The zero-gravity curve and surface and radii for geostationary and geosynchronous satellite orbits.

A geosynchronous satellite orbits the Earthalong a constant longitude. A special case is the geosta-tionary satellite that is located at a constant position abovethe equator. The ideal position of a geostationary satelliteis at the level of zero gravity, i.e. at the geocentric radiuswhere the gravitational force of the Earth equals the cen-trifugal force. These forces must be compensated for sev-eral perturbing forces, in particular for the lunisolar tides.Considering that the gravity field of the Earth varies notonly radially but also laterally, this study focuses on thevariations of zero gravity not only on the equator (for geo-stationary satellites) but also for various latitudes.It is found that the radius of a geostationary satellitedeviates from its mean value of 42164.2 km only within±2m, mainly due to the spherical harmonic coefficientJ22,which is related with the equatorial flattening of the Earth.Away from the equator the zero gravity surface deviatesfrom the ideal radius of a geosynchronous satellite, andmore so for higher latitudes. While the radius of the for-mer surface increases towards infinity towards the poles,the latter decreases about 520 m from the equator to thepole. Tidal effects vary these radii within±2.3km.