SSR Corrections and PPP-RTK: Centimetre-Level Positioning Without Local Network Dependency
Centimetre-precision GNSS positioning has historically followed two distinct approaches: classical RTK, based on the transmission of phase observables from a reference station close to the receiver, and PPP, which operates with satellite clock and orbit corrections without requiring local infrastructure. The PPP-RTK paradigm integrates both approaches through the distribution of corrections in SSR (State Space Representation) format, which models each source of system error separately: satellite orbit and clock errors, ionospheric delay, tropospheric delay, and code and phase biases. This decomposition allows the receiver to resolve phase ambiguities in an absolute manner, achieving centimetre-level precision with convergence times significantly shorter than those of conventional PPP. Compared with classical RTK, the SSR approach eliminates dependence on a base station within a limited operational radius, typically below 30–50 km in order to maintain the spatial correlation of errors. SSR corrections are generated from networks of reference stations with regional or global coverage, processed in computing centres, and distributed to the receiver via low-latency data links, which may include L-band satellite broadcast channels, cellular networks, or aeronautical data links depending on the application. The resulting architecture is scalable and does not require the end user to maintain or know the location of the reference infrastructure. From an operational standpoint, applications with the most demanding integrity and continuous availability requirements are those that impose the strictest constraints on the SSR correction generation and distribution chain. In precision agriculture, tolerable latency is on the order of several seconds, which permits relatively simple distribution architectures. By contrast, autonomous drone guidance systems operating in non-segregated environments, or industrial automation processes with closed control loops, require lower latencies and integrity monitoring mechanisms that supervise the validity of each correction component. Critical infrastructures, such as deformation monitoring of instrumented dams or bridges, add the requirement of service continuity over extended windows. The European regulatory framework, structured around Galileo services and the ongoing standardisation work being carried out by bodies such as RTCM and ISO, establishes the format and interoperability requirements that SSR correction providers must meet in order to ensure compatibility with multi-constellation and multi-frequency receivers. The consolidation of this framework is a determining factor for the institutional adoption of PPP-RTK in regulated sectors, where traceability of the correction and verification of the integrity of the position solution are necessary conditions, not optional ones. Analysis of the complete chain, from the reference network to the ambiguity resolution algorithm in the receiver, is the central element of any rigorous technical evaluation of these systems.
NASSAT - Network Satellite Systems