RELATIVISTIC EFFECTS FOR DOPPLER MEASUREMENTS NEAR SOLAR CONJUNCTION

We consider the relativistic corrections to the Doppler effect in a metric theory of gravity, in the weak field and slow motion approximation, for a source and a receiver at great distances from, but near alignment with, the perturbing body (i.e. near conjunction). We work also in the approximation of a thin screen and paraxial rays, and apply this formalism to an ideal experiment, related to that of the deflection of light rays by the solar mass. We then introduce a differential Doppler technique, which allows some new and interesting developments in the field of experimental gravitation. With a spacecraft equipped with a hydrogen maser on board, one can measure the difference between the fractional frequency shifts in both directions. Near conjunction it is possible to have information, for example, on the angular momentum of the sun or on a possible effect of a privileged cosmological frame of reference. This technique also allows a drastic reduction of the effect due to the plasma of the solar corona. The maximum fractional frequency change induced by the angular momentum of the sun is about 7 x 10(-16), which is barely consistent with the stability of hydrogen masers currently available.