Satellites have a circular path. the earth is an oblate spheroid and is not symmetrical in shape, Those differences need to be calculated and programmed into GNSS units. Also, the earths crust varies in thickness and therefore the surface radius from the earth's centre is not uniform. A theoretical circle is drawn at a constant radius from the earth's centre, mirroring the satellite path, and a measure of elevation is taken from it. Some points on the earth will be below it and other points will be above.. This precise data can be programmed into the onboard GPS for corrections and provide for very accurate 3D approaches without expensive ground based equipment. Equipment such as an ILS can run into the millions in cost and require constant maintenance.
The measurements can be found within the AIP aerodrome data.