The Coordinate Doesn't Start as Latitude and Longitude
Most people picture a GNSS receiver producing latitude, longitude, and elevation directly. In reality, those familiar values are not the first result of the positioning engine. The receiver first determines where it is in three-dimensional space. Only afterwards is that position expressed in the geographic coordinates we recognize.
Imagine freezing the computation immediately after the RTK solution converges. There is no map, no benchmark, and no geoid involved yet. There is only a precisely determined position in an Earth-Centered, Earth-Fixed (ECEF) reference frame.
Why ECEF Matters
ECEF may sound intimidating, but its role is surprisingly simple. It provides a stable three-dimensional coordinate system tied to the rotating Earth. GNSS mathematics works naturally in this system because both the satellites and the receiver can be described using the same geometric framework.
This is an important turning point in our investigation. If the receiver has not yet computed latitude, longitude, or ellipsoid height, it certainly has not measured an elevation above sea level.
ECEF is not another kind of elevation. It is the mathematical workspace where the position is solved before it is translated into forms that humans find easier to interpret.
The rover now has a precise position in ECEF. The next chapter follows that same position as it gets translated into the latitude, longitude, and ellipsoid height most people actually recognize.