Mars Spacecraft Precise Orbit Determination And Phobos Gravity Field Recovery

This dissertation systematically studies the theory,methods,and applications of precise orbit determination and positioning for Mars probes with the background of Chinese upcoming Mars exploration mission.I started the research from the precise orbit determination for Mars Express(MEX),the ESA's first Mars spacecraft,to study the strategy of the orbit determination.The high-precision reconstructed orbit was obtained after comprehensively processing the MEX Doppler tracking data.At the same time,I used the multi-arc two-way Doppler data during MEX flyby Phobos to calculate the gravity field model of Phobos for the first time in the world,and the accuracy of the low-degree gravity field model of Phobos has been improved by two to three.In addition to this,based on the lander positioning problem,a simulation has been did to quantitatively analyze the capability of positioning using the three different measurement modes.Finally,I also studied the relationship between Direct-To-Earth(DTE)configuration and the Mars orientation parameters(MOPs).The methods and accuracy of MOPs determination were discussed.The main research contents are summarized as follows:(1)The theory of precise orbit determination was studied.A set of Mars spacecraft precise orbit determination software was established through development based on LUGREAS.A strictly cross-validation test with GEODYN-II was performed.The results show that the software is reliable and accurate.It has good consistency with GEODYN-II.Subsequently,the MEX two-way Doppler data was processed from the two aspects of short-arc and long-arc determination.The orbit determination strategy and the Wheel-of-Loading(Wo L)events were studied in detail.The accuracy of MEX reconstruct orbits are in line with the accuracy range of the MEX orbit released by ESOC.(2)Although several close spacecraft flybys of Phobos have been performed over the past 40 years,the accuracy of the Phobos gravity field model is still missing.Based on this,I studied the theory of Phobos gravity field recovery.The low-degree coefficients of the gravity field of Phobos were derived from the radio tracking data of two combined MEX flybys,by applying a least squares regularized inverse technique,which introduces as an a priori the gravity field from a shape model based on constant density hypothesis.The result shows that the accuracy of Phobos GM,C₂₀,and C₂₂ is two to three times higher than that of existing models.The estimated C₂₀ value,in contrast to the value of C₂₀ computed from the shape model under the constant density assumption,supports an inhomogeneous density distribution inside Phobos at a confidence interval of 95%,indicating a denser mass in the equatorial region or lighter mass in polar areas.(3)Multi-detector positioning simultaneously are more and more important in Mars exploration.Three new multi-detector tracking modes were proposed and analyzed the positioning capability according to a numerical simulation.The results show that by combining two-way Doppler data with four-way Doppler or simplified four-way Doppler data for precise orbit determination,the accuracy of the reconstructed orbit can improve doubled when the original orbit accuracy is not too high.The lander positioning accuracy is stable at the decimeter level.The addition of same-beam VLBI measurements with a random noise of 10 ps does not significantly improve the orbit accuracy,but it can provide lander position accuracy with tens of meters.(4)Considering the future Mars landing mission,we performed numerical simulations about lander positioning and MOPs determination using DTE radio signals.The results show that it will significantly reduce the MOPs uncertainties performing DTE radio science on a Mars lander.Take the result of a lander located at 20°N as an example,the accuracy of the precession parameter can be improved by 5 to 10 times compared to the present,and the nutation parameter uncertainties will reach 10?30milliarcseconds(mas),and the Length-Of-Day(LOD)and Chandler wobble parameters accuracy can reach to 5?10 mas.This level of accuracy meets the need of studying Martian internal structure and the dynamics of its atmosphere.In addition,by analyzing the solution results of different latitude landers,the two-way range data can provide a good complement to the two-way Doppler data.Therefore,it is necessary to perform two-way range measurement for a high-latitude lander.