How it Works
The GRACE-FO satellites may speed up or slow down for reasons other than changes in Earth’s gravity field. These other forces acting on the satellites are measured using instruments called accelerometers mounted at the center of gravity of each satellite. These instruments allow scientists to distinguish between gravity influences and those caused by air drag in the atmosphere. The three-axis electrostatic accelerometers are similar to the SuperSTAR accelerometers flown on GRACE and were developed by ONERA, a French national research laboratory.
For the accelerometer to measure only non-gravitational forces, it is important that the spacecraft center of gravity be placed at the center of the proof-mass of the accelerometer. The mass-trim mechanism and associated mass-trim electronics serve this function. The six mass-trim mechanisms each consist of a mass moving on a spindle, with each pair providing center of gravity trim along one axis.