  The increasing precision of spacecraft radiometric tracking data experienced
in the last number of years, coupled with the huge amount of data collected and
the long baselines of the available datasets, has made the direct observation
of Solar System dynamics possible, and in particular relativistic effects,
through the measurement of some key parameters as the post-Newtonian
parameters, the Nordtvedt parameter "eta" and the graviton mass. In this work
we investigate the potentialities of the datasets provided by the most
promising past, present and future interplanetary missions to draw a realistic
picture of the knowledge that can be reached in the next 10-15 years. To this
aim, we update the semi-analytical model originally developed for the
BepiColombo mission, to take into account planet-planet relativistic
interactions and eccentricity-induced effects and validate it against
well-established numerical models to assess the precision of the retrieval of
the parameters of interest. Before the analysis of the results we give a review
of some of the hypotheses and constrained analysis schemes that have been
proposed until now to overcome geometrical weaknessess and model degeneracies,
proving that these strategies introduce model inconsistencies. Finally we apply
our semi-analytical model to perform a covariance analysis on three samples of
interplanetary missions: 1) those for which data are available now (e.g.
Cassini, MESSENGER, MRO, Juno), 2) in the next years (BepiColombo) and 3) still
to be launched as JUICE and VERITAS (this latter is waiting for the approval).
