As every spacecraft, Microscope has a function to acquire and keep the desired angular orientation through time. This angular orientation is called attitude and the function is named the ACS (Attitude Control System).
The originality of Microscope is the presence of a continuous control of linear accelerations:
- On board a standard satellite, the linear movement (the orbit) is corrected from time to time by gas jets; but the movement is usually free, subjected to gravitation but also to surface forces (such as aerodynamic forces due to the residual atmosphere, and solar pressure forces due to photon impacts) and other (electro-magnetic) distant forces .
- Things are different aboard Microscope. The linear movement of the satellite is permanently controlled; the disturbing forces are compensated by the nitrogen microthrusters to achieve a purely gravitational movement. This control is ensured along the three linear axes, which is why the term 'drag-free satellite' conveys only a part of the reality. The linear control goes together with the attitude control that is why it is named AACS where the first A refers to the Accelerations control. A 3 axes linear control with such an accuracy will be a world première.
How does the drag-free work?
Permanently, the linear measurements of the accelerometers are processed by the on board computer of the satellite to deduce the forces to be produced by the microthrusters. The commands are sent to the eight microthrusters which modify their thrust consequently. A quarter of second later this process is re-activated and so on for months.
The instrument is used as main sensor for the AACS loop, a star tracker is also used to evaluate the attitude in the long term.
In steady state, the propulsion system just balances the non gravitational forces and the test masses float nearly freely in their cages. Everything goes as if the satellite flew around a test mass used as a guide and corrected its accelerations for the mass to stay at the center of its cage.
The performances required for the AACS are extraordinary: the required acceleration residue is 30 pico-g by Hertz root; under such an acceleration a test mass set free would take more than two hours to travel only one centimeter !