Determination of the attitude of the Ariel III satellite

Abstract

The three radio experiments on the Ariel III satellite require a knowledge of spin axis direction in order to deduce the fields measured by their loop aerials. Solar aspect sensors provide the angle between the direction of the spin axis and the Sun, which defines a conical surface containing the spin axis. Some other frame of reference is required in order to find the exact position of the spin axis on this cone and a method was therefore proposed based on that used on Telstar. Six echelon mirrors were placed on the satellite from which glints of solar reflexions could be monitored by visual, photographic or photoelectric techniques on the ground. At the instant when glints are received the position of the satellite is known in relation to the observer and to the direction of the sun, as also is the angle of the mirror to the spin axis of the satellite; a second cone is therefore determined on which the spin axis lies. From the intersections of these two cones the final attitude of the spacecraft is calculated. Any one observation yields two possible directions, but successive observations enable the true direction to be determined. Excellent results have been obtained and the spin axis direction has been monitored to an accuracy of better than 2°. Results from a photoelectric camera have enabled the coning angle, or angle between dynamic and geometric axes, to be determined to better than 0.1°.