The success of GOCE’s ultra-sensitive gravity measurements depends on finely controlling the satellite’s orbit and speed. The push from the thruster must be just enough to compensate for the tiny amount of drag generated by the few wisps of atmosphere at GOCE’s orbital height. No normal jet engine could do this, but GOCE’s sophisticated electric ion propulsion system can. This cutting-edge system does not burn fuel like a regular rocket motor. Instead, it is supplied with xenon from a 40-kg tank, which is converted to fast-moving ions — naked xenon atoms that have had some of their electrons stripped away by an electric discharge generated from solar energy. The ions are ejected toward the rear, giving a very gentle, steady and smooth thrust. The electric ion propulsion system comprises two redundant thruster units mounted externally on the last panel of the satellite. The thrusters can be throttled between 1 and 20 millinewtons (mN), which will be set automatically, depending on the actual real time drag that the satellite experiences once GOCE is in routine operations.
This is a fantastically small force — a thrust of a few mN is similar to the weight of a few drops of water on Earth. Yet, by thrusting continuously during GOCE’s routine operations, it is sufficient to maintain a ‘freefall’ orbit. The Flight Control Team is now working from the GOCE Dedicated Control Room at ESOC, and has continued with a series of operational checks since the end of the critical Launch and Early Orbit Phase on March 20th. Prior to commissioning the electric ion propulsion, GOCE was in free flight (not powered) and is now at an altitude of approximately 280 km, gently decaying at a rate of about 190 m/day.
GOCE is the first ESA satellite employing drag-free control, in which the satellite is in pure freefall around Earth, and is one of the first-ever satellites to use electric ion propulsion to continually compensate for atmospheric drag. The Drag-Free and Attitude Control Subsystem (DFACS) performs autonomous determination and control of the satellite’s attitude pointing, angular movements and linear and angular accelerations. The DFACS uses, among other inputs, signals from the Electrostatic Gravity Gradiometer (EGG) — the primary on board scientific instrument – to provide information on the drag experienced by GOCE. The propulsion is then varied automatically to maintain the required velocity and altitude. GOCE also orbits at an extremely low altitude, so atmospheric drag and solar activity have important effects on the trajectory. Solar heating causes the upper atmosphere to expand, increasing the amount of drag that GOCE experiences at a given altitude.
(Source: ESA)
