AbstractsCommunication

The aim of this thesis is to investigate a debris-remediation technique where a chaser performs a rendezvous with the debris, establishes a rigid-link connection, and actively de-orbits the debris. Remediation of the space environment becomes an increasing urgency, since the continuously growing space debris population in low-Earth orbit currently poses a serious threat for active satellites. Furthermore, on Earth a threat exists due to uncontrolled re-entry of the debris sooner or later. The thesis addresses various aspects of the rendezvous phase for the debris-remediation technique under consideration. Among others, the aim is to assess passive safety of the required manoeuvres in the rendezvous phase. Also, the thesis explores the possibility of continuous ground communication of the chaser during its final approach with the target. Finally, the thesis studies the illumination conditions. This includes sensor blinding, target face illumination and chaser solar array illumination. ESA's satellite ENVISAT was used as design case. The safety aspects of the rendezvous manoeuvres were assessed by analysing the resulting trajectories after various thruster failures. For the analysis related to communication, the chain of core ESTRACK ground stations (located mainly in Europe) was considered. Furthermore, obstruction of the communication signal by the target was studied. Last, for the illumination conditions, obscuration of the Sun by target was taken into account. In the topic of passive safety, the results indicate that fly-around manoeuvres are preferred in the direction opposite to the natural orbital motion, as these are passively safe. On the other hand, manoeuvres on H-bar and fly-around manoeuvres along the natural orbital motion are found to be passively unsafe. It can be concluded from the communication analysis that the maximum duration of the uninterrupted window amounts up to more than half an hour, using the chain of core ESTRACK ground stations. However, the study on communication blockage shows that frequent communication gaps can occur, with the longest gaps being in the order of one minute in duration. In the field of illumination, it can be concluded that correct target illumination and sensor visibility cannot be guaranteed. Furthermore, the results show that the average solar array area available during final approach varies between 35% and 75%, due to both incorrect pointing by the chaser and obscuration by the target.