Optical tracking of synchronous Earth's satellites for geophysical purposes

Tracking data from synchronous earth's satellites can be analyzed to provide the resonant geopotential coefficients, mainly C22 and S22 which represent the ellipticity of the earth's equator. Very accurate tracking techniques (LASER, Doppler) could in principle be used to improve the present knowledge of these coefficients but nongravitational perturbations on the satellite orbit, in particular solar radiation pressure, cannot be modeled to the same high accuracy unless the satellite is an ad hoc geophysical satellite. On the other hand, geosynchronous satellites currently used for telecommunication purposes can be easily tracked with moderate size large field telescopes measuring the satellite angular coordinates with respect to some catalogue field stars. The accuracy is at least 2 arc sec (≃ 400 m), i.e., much worse than the accuracy of the LASER data but surely comparable to longitude errors due to the radiation pressure model. Moreover, optical tracking does not need any onboard device and is easily accomplished. We conclude that the organization of a worldwide optical tracking campaign would allow an improvement in the knowledge of the long-wavelength geoid by at least one order of magnitude, and this is very interesting for many geophysical purposes.