Abstract
An important concern in spacecraft engineering is attitude determination control and stabilisation (ADCS) - the combination of applied mathematics, classical physics and modern technology which maintains the pointing direction of one or more axes of an Earth-orbiting satellite. This thesis is a detailed study of a particular type of ADCS which exploits the gravity-gradient effect, which is just the weak tendency for an appropriately shaped body to point naturally in preferred directions; reinforced by magnetorquing, which is the active interaction of the geomagnetic field with a switched current passing through coils in the spacecraft body. The advantages of this technology is that it is low-cost, non-consumable and has no moving parts - so constituting no limitation to the satellite's life. The thesis is a detailed study of this form of ADCS with specific application to low Earth polar-orbiting (LEO) satellites, for which it is particularly suitable. The work is also a study in attitude determination based solely on a 3-axis magneometer measurement of the geomagnetic field, which is in principle a simple way, in terms of technology, of determining the attitude of the spacecraft, and from this controlling the attitude, if mediated by an on-board computer implementing appropriate algorithms. The results are for the most part practically based on the author's involvement with two satellites over a six year period with the satellites UOSAT-1 and UOSAT-2, which were designed, built, and continue to be controlled from the University of Surrey. A practical innovation in 2-axis attitude control is described: the active 'delibration' by active damping of a gravity-gradient controlled LEO satellite in an attitude-stabilised state, using a threshold comparison algorithm. A new theory and algorithms are then developed for 3-axis attitude control, based on a complementary use of magnetorquing and gravity gradient. Subject to further development these could alter the general perception of the most effective way of controlling low Earth orbiting satellites.