Gravity Gradient Stabilization of Communication Satellite Systems

Abstract

Abstract The R&D communications satellite payloads launched to date have been spin stabilized with essentially zero gain antennas. Economic studies have shown that the communication efficiency must be improved considerably for operational communication satellite systems to be totally practical. Effective gains of 10 db at medium altitude and 14 db at synchronous altitude can be realized by using a directional antenna and stabilizing the satellite to the local vertical. Two techniques for orienting an antenna beam have raised considerable activity: spin stabilization with a despun antenna, and gravity gradient stabilization of the total spacecraft. The significant difference between the two techniques is that the despun antenna requires active controls, logic, and an expenditure of power while a spacecraft stabilized by gravity gradient requires no active controls and no power. This paper discusses the application of gravity gradient stabilization to communication satellites. Four types of orbits are generally considered: medium altitude, high altitude, near synchronous. The optimum gravity gradient configuration is dependent among other considerations on the operating altitude and stabilization accuracy requirements. Several passively controlled satellites have been flown successfully to date showing feasibility of this concept. These flights include vehicles that are stabilized in two axes (pitch and roll) and others which are also stabilized in yaw. These flights and those planned for the n e a r future can provide engineering design data so that gravity gradient stabilization can be applied to a variety of missions, among which a r e those in the field of communications.