Introduction
Traditional augmentation methods - SBAS, GBAS, RTK - all deliver corrections in Observation Space Representation (OSR): a single blended correction per satellite, valid primarily near the reference station. State Space Representation (SSR) takes a fundamentally different approach, modelling each error source separately and broadcasting them independently. This allows corrections to be applied globally and enables a new generation of high-precision positioning services.
What Is State Space Representation?
In OSR, a reference station combines all error sources into a single pseudorange correction. That correction is only accurate near the reference station because the ionospheric delay, tropospheric delay, and satellite geometry all change with location. Moving even 50 km from the reference degrades OSR corrections significantly.
SSR decomposes the problem differently. A network of reference stations observes the same satellites from many locations simultaneously. From this distributed observation, error components can be separated:
- Satellite orbit corrections: The difference between broadcast ephemeris and the true satellite position, resolved in the radial, along-track, and cross-track directions.
- Satellite clock corrections: The residual clock error after removing the broadcast clock correction - typically at the centimetre level for GPS, a few centimetres for GLONASS.
- Code biases: Hardware-dependent biases in the satellite and receiver that affect pseudorange measurements differently by frequency and signal type.
- Phase biases: Integer ambiguity information that, when broadcast as float or integer values, allows receivers to resolve carrier-phase ambiguities faster.
- Ionospheric corrections: A grid or model of vertical ionospheric delay, separated from satellite and receiver errors.
- Tropospheric corrections: Model corrections for dry and wet tropospheric delay components.
PPP-RTK: Combining Global Precision with Fast Convergence
Precise Point Positioning (PPP) has existed since the late 1990s. Using precise satellite orbit and clock products from organisations like the International GNSS Service (IGS), a dual-frequency receiver can achieve 5 to 10 cm horizontal accuracy globally - but convergence takes 15 to 30 minutes as the receiver estimates ionospheric and tropospheric delays from its own measurements.
PPP-RTK bridges the gap by broadcasting ionospheric and tropospheric corrections as part of the SSR stream, giving the rover receiver the corrections it previously had to estimate slowly from scratch. The result is convergence in 1 to 5 minutes with accuracy approaching conventional RTK - all without needing a nearby base station.
Commercial SSR Services
| Service | Provider | Delivery | Accuracy (H) | Convergence |
|---|---|---|---|---|
| Galileo HAS | EUSPA / EU | Galileo E6B signal (free) | ~20 cm (Phase 1) | ~100 s |
| Trimble RTX | Trimble | Satellite L-band + internet | 2 to 4 cm | ~3 to 5 min |
| TerraStar | Hexagon / NovAtel | Satellite L-band + internet | 2 to 4 cm | ~3 to 5 min |
| Fugro StarFix / MarineStar | Fugro | Satellite L-band | 5 to 10 cm | ~15 min |
| Swift Navigation Skylark | Swift Navigation | Internet (cloud PPP-RTK) | ~5 cm | <1 min |
Galileo HAS - The Free Global Correction Service
Galileo High Accuracy Service (HAS) is a particularly significant development. Broadcast free-of-charge on the Galileo E6B signal, HAS delivers SSR orbit and clock corrections (Phase 1) with plans to add ionospheric corrections (Phase 2). Any Galileo E6-capable receiver can access HAS without subscription fees, making high-accuracy global corrections available to markets that cannot justify commercial L-band subscriptions - agricultural equipment, autonomous vehicles, and drone applications.
SSR and the RTCM 3.3 Standard
SSR corrections are standardised in RTCM 3.3, which defines message types for orbit corrections (MT 1057 to 1068), clock corrections, code biases, phase biases, and ionospheric models. This standardisation allows receivers from different manufacturers to consume SSR streams from different service providers - a critical step toward an open, interoperable ecosystem.
Summary
SSR and PPP-RTK represent the direction of travel for global high-precision GNSS. By separating error sources and broadcasting them independently, SSR corrections enable centimetre-level positioning anywhere on Earth - without the constraint of proximity to a reference station. Free services like Galileo HAS democratise access to this capability, while commercial services push accuracy and convergence time to the limits of current technology.