Optical Measurements on Telstar to Determine the Orientation of the Spin Axis, and the Spin Rate

Abstract

One plane mirror and two faceted mirrors were fitted to Telstar. A photoelectric telescope was mounted on a radar antenna pedestal. Appropriate recording gear was built. The 12-in. aperture Cassegrain photoelectric telescope could be pointed at the satellite directly from prediction tapes, and the pointing accuracy could on occasion be improved by homing on the microwave beacon on the satellite or by applying corrections from visual observation through other telescopes with apertures of up to 6 in. It has been possible to detect the flashes of sunlight reflected from the mirrors on Telstar at slant ranges up to 3,700 miles. Directly from pairs of these observations, or by combining this information with telemetry data from the solar sensors on the satellite, it has been possible to determine the orientation of the spin axis of Telstar and to examine how this is moving in time. This normal precession of the spin axis is due mainly to the interaction of the residual magnetic moment of the satellite with the magnetic field of the earth. It was also possible to study how the spin axis moved when the torque coil was operated. — Measurements have been made of the spin rate of the satellite and of its rate of decay. The spin rate will drop to 1/10 of its initial value in about 750 days. — Photographs of the glints have been taken, and a study is now being made of the feasibility of combining precise photoelectric observation of the time of the flashes with high-precision photographic observation of the instantaneous position of the satellite relative to the fixed star background.