Which geodetic system does Google Earth use?

google Earth uses a "Simple Cylindrical projection," which is also known as Plate Carrée, with a World Geodetic System of 1984 (WGS84) datum.

This is a simple map projection where the meridians and parallels are equidistant, straight lines, with the two sets crossing at right angles.

google Maps uses the Mercator projection also with the WGS84 dataum.

The vertical component (altitude) is measured from the vertical datum, which is the Earth Gravitational Model 1996 (EGM96) Geoid. Altitudes are measured in meters.