The Satellites Never Sent Your Position
A GNSS satellite has no idea a receiver exists on the ground beneath it. It broadcasts exactly two kinds of information, endlessly repeated: an extremely precise time signal, and a description of its own orbital position, called an ephemeris. That is the entire message. Nothing in it refers to you, and nothing in it needs to.
Distance From Timing, Not From a Report
Every GNSS signal is stamped with the instant it left the satellite. The receiver notes the instant it arrived. The difference, multiplied by the speed of light, gives the distance to that one satellite at that one moment. One distance to one satellite narrows your location to the surface of a sphere. A second satellite narrows it further, to a circle. A fourth observation, needed because the receiver's own clock is not perfect, is what finally collapses the possibilities down to a single point in space.
The receiver is doing all of this work itself, continuously, using nothing but arrival times and the computed positions of whichever satellites are currently in view.
GNSS positioning is a geometry problem the receiver solves locally. Each satellite broadcasts a predicted description of its own orbit and a precise timestamp; the receiver uses that broadcast data to solve for its own position and its own clock error at the same time.
The Satellites Themselves Are Moving
There is a detail here worth sitting with for a moment. The satellites a receiver measures against are not fixed towers. A GPS satellite, for instance, moves at roughly 3.9 kilometers per second, close to 14,000 kilometers per hour, completing a full orbit in just under twelve hours. Other constellations orbit at different altitudes and speeds, but the same basic fact holds for all of them. The receiver is not measuring distances to something stationary and well surveyed. It is measuring distances to a set of fast-moving satellites whose positions must be computed, at that exact instant, to within centimeters.
That single fact, that the satellites themselves are in constant, rapid motion, turns out to explain almost everything unusual about how GNSS describes position. We will come back to it directly once we reach the coordinate system GNSS actually computes in.
If a satellite is moving 3.9 kilometers every second, how old can its broadcast orbit be before it becomes useless? This is why ephemeris data always comes with an expiration: a specific window of time the predicted orbit stays valid for.
What the Receiver Actually Has in Hand
At the end of this stage, before any correction, before any reference frame, before any map is involved, the receiver has only this: a set of precise distances, four at minimum and often two or three dozen with a modern multi-constellation receiver, to fast-moving satellites, each one's position computed from its broadcast ephemeris. Everything else in this companion, RTK, ECEF, latitude and longitude, State Plane, a local project grid, is built on top of that one simple measurement.
Satellites broadcast time and a predicted description of their own orbit, nothing more. Your coordinate is a calculation your receiver performs, using those broadcasts as raw material. It is never handed to you directly.
A raw calculation like this is only accurate to a few meters on its own. The next chapter explains why RTK needs a second receiver, sitting somewhere else entirely, before that same calculation can become accurate to a centimeter.