Research On Navigation Method For Mars Entry

Future Mars missions require precision entry navigation capability, especially in the presence of communications blackout. To date, all Mars landers having reached the surface successfully adopt the inertial measurement unit based dead reckoning navigation, which lead to larger landing error ellipse in the order of several hundred kilometers. On the mission of Mars Science Laboratory(MSL), there was a 70-s communications blackout period during atmospheric entry phase. In allusion to the spacecraft communications blackout encountered, this thesis researches the autonomous navigation schemes for atmospheric entry. It’s mainly described as follows:Firstly, by reasonable assumptions and approximations, three-degree and six-degree of freedom dynamic models are established based on the Newton’s laws of mechanics. Then the standard trajectory is calculated through the numerical simulation software, which lays the groundwork for navigation system simulation.Secondly, the atmospheric entry profile can be analyzed by using aero-thermodynamic tools. Such tools make use of composition, pressure, temperature, and density information of the planetary atmosphere as well as a variety of other parameters to output electron number density at the stagnation point or wake region. The Horton program is adopted to predict an upper-bound for any possible blackout period for the case when the gas is in chemical equilibrium. The shock wave equations are established to analyze the planetary atmosphere upstream and downstream. Then the simulation demonstrate the blackout period of MSL.Thirdly, to reduce the error of dead reckoning navigation, the thesis develops the IMU/orbiters integrated navigation algorithm for Mars entry phase, improves Navigation measurements based on IMU and surface radiometric beacons, and proposes innovative attitude observation model based on IMU and range observation model based on orbiters finally. To verify the accuracy and effectiveness of the proposed observation models in the presence of communications blackout, unscented Kalman filter is utilized to demonstrate the navigation performance.Finally, communication channel models between the vehicle and orbiters, which include the wireless fading channel model, plasma attenuation channel model and intermittent observations model, are analyzed in the thesis. The wireless fading channel is modeled as Rice fading. The plasma attenuation channel analyzes the impact of electromagnetic wave power with plasma oscillation frequency and collision frequency. The relationship between transmission failure and signal-to-noise ratio is studied in intermittent observations model. By considering signal fluctuation and transmission failure simultaneously,numerical simulation shows effectiveness of the IMU/radio beacons integrated navigation.