  Recent theoretical works on alternative metric theories of gravity give
greater significance to solar-system tests of General Relativity. In
particular, it is suggested that the post-Newtonian parameter $\gamma$ ought to
be determined with great precision in order to discover possible preferred
frame effects or effects related to a tensor-scalar theory of gravitation. In
this work we focus on a future experiment, based on Doppler tracking of an
interplanetary spacecraft near superior conjunction.
