  The Gravity Advanced Package is an instrument composed of an electrostatic
accelerometer called MicroSTAR and a rotating platform called Bias Rejection
System. It aims at measuring with no bias the non-gravitational acceleration of
a spacecraft. It is envisioned to be embarked on an interplanetary spacecraft
as a tool to test the laws of gravitation.
  MicroSTAR is based on Onera's experience and inherits in orbit technology.
The addition of the rotating platform is a technological upgrade which allows
using an electrostatic accelerometer to make measurements at low frequencies
with no bias. To do so, the Bias Rejection System rotates MicroSTAR such that
the signal of interest is separated from the bias of the instrument in the
frequency domain. Making these unbiased low-frequency measurements requires
post-processing the data. The signal processing technique developed for this
purpose is the focus of this article. It allows giving the conditions under
which the bias is completely removed from the signal of interest. And the
precision of the unbiased measurements can be fully characterized: given the
characteristics of the subsystems, it is possible to reach a precision of 1 pm
s$^{-2}$ on the non-gravitational acceleration for an integration time of 3 h.
