This category of instruments is used here to describe a variety of sensors and supporting systems used to derive information on the Earth’s gravity field, magnetic field or geodynamic activity.
Gravity field measurements from space rely on one of three techniques:
— use of single or multiple accelerometers on one or more satellites to derive gravity or gravity gradient information;
— precise satellite orbit determination (using satellite to ground navigation systems such as GPS and satellite laser ranging systems), and separation of satellite motion, induced by the Earth’s gravitational force alone, from other forces (such as solar radiation and aerodynamic drag);
— satellite to satellite tracking (e.g. by GPS or microwave link) to measure relative speed variations of two satellites induced by gravitational forces.
Satellite-borne magnetometers provide information on the strength and direction of the Earth’s internal and external magnetic field and its time variations.