Study On Scheme Design And Method Of Autonomous Navigation For Mars Atmospheric Entry Phase

Mars landing is one of the hot issues for future deep space exploration,whose atmospheric entry phase is the most complex and critical phase of the entire landing process.A precise autonomous entry navigation has an important effect on the guidance and control system and is one of the key technologies that determine the success of the landing mission.However,the unique flight dynamic environment during Mars entry leads to great challenges for precise autonomous navigation.Such challenges include the lack of navigation information,the strong nonlinearity of the dynamic system,as well as the uncertainty of model parameters.Based on an investigation of pervious researches,a further study on the design of autonomous navigation scheme and state estimation methods for Mars atmospheric entry is carried out.The main research work is as follows:First,focusing on the accuracy requirements of navigation systems for future Mars highprecision landing,research and analysis on the design of autonomous navigation scheme for Mars atmospheric entry phase is conducted.The entry dynamics model of the probe and the measurement model of the available sensors are built,and the measurement characteristics of each sensor are studied,which provides references for the design of subsequent navigation schemes.Based on that,the dynamic pressure measurement is first introduced into the design of Mars entry navigation schemes.The dynamic pressure measurement-aided integrated navigation schemes are then constructed for direct entry and surround entry respectively.An adaptive information fusion strategy is proposed for the surround entry navigation scheme,which can reduce the impact of single sensor failure on the navigation system performance and improve the stability and accuracy and stability of the navigation system.Second,considering the strong nonlinearity and large disturbance of the dynamic system around the peak load in Mars atmospheric entry,a nonlinear differential filtering method based on sensitivity suppression is proposed.The measurement equation of the nonlinearity of the navigation system is established,and the variation of the degree of nonlinearity(Do N)of navigation system over time during the Mars atmospheric entry phase is analyzed.The influence of uncertain parameters on the state estimation accuracy is assessed using the theory of parameter sensitivity.Then,a desensitized first-order and second-order divided difference filtering(DDF)method with analytic filtering gains are derived by minimizing the cost function,which consists of the trace of the posterior estimation error covariance matrix penalized by the trace of the state sensitivity matrix to uncertain parameters.Combined with the Do N of the navigation system calculated on board,an adaptive filtering order selection strategy of the desensitized DDF is proposed.This method reduces the effect of the strong nonlinearity and parameter uncertainty on state estimation.Then,focusing on the state estimation error divergence caused by the large model uncertainty of Mars atmospheric density,a multiple model estimation method based on measurement innovation is proposed.The distribution characteristics of Mars atmospheric density uncertainty with altitude,latitude and longitude are analyzed,and the disturbance model of the dynamics caused by Mars density uncertainty is established.Multiple exponential models are adopted to approximate the distribution of the Mars atmospheric density,and the weights of each model are calculated according to the measurement innovation.At the same time,the weight of each model is updated using the instantaneous gradient ascent procedure.This method can adaptively select the model closest to the true parameter in the model group according to the change of Mars atmospheric density,to achieve the optimal state estimation.Finally,based on the study on theoretical analysis,an autonomous navigation simulation demonstration system is developed,and the performance of different autonomous navigation schemes are compared.Using the modular design concept,the atmospheric density module,the sensor module,the navigation algorithm module,as well as the visual demonstration system are designed independently.In the design of the Mars atmospheric density module,a three-dimensional analytical atmospheric density model is investigated.The atmospheric entry phase navigation performance of two typical landing missions “Mars Pathfinder” and“Curiosity” are analyzed.Based on that,the feasibility,rationality,and effectiveness of the autonomous navigation method proposed in this thesis is validated through the developed navigation simulation demonstration system.