Simulation Methods For Evaluating Mission Reliability Of TT&C System

When executing a spaceflight mission, it needs the tracking, telemetry andcommand (TT&C) system to track, measure, and control the spacecraft to make sure thespacecraft fulfill its scheduled tasks in the pre-designed states. TT&C systemimplements TT&C mission mainly according to the TT&C plan, which includes thewhole TT&C missions that should be carried out during spaceflight mission. TheseTT&C missions’ success directly relate with spaceflight mission’s success or fail. So,quantitatively evaluate those TT&C missions’ reliability has great practical meanings.The involved TT&C resources have complex operational time squence whenexecuting TT&C mission. Both the numbers of participanted TT&C stations anddifferent mission success criterias have influence on the reliability model of TT&Csystem. Because lots of involved TT&C resources, analytical methods are hardlysuitable for evaluating the reliability of TT&C mission, which makes simulation methodbe a possible option. For most TT&C missions, the mission reliabilies are extramly high.To obtain a precise reliability simulation result of TT&C mission, the crude simulationneeds a large given number of simulations, which cost very long computer time.Therefore, the reliability models of TT&C mission and their efficient simulationevaluation methods are studied as main focuses in this thesis. The main work andcontributions include:(1) Reliability models of TT&C mission and their crude simulation methodDifferent influencing factors on the reliability model of TT&C mission, such asnumber of involved TT&C stations, mission success criterias, are analyzed. Then,3different reliability models of TT&C mission are proposed, including single stationTT&C mission, multi phase TT&C mission, multi station complex TT&C mission.Those reliability models are describled in a standard form, and stored and represented inXML form. Based on tool ExSpect, the model base for reliability simulation of TT&Cmission is designed. The crude simulation model for reliability of TT&C mission can bequickly built by selecting, reconfiguring and combining the needed models from themodel base. The procedure for obtaining the unreliability simulation results of these3kinds of TT&C mission reliability model are given respectively. The disadvantage ofcrude simulation for mission reliability evaluation of TT&C system is given throughanalyzing the relationship between its simulation error and simulation number.(2) Efficient simulation methods for static reliability model of TT&C missionFor static reliability model of TT&C mission, two efficient simulation methods,called bounded sampling and approximate zero variance importance sampling (IS), areresearched. After analyzing the factors that influence the efficiency of boundedsampling, a prior information obtaining algorithm is proposed. The non intersection partial minimal cut sets and minimal path sets can be obtained through this algorithm,which could increase the simulation efficiency of bounded sampling. A sequentialmethod for constructing the sample distribution of zero variance IS is given. The formernon intersection partial minimal cut sets obtaining algorim is used to implementapproximate zero variance IS for static reliability model of TT&C mission. The highefficency of bounded sampling and approximate zero variance IS are verified throughexamples, and the domain of applicability and effectiveness of those two simulationmethods are compared too.(3) Efficient simulation methods for dynamic reliability model of single stationTT&C missionAfter analyzing the cause of the difficulty that the crude simulaion method insimulating the occurrence of TT&C mission fail event, then by combining forcing andfailure biasing (CFFB) methods, the next system state happening time for certain systemstate are changed and the proportion of failure transition happens are improved. UsingCFFB simulation, the mission failure event occur more frequently. Compared withcrude simulation method, it is proved that CFFB can greatly reduce the sample variance,which makes simulation results of CFFB have bounded relative error. The concept offailure importance is proposed according the character of reliability model of TT&Cmission. A new scheme for failure biasing is proposed based on the failure importance,which could improve the efficency of CFFB and more suitable for simulating theunreliability of those TT&C missions with high redundancy.(4) Efficient simulation methods for dynamic reliability model of multi-stationsTT&C missionWhen TT&C mission excuted by multi-TT&C stations, the reliability model hastwo types for different mission success criterias. Under a certain start system stateprobability vector, it is proved that the CFFB can get unbiased probability value foreach system state at the phase end. Under this conclusion, the unreliability of type mutiphase TT&C mission can be obtained by repeatedly used CFFB on each phase. Besides,for the special kind multi-phase mission that the required components are same in allphases, it is proved that CFFB can get the mission unreliability at each phase’s end time.For muti-station complex TT&C mission, the analytical model of its missionunreliability can be constructed through analyzing all the possible situations of TT&Cstations and center excuting the mission. For the analytical model, the unkown inputdata can be estimated by the efficient simulation methods proposed in this thesis. Thismixed method that combined analytical model and fast simulation methods offers apossible technique to quickly evaluate the unreliability of this complex TT&C mission.