Which value for the radius of the Earth is used as the datum for satellite altitudes?

Question

Do they use the mean radius of the earth or the equatorial?

Either way, the altitude will never be the actual distance between satellite and sea level, due to oblateness and deviations of the geoid.

Answer 1

Most all satellites these days use the WGS-84 datum and ellipsoid.
Equatorial radius = 6 378 137 m (exactly)
Polar radius = 6 356 752.3 m

WGS-84 is the reference datum for many Earth model, including gravity maps.

If true sea level altitude is important for the satellite mission, then that is available also. The geoid is very well mapped, at wavelengths down to about 500 km. For finer detail, it's pretty much terrain contour maps (computer readable, of course).

Answer 2

To answer your question one would need to know why a satellite engineer would worry about where the surface of the earth is. One would certainly not need to know in general because the important item to understand is where the center of gravity of the earth is. One might need to know the location of a particular point on the earth where a radio antenna exists. One might want to focus a camera on some set of points on the earth but those are specific locations with particular altitudes not average distance from the center of the earth.