Collection of technical papers on guidance and control : AIAA Guidance, Control and Flight Dynamics Conference, Huntsville, Alabama August 14-16, 1967
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The Syncom, Early Bird, and Applications Technology Satellite are spin-stabilized, synchro¬ nous orbit communications satellites with hydrogen peroxide attitude and velocity control systems and an apogee kick motor for insertion into final orbit. Precise attitude determination and control are re¬ quired for alignment of the apogee kick motor prior to apogee boost and in the final orbit where the spin axis is maintained normal to the orbit plane for communications. This paper discusses attitude determination, including the data types and the estimation process, and the use of the hydrogen peroxide control system in changing spacecraft orientation. Next a normal launch sequence is covered, and the inter¬ relation between attitude determination and control during critical mission phases, including real time analysis during reorientation, is explained. De¬ termination and control for final orbit operations, including the long term effect of solar radiation pressure precession, is then discussed. Finally the accuracies achieved by current methods as in¬ dicated by operational experience and new tech¬ niques in attitude determination are covered. I. Attitude Determination Attitude is determined on the ground by telemetered output from on-board sun sensors and by measuring the plane of polarization of the down¬ link signal from the spacecraft communications antenna. Each data type can be used independently for determination of the spin axis direction, and each takes advantage of unique spacecraft charac¬ teristics to allow simple and reliable, yet very accurate, attitude determination. Each sun sensor consists basicallyof a photo¬ voltaic element behind a narrow slit ^aperture. Thus the device, which is mounted on the periphery of the spacecraft, has a fan-shaped field of view which is scanned through 360° by the satellite spin. A measure of the angle between the spin axis and the spacecraft-to-sun line is obtained by mounting two such sensors together, one oriented such that the plane of its fan-shaped field of view contains the spin axis, and the other mounted such that its beam makes an angle of 35° with that of the first, (See Figure 1.) Inspection of this geometry indi¬ cates that in general the two sensors will not "see" the sun simultaneously but will be triggered some time interval apart (except when the sun lies along their line of intersection which is 90° to the spin axis). This time interval may be converted to an equivalent angle through which the spacecraft rotated between pulses since the spin period is known. This angle, designated is related to 0, the angle between the spin axis and the spacecraftto-sun line, by the equation cot 0 = cot 35° sin Thus a single ^ measurement defines a cone of half angle 0 centered on the spacecraft-to-sun line on which the spin axis must lie. FIGURE 1. SUN SENSOR AND CONTROL JET GEOMETRY The second source of data relating to space¬ craft attitude is the linearly polarized radiation from the spacecraft communications antenna. Mounted at the opposite end of the satellite from the apogee motor, this antenna is colinear with the spin axis, so that the electric field vector of the radiated signal lies in the same plane as the spin axis. At a ground terminal, measurement of the angle between the plane defined by the local verti¬ cal and the line of sight to the satellite and the plane of polarization of the down-link signal yields data which, when coupled with station location and orbital parameters, provides an independent in¬ dication of spin axis attitude. The plane of polarization is subject to Faraday rotation by the ionosphere, thus complicating its use. However, this effect can be analytically de¬ scribed with knowledge of the signal's frequency, the earth's magnetic field, and the electron densi¬ ty of the upper atmosphere. Of these the last is least certain. If both a ^2 reading and a polarization angle measurement are taken simultaneously, the cone and plane previously described will intersect in general along two lines, one of which is the spin axis direction. Additional measurements (of either type), separated in time from the initial pair, will