (WAAS, EGNOS, MSAS, GAGAN) cover entire continents using geostationary satellites, providing free corrections with approximately 1-metre accuracy.
- Local systems: GBAS installations at airports cover a radius of approximately 50 km, providing the highest-integrity corrections needed for precision approach and landing.
- Network systems: CORS-based Network RTK covers national or regional areas using a network of reference stations to provide centimetre-level corrections via internet.
Comparison of Major Augmentation System Classes
| System Type | Typical Accuracy | Convergence | Coverage | Cost to User |
|---|---|---|---|---|
| SBAS (WAAS/EGNOS) | ~1 m | Seconds | Continental | Free |
| GBAS | Sub-metre to cm (approach) | Seconds | ~50 km radius | Subscription (aviation) |
| Network RTK / CORS | 1 to 3 cm | Seconds to 1 min | National | Subscription |
| PPP | 5 to 10 cm | 15 to 30 min | Global | Free or subscription |
| PPP-RTK / SSR | 2 to 5 cm | 1 to 5 min | Global/Regional | Subscription |
Integrity - The Safety Dimension
Accuracy alone is not enough for safety-critical applications. Aviation regulators also require integrity - a timely warning when the system cannot be trusted. SBAS and GBAS both provide integrity bounds called Protection Levels. If the true position error exceeds the Protection Level with very high probability, an alarm is raised. This is what makes satellite navigation acceptable for instrument approaches where a missed position error could be fatal.
Summary
GNSS augmentation systems transform satellite navigation from a general-purpose technology into one that is precise, reliable, and trustworthy enough for the most demanding applications. Whether through free continental services like EGNOS, the precision approach guidance of GBAS, or global centimetre corrections from PPP-RTK, augmentation bridges the gap between raw satellite signals and operational requirements. The following lessons examine each major category in detail.