PPP Time Transfer for GNSS: From Post-Processing to Real-Time Operation—A Review

Carrier-phase precise point positioning (PPP) entered Bureau International des Poids et Mesures (BIPM) time computations around 2009 and now yields sub-nanosecond time differences with 10−15–10−16 frequency stability over multi-day arcs, while International GNSS Service (IGS) real-time streams, Galileo High Accuracy Service (HAS), and BeiDou PPP-B2b push toward similar performance at second-level latency. We survey peer-reviewed PPP time-transfer work from 2007 through June 2026 along the product chain from IGS final solutions to state-space-representation (SSR) and broadcast corrections, grouping studies by processing latency, float versus integer ambiguity handling, constellation mix, and receiver-clock models. Post-processed campaigns typically report 0.2–0.4 ns scatter; real-time trials that match the filter clock model to the local atomic reference approach 0.2–0.3 ns root-mean-square error. Open issues include multi-provider datum alignment, outage resilience, real-time integer ambiguity resolution, and kinematic or low-Earth-orbit platforms. The review targets timing laboratories, UTC(k) operators, and GNSS analysts who share the orbit and clock products used in time-scale generation and frequency dissemination.