| In the recent years with the penetration of lunar exploration by human,the requirements of navigation and communication is improved in lunar aerospace mission.The current deep space navigation and communication system has the disadvantages in small coverage,low accuracy and large delay for the lunar space and lunar surface.It cannot satisfy the requirements for the future manned lunar exploration and construction of lunar bases.Therefore,an autonomously operated navigation satellite system that can cover the whole lunar space and lunar surface is required urgently,providing the all-round,high-accuracy,real-time navigation,and communication for the future manned lunar exploration even the construction of lunar bases.Lagrange points is also named of libration points,where a point of mass cannot be fixed due to the instability near libration points.However,the spacecraft can move about these libration points periodically or quasi-periodically by giving a reasonable perturbation.Compared with Keplerian orbit in two-body problem,the orbits near the libration points have longer distance with respect to the primary,longer motion period and larger orbit amplitude,which are appropriate to be the mission orbit of the navigation satellite system.This dissertation theoretically investigates the key technologies in the lunar libration-point navigation satellite system,including the design of the mission orbit,the autonomous navigation technology,the stationkeeping strategy and the constellation design of the navigation satellite system.Firstly,the design of the libration-point orbit is investigated under the ephemeris model.Initializing from the circular restricted three body problem,the equations of motion of the spacecraft near the collinear libration points in the Earth-centered inertial frame and the libration-point-centered synodic frame are derived under the ephemeris model,with the consideration of the eccentricity of the Moon orbit and gravitational perturbations from other celestial bodies in the solar system.According to the equations of motion,the halo and Lissajous orbits about the L1,2 in the Earth-Moon system can be designed using the multiple shooting method,where the initial guess is from the analytical solution in circular restricted three body problem.Compared with designed results of halo and Lissajous orbits in circular restricted three body problem,the periodic halo orbit is reduced to be quasi-periodic orbit in the synodic frame,and the in-plane motion of Lissajous orbit is reduced to be quasi-periodic as well.The designed orbits under the ephemeris model follow the features of the orbits in the real dynamics and is used to be the mission orbits of the lunar libration-point navigation satellite system.Secondly,to solve the autonomous navigation problem of the spacecraft in the libration-point mission,the autonomous navigation scheme based on the X-ray pulsar measurements is introduced in this dissertation.Implementing the Kalman filtering framework,the state equations are the spacecraft's equations of motion under the ephemeris model and measurement equations are the time-of-arrival model of X-ray photons so that the autonomous navigation system for the spacecraft in the libration-point mission is constructed.Based on the standard X-ray pulsar navigation,the system biases including the Earth ephemeris bias,the Sun ephemeris bias,the X-ray pulsar angular bias and the clock bias are analyzed.The state augmented approach is introduced to compensate the system bias in order to guarantee the navigational accuracy.Because the continuous observations of the X-ray pulsar for a certain duration is able to guarantee the navigational accuracy,the abnormal conditions are investigated,including the blockage of the signal by the celestial bodies,limitation of the field-of-view of the X-ray detector and single event effect.To solve the abnormal measurements,a hypothesis-test-based adaptive Kalman filtering algorithm is proposed.The scaled matrix is designed,which modifies the measurement noise matrix adaptively,to guarantee the convergent filtering results under the abnormal conditions.Thirdly,considering the instability of the periodic orbits near the collinear libration points,the stationkeeping strategy in the libration-point mission is investigated.In the conventional Floquet mode strategy,the monodromy matrix is not well-defined for quasi-periodic case.Based on the characteristic that the maximum orbit uncertainty will evolve along the direction of local unstable manifold over time,the covariance-based stationkeeping strategy is proposed.The local unstable direction is defined by the direction of maximum orbit uncertainty so that the stationkeeping maneuver is designed to remove the unstable components.After investigating the stationkeeping strategy based on the Hamiltonian structure-preserved approach,it is found that this strategy cannot overcome the weak instability caused by the orbit eccentricity and gravitational perturbations from other celestial bodies under the ephemeris model.To solve the problem,the modified Hamiltonian structure-preserved approach is proposed.The compensation matrix is constructed by the terms of angular velocities and accelerations so that the orbit-maintenance controller can guarantee the local stability and global stability.Fourthly,to solve the constellation design of the lunar libration-point navigation satellite system,this dissertation takes the halo orbits about the L1,2 in the Earth-Moon system as the mission orbit and proposes 6 schemes of the constellation design of navigation satellite system.Regarding the visibility of the navigation satellites and the geometric dilution precision as the performance indices,it is investigated how the orbit amplitude and constellation geometry influence the navigation performance for the cislunar transfer orbiter,the lunar orbiter,and the lunar surface asset in order to the obtain the optimal constellation design schemes.Finally,to present the true performance of the navigation satellite system under the ephemeris model,the research results of the mission orbit design,the autonomous navigation technology,the stationkeeping strategy and the design of the libration-point navigation constellation system are combined to design the integrated simulation: the autonomous navigation module supplies the navigation-error-included position and velocity information for the stationkeeping module and navigation satellite system;the stationkeeping module designs and performs the stationkeeping maneuvers based on the position and velocity input from the autonomous navigation module,in order to realize the long-term on-orbit run of the navigation satellite system;on the basis of autonomous navigation results,the navigation satellite system navigates the cislunar transfer orbiter,the lunar orbiter,and the lunar surface asset.The simulation results including the autonomous navigational errors,the stationkeeping errors,the total stationkeeping cost of the navigation satellite system,and the navigational errors for the lunar explorers are presented in order to verify the autonomous and navigational performance of the navigation satellite system. |
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