Mission Reliability Evaluation Of Space TT&C Systems With Time Redundancy

As an important part of aerospace engineering,the space tracking,telemetry and command(space TT&C)system establishes a link between the ground and the spacecraft through its TT&C station,which ensures the normal operation of the spacecraft during its entire life.As the only connection between the space and the earth,the system is indispensable for space missions,and the assessment of its mission reliability has always been an important part of aerospace engineering practice.Missions of the space TT&C system are described by the space tracking plans,which details the the visible time windows involved in the mission,the the TT&C services operated in each time window,and the TT&C equipment supporting these services.Previously,mission reliability assessment of the system is generally based on a given space tracking plan.However,time redundancies always exist in the execution of TT&C mission.In practice,some TT&C services failed in the current time window can be re-executed in a later time window(called time redundancy between phases),and the success of the service may only require the system to keep operational for a given time interval,instead of working throughout the time window(called time redundancy inner phase).These time redundancies have rarely been described by the space tracking plan.In order to obtain a more accurate result,this paper studies the mission reliability evaluation of space TT&C system,whose space tracking plan contains the time redundancy.The research is developed as follows:(1)The descriptive modeling on space TT&C system with time redundancy.The formal description on space tracking plan is first completed after ascertaining the composition of the system,and the type of TT&C services and missions.The fundamental forms of time redundancy in TT&C mission are then clarified and attached to the formal description of space tracking plan.Based on this,the formal space tracking plan is transformed to a phased mission system with time redundancy(TR-PMS)through the division on visible time windows.(2)The mission reliability evaluation of space TT&C system under a single spacecraft scenario,whose TT&C mission has time redundancy between phases.The evaluation process takes the model of PMS with backup phases as input.Because the failure of current phase may result in a configuration change of later phase in this PMS,the event tree(ET)model is chosen for its solution,and a static combinatory modeling method is proposed to model the mission reliability of the PMS.Based on this,a binary decision diagram(BDD)-based approach is applied for its quantification.Considering its non-coherent model structure,a multi-phased BDD modeling method based on the single phase BDD and its negation form is proposed,which has a better performance than other alternative solutions.(3)The mission reliability evaluation of space TT&C system under a multispacecraft scenario,whose TT&C mission has time redundancy between phases.The evaluation process takes the model of PMS with shared backup phases as input.Because the occupation of shared backup phase by different TT&C services is decided by their failure sequences in this PMS,the dynamic logic gate of the dynamic fault tree(DFT)is used,and a multi-phased DFT modeling approach is proposed to model the mission reliability of the PMS.Based on this,a sequential BDD(SBDD)-based method is applied on this DFT for its quantification.Considering that the DFT structure is highly coupled,an improved SBDD generation method is proposed,which effectively reduces the scale of SBDD model and improves its computational efficiency.(4)The mission reliability evaluation of space TT&C system under a multispacecraft scenario,whose TT&C mission has time redundancy both inner phase and between phases.The evaluation process takes the model of TR-PMS as input.Considering that the analytical method has difficulty in the mission reliability evaluation of TR-PMS,an extended object-oriented Petri net(EOOPN)simulation method is selected for evaluating the mission reliability of the PMS.Simplification on EOOPN transition and standarazation on the data structure of EOOPN token are made to overcome the deficiencies in the existing EOOPN definition.Based on this,a three-level hierarchical modeling approach is proposed to establish the EOOPN model for TR-PMS mission reliability.Compared with other alternative Petri net models,the constructed EOOPN is more concised in model structure and smaller in model scale.An EOOPN simulation tool is developed based on the event scheduling method,and a simplified example of satellite launching TT&C mission is used for method illustration and verification.Moreover,under the condition that the component is unrepairable and the mission has no time redundancy inner phase,the mission failure probabilities of the example system in several different assumptions of time reduandancy between phases are obtained through the EOOPN simulation method.Result comparison is made with the static combinatory modeling method and the multi-phased DFT modeling approach,and the agreement is found to be good.