In this paper, we investigate a practical strategy for de-orbiting the retired satellite in low-Earth orbit for the space debris mitigation. The only means available onboard the spacecraft for performing the task is the chemical propulsion system with limited propellant provided. It is proposed to reduce the orbital perigee to reach a certain level where the atmospheric drag can play its role in lowering the satellite altitude, and eventually bringing it to re-entry within a defined period of time. The required delta-V is divided into a series under the constraints on the propulsion system and orbit control manoeuvre implementation. The results from the flight dynamics simulator suggest that a fraction of the remaining propellant available on the demonstrating mission, the Thaichote satellite, would be sufficient to accomplish the task. The strategy implementation will be another vital step in transferring the spacecraft to a safe passive state, where the fuel tank is empty, all batteries are discharged and all electronic devices are deactivated.

In this paper, the development of an antenna pointing control system for the operation of satellites orbiting in Low-Earth Orbit is presented. A high-fidelity orbital dynamics model is developed for the prediction of the satellite's position and velocity. It includes higher geo-potential harmonics of the non-spherical Earth, the gravitational attraction from the Sun and the Moon, and other non-conservative forces, such as atmospheric drag and solar radiation pressure. A high precision orbit integrator based on the Burlish-Stoer algorithm plays a crucial role in integrating the equations of motion and find the satellite's states advance in time. The states from the EarthCentered Inertial Coordinate are transformed to the Topocentric Horizon Coordinate to find the satellite's motion viewed from the control station on the ground. The ephemeris file that comprises vital information for the satellite tracking, such as azimuth and elevation angles, can be generated when the pass is detected. The antenna pointing control system then tracks the predicted trajectory. The proposed system is verified during the operation of the Thaichote satellite. It is shown that the predicted orbital states are very close to those determined from the onboard GPS receiver. It exhibits only some small residuals in position and velocity. The tracking performance is also shown through the high signal level received from the S-band antenna throughout the tracking course.

In this paper, we investigate the algorithm for autonomous station-keeping for communication satellites in geostationary orbit. An impulsive optimal feedback controller, based on the pseudospectral method is proposed. The receding horizon concept is employed to control the satellites toward its predefined slot. Computer simulation results show that the closed-form controller based on state transformation matrix is asymptotically stable with fast error suppression ability, despite the effects from perturbing forces, especially the attractions from the non-spherical Earth, and the third body, i.e. the Sun and the Moon. The fuel expenditure in the proposed algorithm is optimized along the control horizon. Furthermore, it requires less computational burden than general optimization methods, hence it is suitable for online implementation.