Study The Guidance, Autonomous Navigation And Control Of Lunar Soft Landing

On October 24, 2007, the successful launching of Chang'E-1 satellite became a mile-stone of Chinese Lunar Exploration Programme (CLEP) which takes three steps: orbiting,soft-landing and return. In the future, China will establish bases on the near side or farside of moon to supply plentiful resource for the earth. Therefore, it is required that thelunar probe can achieve pin-point soft-landing and autonomous navigation without rely-ing on the ground stations. With the support of the Chinese Science Nature Foundation -the Fundamental Research of the Key Technology for Lunar Probe's Modeling, Sensing,Navigation and Control, this dissertation studies the key technologies on guidance, nav-igation and control (GNC) system of lunar probe from lunar orbit to the moon's surfacefor future lunar exploration missions.Firstly, this dissertation introduces preliminary knowledge of the GNC system of thelunar probe from lunar orbit to lunar surface. As essential elements of dynamical sys-tems, the definitions of time system and reference coordinates are given. The calculationmethod of sun and moon ephemeris is given. The orbit dynamics, rotational kinematics,attitude dynamics and mass ?ow equation for different phases from lunar orbit to lunarsurface are established.Nextly, autonomous navigation methods of lunar orbit, which play important rolesfor pin-point landing, based on the orientation information of sun-earth-moon are studied.Nadir vector determination is an important element in autonomous navigation algorithm.Therefore, according to the principle of imaging, algebraical constraints of the images ofthe lunar edge are established and the nadir vector is determined from these algebraicalconstraints, of which the in?uence of lunar oblateness is considered. Combined with theinformation from sun and earth sensors, a high accuracy autonomous navigation with theconsideration of the lunar oblateness is proposed. Numerical simulations show that theproposed navigation algorithms can correct lunar oblateness effectively and achieve highaccuracy navigation.Subsequently, the optimal impulse maneuvers between two lunar orbits are studiedsince the probe is required to transfer from initial lunar orbit to another orbit which is moresuitable for landing sometimes. General time-free two-impulse optimal transfers between two orbits are studied. Using the powerful tools of primer vector theory and Mathieutransformation, the two-point boundary problem with variable boundary is converted to27 algebraical equations with 27 unknown constants. The 27 unknown constants can besolved by a nonlinear Least-Squares method which implies that the two impulse trajectoryis determined. The method is testified by numerical simulations. It should be notedthat the method can be also expanded to multiple-impulse trajectories. However, thecomputation time would increase for multiple-impulse trajectories.Thereafter, the GNC system with embedded autonomy in the powered descent phaseis studied. Based on reasonable assumptions, a sub-optimal analytical trajectory is found.A robust adaptive controller is designed to track the reference trajectory. Such a controllercan cope with uncertainty of mass consumption, inertia matrix, thrust misalignment andthrust failures. In additional, An aid-Inertial Navigation System (A-INS) is designed.Inertial Navigation System (INS) is adopted to estimate the probe's orbit and attitudeinformation, and external altimeter and velocimeter are used to estimate the inertial er-rors. The analytical solution, robust controller and autonomous navigation method enablesystem's embedded autonomy. The design is also verified by numerical simulations.Finally, the GNC system for the terminal descent phase is proposed. A polynomialquadratic optimal guidance law is derived for time-free and time-fixed descent problemwhich can ensure the pin-point landing. A polynomial guidance law with attitude con-straint is also designed which can ensure pin-landing with vertical attitude. The proposedattitude controller in power descent phase can be also used for terminal descent. AnA-INS system is designed. INS system is used to estimate probe's orbit and attitude in-formation, and external lunar imaging sensor is used correct the inertial errors. Numericalsimulations show that the GNC system for terminal descent phase can ensure pin-pointsoft landing with almost vertical attitude.