TABLE OF CONTENTS Attitude control deals with the orientation of the spacecraft axes with respect to an inertial reference frame. An instantaneous spacecraft attitude is commonly described by a pitch angle, a roll angle, and a yaw angle. The most frequently used inertial reference frame is shown in Fig. 5.1. The +X vector is parallel to the spacecraft velocity vector. The +X and +Z axes are contained in the orbit plane, as is the velocity vector. The +Z is "up," opposed to the gravity vector. The +Y vector is perpendicular to the orbit plane. Spacecraft attitude is measured as an angular deviation of the spacecraft body axes from these inertial coordinates. The attitude control system (ACS), controls the spacecraft body axes such that the errors in pitch, yaw, and roll angles are within defined limits. Note the distinction between attitude control, which deals with rotation about spacecraft axes, and guidance and control, which deals with an objects position in geographical coordinates, usually latitude and longitude.
Any uncontrolled body in space, for example, an asteroid, will tumble about all axes in response to natural forces, notably solar pressure, gravity gradients, and magnetic torques. Sputnik I, the first spacecraft, was allowed to tumble in this way. However, natural tumbling is not normally acceptable for a spacecraft because solar panels must be pointed at the sun for power, antennas must be pointed at...
