  Radio tracking of interplanetary probes is an important tool for navigation
purposes as well as for testing the laws of physics or exploring planetary
environments. The addition of an accelerometer on board a spacecraft provides
orbit determination specialists and physicists with an additional observable of
great interest: it measures the value of the non-gravitational acceleration
acting on the spacecraft, i.e. the departure of the probe from geodesic motion.
This technology is now routinely used for geodesy missions in Earth orbits with
electrostatic accelerometers. This article proposes a technological evolution
which consists in adding a subsystem to remove the bias of an electrostatic
accelerometer. It aims at enhancing the scientific return of interplanetary
missions in the Solar System, from the point of view of fundamental physics as
well as Solar System physics. The main part of the instrument is an
electrostatic accelerometer called MicroSTAR, which inherits mature
technologies based on Onera's experience in the field of accelerometry. This
accelerometer is mounted on a rotating stage, called Bias Rejection System,
which modulates the non-gravitational acceleration and thus permits to remove
the bias of the instrument from the signal of interest. This article presents
the motivations of this study, describes the instrument, called GAP, and the
measurement principle, and discusses the performance of the instrument as well
as integration constraints. Within a mass of 3.1 kg and an average consumption
of 3 W, it is possible to reach a precision of 1 pm s$^{-2}$ for the
acceleration measured with an integration time of five hours. Combining this
observable with the radio tracking data, it is therefore possible to compare
the motion of the spacecraft to theoretical predictions with a precision
improved by at least three orders of magnitude with respect to existing
techniques.
