The Coordinate on the Screen
You occupy a point in the field. The receiver acquires satellites, resolves RTK, and a coordinate appears on the data collector: a latitude and longitude, or perhaps a State Plane northing and easting, precise to a fraction of an inch.
That number is the end product of a long chain of observations, comparisons, and transformations. This companion follows that chain, one deliberate stage at a time, starting with the satellites themselves.
A displayed coordinate looks like a single, instantaneous fact. It is actually the end product of a long chain: raw signals, RTK corrections, a geocentric solution tied to a reference frame and a specific moment in time, a map projection, and often a local site adjustment, each one applied once, in order.
One Point, One Journey
This companion follows a single physical point through that entire chain, from the moment a satellite signal leaves orbit to the moment a number appears in front of you. At every stage we will ask the same question this series always asks: did the point move, or did only its description change?
Almost always, the answer is that the point never moved at all. What changed was the language used to describe it, and the number of times that description was translated before it reached the screen.
Every coordinate on a screen has a biography. It was observed, corrected, solved, dated, expressed, projected, and sometimes adjusted again for a local project. Knowing that biography is what separates a real position error from an apparent one.
One Method Among Several
Real-time GNSS correction is not a single technique. RTK, using a nearby base or a network correction, is only one member of a larger family that also includes PPK, PPP, PPP-RTK, and corrections delivered as SSR rather than the older observation-space format RTK relies on. Each of those methods solves a related problem in its own way, with its own chain of reference information and its own failure points.
This companion follows the classic real-time RTK chain specifically: a base or network correction source, a rover, and a carrier-phase solution resolved in the field. That choice keeps one book from trying to be five books at once. PPK, PPP, PPP-RTK, and SSR corrections each deserve the same careful, one-point-at-a-time treatment, and later companions in this series will give it to them.
Everything in this companion assumes traditional RTK: a base or network correction source compared against a rover in real time. PPK, PPP, PPP-RTK, and SSR corrections follow different chains and are covered in their own future companions.
Why This Matters Before You Distrust a Number
When a coordinate looks wrong, most users start with the receiver. Was it really FIX? Was the antenna height entered correctly? Those are reasonable first questions, but they only cover the first few links in a much longer chain. A perfectly good RTK solution can still produce a coordinate that looks wrong several steps later, after a reference frame, an epoch, a projection, or a site calibration has changed how that same point is described.
Before deciding a coordinate is wrong, it helps to know how many hands it passed through to get to the screen. Most apparent errors live in that handoff, not in the original measurement.
In the next chapter we start at the very beginning of the chain: what a satellite actually broadcasts, and what it very deliberately does not.