The Advanced Telescope for High-Energy Astrophysics (ATHENA) has been selected as the second large (L-class) mission designed to address the Cosmic Vision science theme “The Hot and Energetic Universe”. The European Space Agency has produced a Mission Observation Plan which contains objectives and constraints of the mission. In addition, this plan includes a representative sequence of targets, the Mock Observation Sequence, which defines the order in which ATHENA has to observe the targets. The feasibility of the Mock Observation Plan has to be assessed and the Mission Timeline Simulator has been designed for this purpose. It is a tool developed by Airbus DS that includes all the spacecraft operations. The analysis of the Mock Observation Plan in the Mission Timeline Simulator showed that the ATHENA’s line of sight points into the sun during some observations. This would lead to a loss of the whole mission since sunlight damages the instruments. Consequently, a new Observation Sequence has been produced by Airbus DS, in which the spacecraft does not point to the sun. However, this new sequence does not meet one important requirement: the operational availability shall be above 90%. The operational availability is defined by the percentage of the total time which is used for science observation. The version at the start of the thesis of the artificial case study but representative to the ATHENA mission achieved a value of 87.3 %. One of the main objectives of this thesis is therefore to obtain a sequence that meets the operational availability requirement with the final goal of generating a functional timeline of several days. If any interrupt occurs to the timeline (Targets of Opportunity, safe mode events, etc.), the spacecraft shall be able to find the way back to the optimal timeline or replan a new one. The current status includes these events but the timeline does not get interrupted. In this aspect, the currently implemented Greedy algorithm, that chooses the observation sequence, must be extended in branch and horizon. In order to achieve these objectives, a Beam Search algorithm has been developed and implemented in the Mission Timeline Simulator. The operational availability has been increased by incrementing the width search (branching factor). The depth search (horizon factor) allows the user to restrict the longitude of the timeline in terms of number of targets. A first implementation not including interrupts was analysed and an operational availability higher than 90% was achieved. A second implementation with a functional timeline considering interrupts shows a decrease in the operational availability, which does not reach 90%; nevertheless improving the results of the previous version with a final operational availability of 89.24%. Note that this is a representative case study of the mission. The Beam Search algorithm has been demonstrated to be suitable for the defined problem and a future sensitivity analysis of different parameters, such as depth and width search, could increase further the performance of the algorithm and therefore the simulation results. / ATHENA mission
| Identifer | oai:union.ndltd.org:UPSALLA1/oai:DiVA.org:ltu-90060 |
| Date | January 2022 |
| Creators | Pérez Cámara, Flavia |
| Publisher | Luleå tekniska universitet, Institutionen för system- och rymdteknik |
| Source Sets | DiVA Archive at Upsalla University |
| Language | English |
| Detected Language | English |
| Type | Student thesis, info:eu-repo/semantics/bachelorThesis, text |
| Format | application/pdf |
| Rights | info:eu-repo/semantics/openAccess |
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