  The radioscience experiment is one of the on board experiment of the Mercury
ESA mission BepiColombo that will be launched in 2014. The goals of the
experiment are to determine the gravity field of Mercury and its rotation
state, to determine the orbit of Mercury, to constrain the possible theories of
gravitation (for example by determining the post-Newtonian (PN) parameters), to
provide the spacecraft position for geodesy experiments and to contribute to
planetary ephemerides improvement. This is possible thanks to a new technology
which allows to reach great accuracies in the observables range and range rate;
it is well known that a similar level of accuracy requires studying a suitable
model taking into account numerous relativistic effects. In this paper we deal
with the modelling of the space-time coordinate transformations needed for the
light-time computations and the numerical methods adopted to avoid rounding-off
errors in such computations.
