An attitude determination routine based on geometric relations coupled with an orbital position estimator is designed. The proposed determination algorithm utilizes Earth position and magnetic field vector measurements. Orbital position data is provided by an extended Kalman filter (EKF) estimation of the Keplerian orbital elements. This estimator uses only measurements of the magnitude of the Earth's magnetic field. Coupling the orbital position estimator and attitude determination routine results in a fully autonomous satellite navigation system. The proposed attitude determination routine significantly reduces the computational efforts necessary to accurately estimate the attitude. It also eliminates the need of error-prone gyros while only requiring a nadir vector measurement. Simulation of the proposed system on the CATSAT (cooperative astrophysics and technology satellite) model results in accurate orbital position and attitude estimates for secondary fault detection and isolation implementation.