What is the approximate altitude required for a geostationary satellite (GEO)?

Approximately 35,786 kilometers above the equator

For a satellite to achieve a Geostationary Earth Orbit (GEO) and appear fixed above a single point on the ground, it must be placed at a very specific altitude, calculated to be approximately 35,786 kilometers above the equatorial line. At this precise height, the speed required for the satellite to maintain orbit results in an orbital period that exactly matches the Earth's rotational period, which is one sidereal day (23 hours, 56 minutes, and 4 seconds). This altitude is crucial because if the satellite were significantly lower, it would orbit faster than the Earth rotates, causing it to drift eastward relative to the ground. If it were much higher, it would orbit slower, causing it to drift westward.