This paper proposes a fault-tolerant finite-time controller for attitude tracking control of rigid spacecraft using intermediate quaterion in the presence of external disturbances, uncertain inertia parameter, and actuator faults. First, a novel non-singular fast terminal sliding mode control law is derived using intermediate quaternion, free of singularity, ambiguity, and unwinding phenomenon. Second, a proposed controller is developed by combining the continuous non-singular fast terminal sliding mode method with the finite-time disturbance observer. The key feature of the proposed control strategy is that it globally stabilizes the system in finite time, even in the presence of actuator faults, inertia uncertainty and external disturbances. Simulation results are presented under actuator constraint to show the desirable properties and the superiority of the proposed controller compared to an asymptotic controller and non-singular fast terminal sliding mode controller.

Enhanced real-time onboard orbit determination for low-Earth-orbit satellites is essential for autonomous spacecraft operations. However, the accuracy of such systems is often limited by signal propagation delays between GPS satellites and the user spacecraft. These delays, primarily due to Earth’s rotation and ionospheric effects become particularly significant in high-dynamic LEO environments, leading to considerable errors in range and range rate measurements, and consequently, in position and velocity estimation. To mitigate these issues, this paper proposes a real-time orbit determination algorithm that applies Earth rotation correction and dual-frequency (L1 and L2) ionospheric compensation to raw GPS measurements. The enhanced orbit determination method is processed directly in the Earth-centered Earth-fixed frame, eliminating repeated coordinate transformations and improving integration with ground-based systems. The proposed method employs a reduced-dynamic orbit determination strategy to balance model fidelity and computational efficiency. A predictive correction model is also incorporated to compensate for GPS signal delays under dynamic motion, thereby enhancing positional accuracy. The overall algorithm is embedded within an extended Kalman filter framework, which assimilates the corrected GPS observations with a stochastic process noise model to account for dynamic modeling uncertainties. Simulation results using synthetic GPS measurements, including pseudoranges and pseudorange rates from a dual-frequency spaceborne receiver, demonstrate that the proposed method provides a significant improvement in orbit determination accuracy compared to conventional techniques that neglect signal propagation effects. These findings highlight the importance of performing orbit estimation directly in the Earth-centered, Earth-fixed reference frame, utilizing pseudoranges that are corrected for ionospheric errors, applying reduced-dynamic filtering methods, and compensating for signal delays. Together, these enhancements contribute to more reliable and precise satellite orbit determination for missions operating in low Earth orbit.