  Space provides unique opportunities to test gravitation. By using an
interplanetary spacecraft as a test mass, it is possible to test General
Relativity at the Solar System distance scale. This requires to compute
accurately the trajectory of the spacecraft, a process which relies on radio
tracking and is limited by the uncertainty on the spacecraft non-gravitational
acceleration.
  The Gravity Advanced Package (GAP) is designed to measure the
non-gravitational acceleration without bias. It is composed of an electrostatic
accelerometer supplemented by a rotating stage. This article presents the
instrument and its performances, and describes the method to make unbiased
measurements. Finally, it addresses briefly the improvement brought by the
instrument as far as orbit reconstruction is concerned.
