A BALLOON-BORNE THREE AXIS STABILISED PLATFORM FOR LARGE ASTRONOMY EXPERIMENTS

A prototype 3-axis stabilised balloon platform has been developed by the Appleton Laboratory of the Science Research Council for the use of UK astronomy groups. The platform is designed to carry experiments weighing up to 500 kg at altitudes of about 40 km and point them with an accuracy of better than one minute of arc. It was successfully flown in September and October 1978. The main features of the platform are outlined and the position and stabilising sensors which use both terrestrial and celestial frames of reference are described. The paper discusses the problems specific to controlling a platform of this type when attached to a large balloon by some 100 m of flexible suspension. The difficulty of predicting or simulating the behaviour of such a suspension system has led to a design in which many of the control loop parameters may be adjusted by command from the ground. The design measures used to overcome such problems as suspension resonance, pendulum motion, bearing friction and inter-axis coupling are described. Preliminary results from ground tests and from the two flights which took place in Texas are presented and discussed and the paper concludes with a summary of achievements to date and suggestions for further improvements to the control system.