Guidance And Control Law For Asteroid Soft Landing In Power Decent Phase

In recent years, with the progress of science and technology of the aerospace, deep space exploration activities flourish all over the world. Due to the potential of hitting the earth and the unique scientific value, the asteroid detection becomes the research focus of the deep space detection. Detecting forms include flyby, orbit detection, impact, landing on the asteroid surface and sampling return to the earth now. Landing softly and sampling by the instruments onboard is the most ideal and important mean to gather scientific data among the forms. On account of the unique properties of asteroid, including its small mass, irregular shape, weak gravitational field and complicated self-rotation, the landing guidance must be high precision, strong robustness and autonomy for achieving the landing mission. In the background of high precision pin-point soft landing mission for asteroid, this paper is mainly focus on the guidance in the power decent phase.Firstly, the orbit constraints in asteroid power decent phase are analyzed and the motion equation of asteroid lander is established. Among the modeling approaches of steroid’s gravity field, spherical harmonic expansion model and the polyhedron model are detailed studied to model gravity field of the irregular shape asteroid.Secondly, the in-depth study of calculation and optimization problems of the asteroid gravity field sphere harmonic coefficients is presented. To get the high precision global sphere harmonic coefficients, the partition form of polyhedron model and selection method of test points are discussed. In order to establish the gravity field model of the local space, the concept of local sphere harmonic coefficient is proposed, choosing the test points located in the detector’s major operation area. Then, a new performance index to measure the effectiveness of local coefficients is constructed.Thirdly, this paper develops explicit guidance laws in the power decent phase for asteroid landing detector. Considering the uncertainty of the asteroid dynamics environment and external disturbance, two autonomous and robust guidance laws based on sliding mode variable structure are presented to realize the pin-point landing mission safely and accurately. The high-order sliding guidance law is presented to reduce order of the control system, simply the control problem and achieve landing at the fixed time. The multiple sliding surfaces guidance law is designed to give full play to the control of the actuator and realize fast landing targetFinally, this paper plans the nominal trajectory and develops a tracking guidance law in the power decent phase. Based on the nominal trajectory in a polynomial form, a nonlinear generalized predictive guidance law with analytic expression and low computational complexity are designed which tracks the nominal trajectory in short period with a certain anti-interference ability meanwhile and contains as few adjustable guidance parameter s as possible.