Gravity-Gradient Stabilization of Earth Satellites

Abstract

The possibility of using the earth’s gravity field for vertical stabilization of near-earth satellites has intrigued theoreticians for many years. The very small stabilizing torque available, and the lack of a natural damping mechanism have been recognized as the major problems. The satellite 1963-22A launched in June 1963 was the first orbiting vehicle to successfully achieve passive gravity-gradient attitude stabilization. The initial spin of the satellite was removed by mechanical and magnetic despin devices. An internal electromagnet was then energized to align the satellite along the local magnetic field direction. Twelve hours later when the satellite passed over the north magnetic pole it was vertical with the correct side facing earthward. A 100 foot boom was then extended from the satellite and the magnetic dipole was turned off. Libration damping was provided by the combination of a lossy spring and magnetic hysteresis rods. Within a week after boom deployment the peak angle of oscillation of the satellite was damped below 15 degrees. By June 1964 four other satellites achieved gravity-gradient stabilization using passive damping techniques. The satellites 1963-38B and 1963-49B used the combination of a lossy spring and magnetic rods in a configuration that was virtually identical to the satellite 1963-22A. The satellite 1964-26A achieved gravity stabilization using only 0.6 pound of magnetic hysteresis rods to damp the peak libration angle from 40 degrees to 10 degrees in a period of approximately 4 days. This lightweight, simple and completely passive magnetic damping technique shows great promise for future applications for near-earth orbiting satellites. At satellite altitudes above 5000 miles, vector magnetometers with current amplifiers can be used to enhance the damping capability of the magnetic rods. This semi-passive technique with no moving parts should be suitable for satellites even at synchronous altitude. Thermal bending of the extendible booms has been observed on several satellites. A high frequency, dynamic boom bending has been observed for unplated booms with an amplitude of ±12 degrees. With silver plated booms, the dynamic thermal bending was reduced to less than ±1/4 degree.