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A new strapped-down system for on-board, real-time spacecraft attitude determination is discussed. The electro-optical system is capable of sub-ten-arc second precision with no moving parts. The light-sensitive element is an array-type Charged Coupled Device (CCD) having about 2 x 10 to the 5th power silicon pixels. Parallel, high speed analog circuits scan the pixels (row by row) to locate and A/D convert only those pixel response values (about 100 to 200 per scan) about a preset analog threshold. Angular rate measurements from conventional rate gyros are used to estimate motion continuously. Three intermittently communicating microcomputers operate in parallel to perform the functions: (i) star image centroid determination, (ii) star pattern identification and discrete attitude estimation (subsets of measured stars are identified as specific cataloged stars), (iii) optimal Kalman attitude motion estimation/integration. The system is designed to be self-calibrating with provision for routine updating of interlock angles, gyro bias parameters, and other system calibration parameters. For redundancy and improved precision, two optical ports are employed. This interim report documents Phase I of a three phase effort to research, develop, and laboratory test the basic concepts of this new system. Included in Phase I is definition, formulation, and test of the basic algorithms, including preliminary implementations and results from a laboratory microcomputer system. (Author)