ELECTROSTATIC ACCELEROMETERS FOR THE EQUIVALENCE PRINCIPLE TEST IN-SPACE

The concept of the three-axis electrostatic accelerometers based on th e full electrostatic suspension of one unique proof mass is very suita ble for space applications requiring very high resolution of accelerat ion measurement or drag-free control of satellite. This concept has be en tested in orbit with the accelerometer CACTUS from ONERA in the lat e seventies and recently with the accelerometer ASTRE on board Columbi a shuttle in June 1996. The accelerometer outputs are derived from the measurement of the electrostatic forces, necessary to maintain the ma ss motionless at the centre of the accelerometer cage. The relative te st-mass position and attitude are servo-controlled from measurements o f capacitive sensors exhibiting resolutions of better than 10(-10) mHz (-1/2) depending on the geometrical configuration. The test of the wea k equivalence principle can be performed in orbit on board a drag-free satellite with two concentric electrostatic accelerometers including two cylindrical test masses made of different materials. The measured common acceleration is controlled to null along the three directions b y the drag compensation system of the satellite. The differential acce leration is detected at the orbital frequency (or around the satellite spin frequency) along the common revolution axis with an expected res olution of 10(-14) ms(-2) Hz(-1/2). The differential disturbing accele ration induced by magnetic, electric and thermal disturbances must be limited to this value thanks to the 4 K environment of the sensor-head . The present definition of such an instrument is presented and the ex pected performances are detailed.