The Two Questions, Revisited
With RTK, PPK, PPP, and PPP-RTK each covered on their own terms, the two-question framework from earlier in this companion can now hold all four at once, and the whole family stops looking like a list of unrelated tools.
RTK, PPK, PPP, and PPP-RTK are four distinct combinations of the same two answers: when the correction is applied, live or delayed, and where the reference information comes from, a nearby station or precise global products with no local reference at all.
Placing All Four on the Grid
RTK answers both questions the same way it always has: live, and dependent on a nearby physical or virtual reference. PPK keeps that same nearby reference but moves the answer to the first question, choosing delayed over live. PPP answers the second question the opposite way from RTK, requiring no nearby reference at all, and can itself be run either live, using a real-time stream of precise products, or delayed, in full post-processing, making it flexible on the first question in a way the others are not. Galileo HAS and PointPerfect Global, both covered earlier in this companion, are concrete occupants of the live side of this cell; CSRS-PPP, Natural Resources Canada's free online processing service, occupies the delayed side the same way. PPP-RTK stays live like RTK, but replaces RTK's dense local reference with a sparser regional network delivering SSR corrections, answering the second question with something between RTK's tight local dependency and PPP's complete independence. PointPerfect Flex's native SPARTN option occupies the PPP-RTK cell the same way, though its separate RTCM option, being an observation-space product rather than a state-space one, actually belongs back in the RTK cell instead, a reminder that a single brand name does not always sit in a single cell on this grid. Galileo HAS itself illustrates that a single service can eventually straddle both cells: its currently operational Service Level 1 is genuinely global PPP, while its planned Service Level 2, adding regional atmospheric corrections for the European Coverage Area, will function as PPP-RTK once deployed.
| Method | When (Timing) | Where (Reference Source) | Concrete Example |
|---|---|---|---|
| RTK | Live | Nearby reference, physical or virtual | Local base, statewide CORS, PointPerfect Flex (RTCM/OSR) |
| PPK | Delayed | Nearby reference, logged rather than streamed | NGS OPUS, AUSPOS, RTKLIB |
| PPP | Live or delayed | No nearby reference needed; global precise products | Live: Galileo HAS Service Level 1, PointPerfect Global. Delayed: CSRS-PPP |
| PPP-RTK | Live | Sparse regional network (SSR) | PointPerfect Flex (SPARTN/SSR); Galileo HAS Service Level 2 (planned, Europe) |
PPP is the one method in this family capable of answering the first question either way, live or delayed, while always answering the second question the same way: no nearby reference required, only precise satellite information.
Why This Grid Is the Real Takeaway
None of these four methods is a strictly better or worse version of another. Each sits at a different, deliberate point on the same two-question grid, and the right choice depends entirely on which answers a given project can actually afford, not on which method happens to be newest or most familiar.
Newer is not automatically better in this family. PPP-RTK is the most recently developed method here, but RTK remains the correct choice for a huge share of everyday work, precisely because its dependence on a nearby reference is not actually a limitation when that reference is easy to provide.
A Grid Worth Keeping in Mind
Any new correction method that emerges after this companion is written can very likely be placed on this same grid, answering the same two questions in its own particular way. The specific acronyms may change. The two questions underneath them are unlikely to.
A new correction service is announced, described only as faster than PPP but not requiring dense local infrastructure. Before learning anything else about it, what two questions would immediately help place it? Whether it operates live or with some delay, and exactly how sparse its supporting reference network can be, the same two questions this entire companion has been built around.
Every correction method in this companion, and very likely any that follow it, can be placed by answering two independent questions: live or delayed, and dependent on a nearby reference or not. RTK, PPK, PPP, and PPP-RTK occupy four distinct, deliberate positions on that same grid.
A grid this clean invites an obvious next question: given a specific project, which position on it is the right one? That question turns out to be harder, and more honest, than it looks.