Precision Control, Knowledge and Orbit Determination on a Small Spacecraft Bus: The OrbView-4 Attitutde Control System

High resolution, Earth imaging missions place stringent requirements on the spacecraft Attitude Control System (ACS) in terms of pointing accuracy, attitude knowledge, and orbit knowledge. For OrbView-4, a body scanner, the spacecraft must also be agile to maximize the number of targets that can be imaged each orbit. These requirements dictate a robust, high performance design that allows rapid target acquisition and high line scan rates for mapping while maintaining precise knowledge and control of the camera line of sight vector. The OrbView-4 ACS combines state of the art components with robust algorithm design to meet these objectives. Equipment selection, attitude determination, attitude control, and GPS based onboard orbit determination are discussed as the elements of an ACS design for an advanced Earth imaging spacecraft. An overview of the OrbView-4 mission is presented along with a discussion of the requirements imposed on the ACS. The system architecture is described in terms of its hardware and software elements. The design of the attitude determination, orbit determination and attitude control algorithms are discussed, and the results of a system performance analysis are summarized.