Research On Design And Optimization Method Of Autonomous Navigation Scheme For Mars Entry Phase

The Mars entry phase is the most dangerous and complicated period in the Mars landing exploration mission. The autonomous navigation during Mars entry is one of the key technologies determining the success or failure of a mission. The specialty in the dynamic environment poses great obstacles to an accuracy autonomous navigation. The poor observability of navigation measurements as well as the nonlinearity and uncertainty of dynamic system are two main technical challenges. According to these challenges, the paper focuses on the performance improvement of autonomous navigation during Mars entry phase. Based on the investigation on previous research results, a further research on the design and optimization method of autonomous navigation scheme for Mars entry phase is carried out. The main studies and conclusions are as follows.First of all, the impact of disturbance sources(e.g initial state errors, atmospheric density uncertainty, gravity acceleration uncertainty, and lift and drag coefficients uncertainties) on the statistical properties of spacecraft’s states is analyzed. Meanwhile, the estimation error lower bounds of navigation based on radiometric measurements are analyzed. It is concluded that the atmospheric density is a major disturbance during the Mars entry phase. The performance of entry navigation can be significantly improved by introducing the radiometric measurements. Furthermore, the nonlinear propertiy of dynamic system is also a contributor to the estimation accuracy.The concept of observability is introduced to the optimization of beacon configuration. The configuration of beacons and navigation orbiters is optimized based on certain entry trajectory, which improves the navigation accuracy in the entry phase. According to this concept, an observability-based optimization method for beacon configuration, which can be sovlved by the genetic algorithm, is proposed. In such method, the observability of navigation system is chosen as the objective function, and the line-of-sight visibility is also considered. Based on these researches, novel observability analysis methods based on quadratic approximation and Fisher information matrix are proposed. Therefore, the relationship between the observability and the configuration of beacons is analytically investigated, and the theoretical optimal configurations corresponding to different number of beacons are obtained.An observability-based trajectory method is also proposed, in which the observability of navigation system is chosen as the objective function and multiple path constraints and boundary constraints are considered. The optimal condition of obtaining navigation information and the improvement of navigation performance can thus be achieved by optimizing the entry trajectory. Based on such investigation, a novel robust trajectory optimization method is proposed, in which the disturbances of atmosphere density and initial states are considered, and the polynomial chaos is used to describe the uncertainty propagation of states and constraint functions. Therefore the constraint violation, which is common for traditional method based on deterministic dynamic model, can be avoided by the proposed robust approach. However, the receiving benefit of observability is not as obvious as the beacon configuration optimization due to the limited control capability.Based on such researches, an accelerometer calibration method using attitude and angular rate information is firstly proposed. The output model of accelerometer is derived according to the relationship between the acceleration and the angular rate and angular acceleration. The optimal estimation algorithm is thus developed based on the measurements of attitude and angular rate. The accelerometer can be calibrated without the use of position measurements. Meanwhile, a nonlinear square-root filter is also given. The nonlinearity and disturbance of dynamic system are considered, and the equations of square-root filter are also derived based on the polynomial chaos. The proposed nonlinear filter can surpass the estimation capability of tradition filters.Finally, the prospect of navigation scheme for Mars entry is proposed which consists of the navigation schemes based on inertial measurement unit, radiometric measurements, and Mars navigation network. Meanwhile, a simulation system of autonomous navigation for Mars entry is developed using the MATLAB GUI. The simulation of navigation scheme based on radiometric measurements can thus be performed.