The Status of Passive-Gravity-Gradient Stabilization

Abstract

In the last few years significant advances have been made in the theory and implementation of passive attitude control systems for earth pointing satellites. In each of the systems that have evolved, the gravity-gradient provides a preferred orientation such that the axis of minimum moment of inertia points toward the center of the earth. Many techniques have been developed for damping attitude motions about this preferred orientation. This is usually accomplished by absorbing the energy of the relative motion between the stabilized body and one or more auxiliary bodies. The various damping techniques differ basically in the ambient field that determines the position of the auxiliary bodies. Systems have been devised in which the auxiliary bodies are oriented by solar, magnetic, aerodynamic, or gravity fields. Hardware for some of these systems is in an advanced state of development.

The choice of a system for passive attitude control will depend upon the orbital altitude and inclination, the anticipated eccentricity, and acceptable steady-state pointing accuracy, whether or not control about the earth-pointing axis is required, the acceptable time to damp transient motions, and the probability of successful deployment of the system. The manner in which the choice of the ambient field utilized to effect the damping enters into the fulfillment of these requirements is discussed to point out the advantages and disadvantages of each system.

To date, passive gravity-gradient stabilized satellites have been flown only at low orbital altitudes. This flight experience is discussed and the performance compared with theoretical predictions. For many applications, it is desirable to place earth-pointing satellites at much higher altitudes, particularly in stationary, or synchronous, orbits. An experimental flight program which could prove the practicability of passive stabilization at this altitude is described.