I might be wrong but GPS satellites don’t send altitude, geometric altitude is computed by the GPS receiver by comparing the 3D position with the WGS-84 ellipsoid. All satellites send almanac including UTC time, and at least 3 satellites are required for any 2D position, 4 satellites for a 3D position and 5 for RAIM. And all satellites know exactly where and in what orbit they are, thats kind of the working principle of the whole GPS system. And I thought all GPS satellites are in MEO and not in geostationary orbit.
Correct me if I’m wrong.