Research Of Drag-Free And Attitude Control System For The Next Generation Gravimetric Satellite

Satellite to Satellite Tracking in Low Earth Orbit(SST-LL)missions are a powerful tool for continuously monitoring the Earth's gravity field for a long time.15 years has pasted from the launch of the first generation of GRACE satellite Recently,faster and more accurate gravity field monitoring data are becoming demanding in earth science field.With the development of ranging technology and new satellite platform,the next generation of SST-LL missions have entered the engineering research stage.The mission will employ 10 nm level laser interferometric ranging system instead of the KBR ranging that used in the GRACE mission,and use drag-free system to keep the spacecraft in the orbit below 400 km altitude to get stronger signals from the gravity field.The new mission calls for higher requirements on the control system design for the satellite drag-free and attitude control,which includes:the electrostatic accelerometer needs to be controller up to the sensor noise floor;drag-free control system using proportional electric propulsion must be employed in order to improve the platform drag environment;attitude control accuracy should be at least two orders of magnitude better to meet the laser pointing requirement.Additionally,flowing demands should be kept during the control system design:1)the control algorithm should be practically simple and reliable for space engineering;2)the design of controller and parameters should focus on frequency domain performance target;3)design results should be validated by numerical simulation in the closed loop system.In order to meet the above challenges,firstly the basic information of the next generation gravimetric missions is introduced,and then the satellite orbit,attitude dynamics and its environmental disturbance,as well as the control system hardware have been modeled,based on which a numerical simulation platform has been built.The platform can not only simulate the orbit and attitude dynamics of satellites,but also calculate the relative attitude of the twin space crafts.The formation disturbance from high-order Earth's gravity field are also included.Additionally,the characteristics of all sensors and actuators of the control system are described in the platform.which makes a reliable foundation for control system validation.In order to meet the challenge of controller design,a discrete state space controller based on disturbance estimation and rejection called EMC(Embedded Model Control)are introduced.The innovatively use of non-smooth optimization for structured parametric controller tuning based on H-infinity technology makes it possible to directly design the frequency domain H? loop shape of EMC without tedious pole-location and dynamic equation derivations.Practical design steps explored here could give out a set of parameters list.The controller corresponding to the tuned parameters in the list have similar frequency domain curve of the transfer function along the frequency axis,it greatly simplifies the in-field adjustment process of the controller.The next generation space borne electrostatic accelerometer control requirements are derived from its measurement noise target.Using the accelerometer as an example control plant,the EMC design method and parametric optimization steps have been introduced,and a list of parameters are provided.The list is then simulated an end to end numerical simulator,frequency domain performance analysis gives an optimal selection of parameters.The advantages of EMC controller are fully demonstrated by simulation results,including smooth transition between different control parameters are,and low noise acceleration measurement output port.The two drag-free control modes are introduced and compared before the controller design.Then the drag-free controller based on EMC with non-smooth frequency-domain optimizations have been designed and validated on the simulation platform,and the residual acceleration noise of satellite under drag-free control has reached 3 nm/s2/Hz1/2 level.The simulation results indicate that the drag-free control requirements could be well met for three main formations.The last section of this work designed the attitude controller.Firstly,the complete structure is build including the angular acceleration inner loop and the attitude states predictor with feedback gains.Then after detailed discussion of each sub blocks,a framework of the whole controller has been built using EMC with parameters tuned by non-smooth optimization.The closed-loop simulation shows that the attitude controller could meet the requirements of laser pointing in the scientific measurement mode of the mission,and the residual attitude disturbance of the satellite can reach a level of better than 5?rad/Hz1/2,satisfying the attitude control requirement of laser interferometry.The numerical simulation shows that the angular acceleration measurement in the attitude control can effectively reduce the bandwidth of the attitude loop and obtain a smoother control torque output.The advanced-EMC controller developed in this work has simple structure and physical meaning,and its frequency domain performance could be easily adjusted.All the advantages of the novel controller designed in this work promise that it could be directly deployed in the satellites for missions that involves high precision laser ranging,including the measurements of Earth's gravity field and the space borne gravitational wave detection.