TABLE OF CONTENTS Virtually all but the simplest spacecraft employ means of active attitude control using such actuators as control momentum gyros, reaction wheels, offset thrusters, and magnetic torque rods. Attitude control is almost always performed by a closed-loop system onboard the spacecraft; especially for low Earth orbit (LEO) spacecraft, where ground contacts are often limited to a few minutes per day. Only unusual events such as a momentum unload or spacecraft slew maneuver are commanded from the ground, and even then, the onboard system is usually responsible for some level of automatic control. Closed-loop attitude control, of course, requires sensor feedback of the vehicle orientation. This has traditionally been provided by such low-cost sensors as magnetometers, horizon sensors, and sun sensors, or more expensive high-performance instruments including gyroscopes and star trackers. Recently developed GPS attitude determination systems provide an opportunity to use this new technology in attitude control system designs.
As described in Chapter 19, this volume, precise measurement of differential carrier phase between multiple antennas may be used to determine the attitude of a vehicle. Thus, installing a GPS receiver onboard a spacecraft affords the opportunity to use a single lightweight, low-cost sensor for a multitude of functions: position, velocity, attitude, attitude rate, and time. This consolidation of resources is likely to lead to an overall savings in cost, power, weight,...
