This thesis evaluates communication architectures of space missions with respect to their suitability for a given application. With new applications, like real-time teleoperated spacecraft for orbital maintenance tasks, gaining attention, the communication systems of such missions are facing new requirements. Parameters not relevant for existing missions need to be included in the design process of future spacecraft. With strict real-time requirements for such possible teleoperation missions, the communication architectures need to be evaluated with a coherent parameter set that includes all relevant aspects. To achieve this goal, a model is proposed that allows the comparison of different communication architectures with respect to their Quality of Service (QoS), a common metric for information transmission evaluation. In the first step existing QoS parameters sets, mostly from terrestrial communication systems, are researched and extended as space communication systems face different technological challenges not reflected in these definitions. Besides the link performance measured in time delay, jitter, bandwidth and error ratio the aspect of link availability is included in the extended QoS definition. Long-duration disconnections due to orbital dynamics thus become part of the QoS set (total access time and mean contact length). In the second step the determined QoS parameters need to be related to the user requirements for specific missions. To define this relation, utility functions are used that relate a physical parameter (e.g. time delay) to a utility value between zero (= no utility) and one (= maximum utility). A mathematical relation is proposed that defines the drop in utility from ideal communication conditions (i.e. no time delay/jitter, unlimited bandwidth, no transmission errors). The used function is based on literature values and experimental data acquired with a simulation environment for orbital proximity teleoperation. The mathematical relation includes a scaling factor denoted ku that yields the thresholds of a 50% utility drop with respect to ideal communication conditions. With the relevant QoS parameters defined and the utility functions determined, possible communication architectures can be compared in the third step of this thesis. Different architectures were modeled with varying selections of space based data relay nodes (SDRN), bandwidth, frequency, and ground based data relay nodes (GDRN), with each architecture yielding specific QoS values. In a final step the determined link performance and availability values were traded against first order cost estimations, to include financial constraints. This four-step approach was applied in three different case studies to demonstrate the model's usability. With the origin of the research work being real-time teleoperation (RTTO), the two main case studies conducted were related to inspection and manipulation type RTTO missions. For the inspection case study experimental data and literature yielded k factors of 2.44 seconds for time delay and 0.866 Mbit/s for bandwidth. For the manipulation case study ku factors of 0.671 seconds and 1.74 Mbit/s were determined based on literature values only. The link availability ku factors for both case studies could only be modeled with a simplified theoretical model. The communication architecture evaluation for both RTTO case studies showed a distinct difference between performance biased and cost restricted emphasizes and allowed a general ranking of importance of the different architectural options. Following the model's results the most important parameter is the SDRN selection, followed by the bandwidth, GDRN and frequency. The applicability of a non-RTTO scenario is analyzed, using earth observation missions as third case study. The results show only simplified evaluations can be carried out for this case study, as the model is focused on real-time aspects and does not reflect parameters important for the earth observation application (e.g. satellite to ground target viewing geometry).


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    Title :

    Optimal quality of service for space communication architectures with focus on real-time teleoperated spacecraft


    Contributors:

    Publication date :

    2014


    Size :

    178 Seiten, Bilder, Tabellen, 215 Quellen



    Type of media :

    Theses


    Type of material :

    Print


    Language :

    English