State Space Representation (SSR) is a correction format methodology where individual GNSS error sources, satellite orbits, satellite clocks, ionospheric delays, tropospheric delays, and signal biases, are modeled and transmitted separately rather than combined into aggregate corrections. This disaggregated approach enables more flexible and scalable correction services, particularly for wide-area PPP-RTK applications, though it requires more sophisticated processing in the rover receiver.
In SSR architecture, correction providers compute and transmit separate correction streams for each error component. Satellite orbit corrections describe differences between broadcast and precise ephemerides. Satellite clock corrections describe high-rate clock error variations. Ionospheric corrections provide regional models of ionospheric delay, typically as grid values or polynomial parameters. Tropospheric corrections model wet delay variations (dry delay is more predictable). Phase bias corrections enable integer ambiguity resolution at the rover.
The rover receiving SSR corrections must reconstruct and apply appropriate corrections for its specific location. For atmospheric corrections, this involves interpolating grid-based or parametric models to the rover’s position. For satellite-specific corrections, this means applying orbit and clock adjustments to all observed satellites. This computational load transfers complexity from the correction provider to the rover, but enables efficient service scaling, the same SSR stream serves unlimited users across the coverage area.
SSR contrasts with Observation Space Representation (OSR), the traditional RTK approach where corrections are pre-combined and specific to a particular reference station or virtual location. OSR is computationally simpler for rovers but requires either physical proximity to base stations or complex virtual reference station generation for each user. SSR’s flexibility makes it the foundation of modern PPP-RTK services that provide centimeter-level accuracy across continental scales, represented in RTCM 3.x message types 4062 and related definitions.
