Two Independent Questions: When and Where
Every correction method in this companion can be placed using just two questions, asked independently of each other. Once those two questions are clear, RTK, PPK, PPP, and PPP-RTK stop looking like an arbitrary list and start looking like four different, logical answers to the same two-part decision.
Every correction method answers two independent questions: does the correction happen live or after the fact, and does the position rely on a nearby reference station or on precise global satellite products with no local reference at all.
The First Question: When
Some methods deliver a usable position the moment it is needed, in the field, in real time. Others deliberately wait, collecting raw observations now and computing the actual position later, once better information becomes available. Neither choice is automatically superior. Real-time delivery is essential when a decision has to be made on the spot. Waiting is often rewarded with better precision, for reasons the next two chapters explain in detail.
Real time and delayed are not a spectrum from worse to better. They are a genuine trade: immediacy in one direction, and often improved precision in the other.
The Second Question: Where
Some methods depend on a nearby reference, a physical base station or a network's modeled virtual station, close enough that shared atmospheric and orbital error actually cancels through comparison. Other methods use no local reference point at all, instead relying on precise, independently computed descriptions of exactly where every satellite was and how its clock behaved, modeling the remaining error directly rather than canceling it by comparison.
A method that uses no nearby reference station is not skipping a step that other methods perform. It is solving the same underlying problem, satellite and atmospheric error, through direct modeling instead of through comparison against a reference.
Placing Every Method on This Grid
RTK answers both questions the same way: live, and dependent on a nearby reference. PPK keeps the nearby reference but moves the correction to after the fact. PPP moves in the opposite direction, staying live in the sense that a position is available continuously, but abandoning the nearby reference entirely in favor of precise satellite products. PPP-RTK, the newest of the four, tries to answer both questions favorably at once, live and without needing a dense local reference network, using a correction format built specifically to make that combination possible.
A survey method uses no live correction link and no nearby base station, collecting raw data now and computing a position after the fact using precise satellite products downloaded later. Which method does this describe? A form of PPP applied after the fact, sometimes called post-processed PPP, combining the delayed timing of PPK with PPP's independence from a local reference.
Every correction method answers two independent questions: when the correction happens, live or delayed, and where its reference information comes from, a nearby station or precise global satellite products alone. Placing RTK, PPK, PPP, and PPP-RTK on this simple grid explains the entire family at once.
With that framework in place, the next several chapters walk through each method in turn, starting with the one that changes only a single answer from RTK: PPK.