PPP-RTK and SSR: Closing the Gap
RTK converges almost instantly but needs a nearby reference. PPP needs no nearby reference but converges slowly. PPP-RTK is a newer approach built specifically to close that gap, borrowing pieces from both methods rather than accepting either trade-off in full.
PPP-RTK uses a regional network to broadcast detailed, separated information about satellite orbit, satellite clock, and atmospheric conditions, allowing a receiver to converge on a PPP-style solution in a few minutes instead of tens of minutes, without needing a dense local RTK network nearby.
The Insight Behind the Approach
A large share of PPP's convergence delay comes from having to estimate atmospheric conditions from scratch, using nothing but the lone receiver's own observations. A regional network of reference stations, considerably sparser than what dense local RTK coverage requires, can estimate those same atmospheric conditions across a wide area and broadcast that information directly. A receiver that already knows roughly what the atmosphere is doing overhead does not need to spend nearly as long estimating it independently.
PPP-RTK does not require reference stations densely spaced enough for traditional RTK's shared-error cancellation to work directly. It only requires enough stations to model regional atmospheric behavior accurately, a considerably lighter infrastructure requirement.
A New Way of Describing the Correction
Making this work requires broadcasting the correction in a fundamentally different format than traditional RTK uses, called SSR, short for state-space representation. Rather than bundling every error source into one combined number for one particular reference station, the way older formats do, SSR describes each error source separately: satellite orbit error, satellite clock error, and atmospheric delay, each as its own distinct, physically meaningful component.
PPP-RTK is not simply PPP with a shorter wait built in by better marketing. It represents a genuine architectural shift, using a new correction format specifically designed to let a widely spaced network supply the atmospheric information a lone PPP receiver would otherwise have to estimate entirely on its own.
What This Buys in Practice
A PPP-RTK receiver, supplied with SSR corrections from a regional network, can often reach centimeter-level precision within a few minutes rather than tens of minutes, while still requiring nothing resembling the dense reference station spacing traditional network RTK depends on. It aims to deliver something close to RTK's convergence speed and PPP's freedom from a nearby, tightly spaced reference network, at once.
A region has scattered reference stations spaced far too widely apart for traditional network RTK to work well, but close enough together to characterize regional atmospheric behavior reasonably. Is that region a candidate for PPP-RTK service? Very plausibly, since PPP-RTK's infrastructure requirement is built around modeling the atmosphere regionally, not around the dense spacing traditional RTK needs.
A Real Service Built This Way
PointPerfect Flex, a commercial correction service from u-blox, is a concrete, currently operating example of this approach, offered natively in SPARTN (Secure Position Augmentation for Real-Time Navigation), a compact format built specifically for separated, state-space corrections. Under the same PointPerfect Flex name, u-blox also offers a second, separate correction delivered over standard RTCM, one that Thingstream, u-blox's own service delivery platform, documents as a genuinely distinct observation-space product rather than the same SSR correction repackaged for compatibility.
A single commercial product name does not guarantee a single underlying architecture. PointPerfect Flex's SPARTN option and its RTCM option are documented as two separate correction services, SSR and OSR respectively, offered side by side under one brand rather than one correction delivered in two formats.
PPP-RTK combines a regional network's atmospheric modeling with PPP's underlying approach, delivered through a new correction format called SSR, to reach centimeter precision in minutes rather than tens of minutes, without requiring the dense reference spacing traditional RTK depends on.
SSR was mentioned as a new correction format without yet being explained on its own terms. It deserves that explanation directly, since it is fundamentally different from the correction format the rest of this series has assumed until now.