Acquisition and Tracking Strategies for Satellite to Ground Optical Communication Systems

Optical wireless communication (OWC) offers a promising alternative to radio frequency (RF) based communication because it can support the increased demand for bandwidth in modern networks. This thesis examined three strategies that could be implemented to improve or simplify the design of a ground and satellite optical communication link. The acquisition of a laser beam emitted from a space orbiting satellite was examined. Atmospheric conditions and how they affect beam refraction was modeled using beam geometry and the refractive properties of air. Simulation results indicate that a beam with a large zenith angle is refracted to a higher degree than a beam with a smaller zenith angle. Beam refraction of an emitted beam with zenith angles of 61º and 82º reached the Earth surface with a peak power of 1179 photons/bit and 305 photons/bit respectively. Initial orbit estimation methods were examined, and it was found that Gauss’ Angles Only method was able to predict the azimuth and elevation of a target satellite with an average error of 6.38e−1. Which were positive results, and indicated that the Gauss method would be useful for initial orbit determination of an emitting satellite. Finally, a Extended Kalman Filter (EKF) state estimator was designed to evaluate whether the use of a Kalman filter is suitable for orbit determination when only using the angular observations that are available at an optical groundstation. Results indicated that when measurement errors of ±0.3 degrees were introduced into the system, position error state estimates reached a maximum of 6.9 km and 0.013 km/s. When the EKF was given smaller measurement errors of ±0.1 degrees, the errors in the state estimates were found to be a maximum of 1.4 km and 0.002 km/s. The results from the simulation for the state estimator indicated that an EKF can be applied to track the motion of a target satellite

Acquisition and Tracking Strategies for Satellite to Ground Optical Communication Systems

Optical wireless communication (OWC) offers a promising alternative to radio frequency (RF) based communication because it can support the increased demand for bandwidth in modern networks. This thesis examined three strategies that could be implemented to improve or simplify the design of a ground and satellite optical communication link. The acquisition of a laser beam emitted from a space orbiting satellite was examined. Atmospheric conditions and how they affect beam refraction was modeled using beam geometry and the refractive properties of air. Simulation results indicate that a beam with a large zenith angle is refracted to a higher degree than a beam with a smaller zenith angle. Beam refraction of an emitted beam with zenith angles of 61º and 82º reached the Earth surface with a peak power of 1179 photons/bit and 305 photons/bit respectively. Initial orbit estimation methods were examined, and it was found that Gauss’ Angles Only method was able to predict the azimuth and elevation of a target satellite with an average error of 6.38e−1. Which were positive results, and indicated that the Gauss method would be useful for initial orbit determination of an emitting satellite. Finally, a Extended Kalman Filter (EKF) state estimator was designed to evaluate whether the use of a Kalman filter is suitable for orbit determination when only using the angular observations that are available at an optical groundstation. Results indicated that when measurement errors of ±0.3 degrees were introduced into the system, position error state estimates reached a maximum of 6.9 km and 0.013 km/s. When the EKF was given smaller measurement errors of ±0.1 degrees, the errors in the state estimates were found to be a maximum of 1.4 km and 0.002 km/s. The results from the simulation for the state estimator indicated that an EKF can be applied to track the motion of a target satellite

Point-to-Point Motion Planning of a Free-Floating Space Manipulator Using the Rapidly-Exploring Random Trees (RRT) Method

SUMMARYIt is usually proposed to use a robotic manipulator for performing on-orbit capture of a target satellite in the planned active debris removal and on-orbit servicing missions. Control of the satellite-manipulator system is challenging because motion of the manipulator influences position and orientation of the chaser satellite. Moreover, the trajectory selected for the capture manoeuvre must be collision-free. In this article, we consider the case of a nonredundant manipulator mounted on a free-floating satellite.We propose to use the bi-directional rapidly-exploring random trees (RRT) algorithm to achieve two purposes: to plan a collision-free manipulator trajectory that, at the same time, will result in a desired change of the chaser satellite orientation. Several improvements are introduced in comparison to the previous applications of the RRT method for manipulator mounted on a free-floating satellite. Feasibility of the proposed approach is demonstrated in numerical simulations performed for the planar case in which the chaser satellite is equipped with a 2-DoF (Degree of Freedom) manipulator. The obtained results are analysed and compared with the results obtained from collision-free trajectory planning methods that do not allow to set the desired final orientation of the chaser satellite.

Navigation for Satellite Formation Flying

This paper deals with the case of a target satellite in an unknown orientation and location with respect to the master satellite. Feature based monocular pose estimation vision system was presented. The results of analysis, implementation and testing of simulation intended for vision-based navigation applications such as rendezvous of satellites and formation flying are shown. The mobile robot was used as the platform for the vision system. Pose estimation algorithms were implemented in Matlab environment. It was obtained that the proposed method is robust on varying and low light conditions.

Safe path planning for free-floating space robot to approach noncooperative spacecraft

In order to eliminate plume impingement on the target satellite, a free-floating space robot (also known as a chaser), which has the advantage of executing on-orbit service, is always used. This paper develops a path planning method for a safe rendezvous of chaser with a noncooperative target satellite in orbital coordinates. Safety principles for rendezvous in terminal approaching phase are proposed. Grasp points on the target satellite are analyzed and classified into two categories, and a moving ellipse trajectory is adopted to approach a rotating and uncontrolled target satellite. This method guarantees that the chaser can successfully escape if unexpected error occurs or capture fails. The simulation results show that, with this novel autonomous rendezvous method, the chaser can approach the noncooperative target satellite along the designated trajectory in any quadrant of the orbital plane.