Attitude control for GRACE the first low-flying satellite formation

The GRACE mission - "Gravity Recovery And Climate Experiment" - is a scientific co-operation between the USA and Germany. The two identical spacecraft were designed and built by Astrium in Germany. All operations are carried out at the German Space Operations Centre (DLR-GSOC), whereas the scientists are from the university of Texas in Austin and the Geoforschungszentrum Potsdam. The complete on-board science instrumentation is under the responsibility of JPL (Jet Propulsion Laboratory, USA). The GRACE mission has as primary objective the measurement of the Earth's gravitational field. It is also going to deliver so-called occultation measurements, which are based upon the attenuation of the signals from setting GPS satellites to yield information on the atmosphere. The twin spacecraft were successfully launched on March 17(th) 2002 by a Russian Rockot launcher. GRACE is not only the first dual-satellite mission operated by GSOC, but also the first formation-flying occurring at an altitude below 500 km. The fact that the satellites themselves and their constellation are the scientific experiment, so to speak, poses stringent demands on attitude and orbit control. The emphasis in this paper is on the aspects of attitude control, leaving the orbit part of AOCS to "flight dynamics" specialists. The principle of attitude control in the several modes is briefly discussed, and the various sensors and actuators are presented. The focus hereby lies on the so-called "science" mode, in which the link between the two satellites is maintained by accurately pointing them at each other. Shortly after launch the twin spacecraft ceased to be identical and each required and still needs a separate treatment. This is discussed in the last section, together with the solutions installed to guarantee a maximum time in science mode for each of the satellites, thus optimising the scientific return from the formation as a whole.