ECEF: The Earth's Coordinate Language
The raw coordinate system a GNSS receiver actually computes in is called ECEF, short for Earth-Centered, Earth-Fixed. Both halves of that name are doing real work, and neither is arbitrary. To see why, it helps to start not with the receiver, but with the satellites.
ECEF places its origin at the center of mass of the Earth because that is not a convenient choice, it is the one point every GNSS satellite's orbit already revolves around.
Every Orbit Points at the Same Spot
A satellite in free fall around a much larger body traces an ellipse, and that ellipse has the larger body's center of mass sitting at one of its two foci. This is Kepler's first law, and it applies just as cleanly to a GNSS satellite as it does to the Moon. A GNSS satellite's mass is negligible next to the Earth's, so for every practical purpose that focus is the Earth's own center of mass.
This is true for every satellite in the constellation, regardless of which orbital plane it occupies or how it was launched. Every one of their elliptical paths is pinned, geometrically, to the same single point. Placing a coordinate system's origin there is not a design flourish. It is choosing the one location that every satellite in the sky already agrees on.
GNSS satellites are not centered on Earth's geographic middle or its geometric surface. They orbit the center of mass, the single point that gravity itself uses as the pivot for every satellite's ellipse.
Satellites That Are Genuinely Moving Fast
It is worth pausing on just how much motion this coordinate system has to keep up with. A GPS satellite orbits at an altitude of roughly 20,200 kilometers, traveling at about 3.9 kilometers per second, somewhere near 14,000 kilometers per hour. It completes a full orbit in 11 hours and 58 minutes, a period deliberately chosen to be almost exactly half a sidereal day, so that the satellite's ground track repeats at the same sidereal time every day. None of the satellites a receiver depends on are holding still. Their exact position must be computed, at each specific instant, to within centimeters.
A GPS satellite covers about 3.9 kilometers every second. Over the roughly sixty to ninety milliseconds a signal takes to reach a receiver, does the satellite's position from an instant ago still describe where it is now? Not quite, which is one reason GNSS processing works in extremely precise fractions of a second, not whole ones.
Fixed Enough to Be Useful on the Ground
Centering the coordinate system on Earth's mass solves half the problem. The other half is rotation, and this is where the second half of the name, Earth-Fixed, earns its place.
Watch the GNSS constellation from a point that does not rotate with the Earth, hovering motionless in space, and each satellite traces a clean, repeating ellipse around the center of mass, exactly the textbook picture. Now watch the same constellation from a point standing on the Earth's surface, rotating along with the planet once every day. From that vantage point, the satellites no longer look like they are following simple ellipses at all. They appear to sweep across the sky in long arcs, rising in the west and setting in the east — the reverse of how the Sun and stars move — because these satellites complete an orbit in about half a day and outpace the Earth's own rotation beneath them.
A useful coordinate system for a receiver bolted to the Earth's surface needs to rotate along with that surface. If it did not, a receiver sitting motionless on a fixed monument would appear to be racing around the Earth's axis at more than 1,600 kilometers per hour at the equator, simply because the coordinate system itself was refusing to turn with the planet. Fixing the frame to the Earth's rotation is what allows a stationary receiver to report a stationary coordinate.
Earth-Centered anchors the coordinate system to the one point every orbit already revolves around. Earth-Fixed makes that same coordinate system spin with the planet, so a receiver that has not moved reports a position that has not moved either.
A Familiar Irregularity, Seen From Orbit
Readers of the first companion in this series will recognize a theme returning here from an unexpected direction. The same uneven distribution of mass inside the Earth that reshapes the geoid, the mountain ranges, the dense rock, the ancient structures deep in the crust, also tugs slightly on every satellite's orbit as it passes overhead. The irregularity that bends a gravity-based elevation surface is the same irregularity that perturbs a satellite's path through space. It is one physical fact showing up in two very different parts of this series.
A Correction Worth Naming, Even Briefly
There is one more consequence of all this motion worth knowing about, because it is a clean example of a theme this whole series returns to again and again: a correction that must be applied exactly once, at the right step. Because a signal takes some tens of milliseconds to travel from satellite to receiver, and because the ECEF frame itself is rotating with the Earth during that entire interval, the frame has turned slightly between the moment the signal left and the moment it arrived. Precise GNSS processing accounts for this explicitly, often called an Earth-rotation or Sagnac correction, so that a satellite's position is expressed in the frame as it existed at the correct instant, not the frame as it existed a fraction of a second earlier.
ECEF is not a single, fixed grid painted onto an unmoving Earth. It is centered on a real physical point, the Earth's center of mass, and it rotates continuously with the planet. Both properties are essential, and both come directly from the physics of orbiting satellites and a spinning Earth, not from an arbitrary convention.
ECEF is Earth-Centered because every satellite orbit already revolves around the Earth's center of mass, and Earth-Fixed because a coordinate system useful to a stationary receiver has to rotate along with the ground that receiver is standing on.
ECEF answers where the origin sits and how the frame moves. It does not yet answer a more subtle question: which ECEF. There is more than one, and the difference between them is the subject of the next chapter.