Accuracy Analysis Of Orbit Determination For Chinese Lunar Exploration Mission

In January 2004, Chinese Lunar Exploration Mission 'Chang Er 1'started formally, which indicated the deep space exploration of China step into actual operation stage. This mission will be divided into 3 stages: round, fall and return. With the development of Chinese spaceflight business, accuracy requirement of orbit determination for the spacecraft will be more and more high. Currently, Chinese rocket can ensure to send the spacecraft with total weight of about 2.5 tons to 380,000 kilometers, but the success of the spacecraft's entering accurately Mission orbit depends on the precise determination of orbit and orbit prediction provided by the ground TT&C. system. After many arguments, Chinese Lunar Exploration Mission decides the task of tracking the spacecraft is mainly dependent on the joint tracking network of United S-Band(USB), and Very Long Baseline Interferometry (VLBI)as a supplement. This paper focus as on the Accuracy Analysis of orbit determination for the on-going Chinese lunar exploration mission 'Chang Er 1', using the joint tracking network of USB and VLBI with real network geographic distribution scenario and considering both systematic and stochastic measurement noise. The whole flight orbit of the " Charng Er 1' includes Phasing orbit, Translunar orbit and Mission orbit. Since different orbit has different orbit characteristic, this paper analyses the accuracy of orbit determination for Translunar orbit and Mission orbit separtely. We analyse various error sources that would affect the orbital determination accuracy of the lunar orbiter, moreover, the influences of observation accuracy and observation data types on accuracy of orbit determination have been investigated. Specially, the lunar gravity field which is the leading error source for the mission orbit of a lunar spacecraft is discussed, The reduced dynamics method is used to absorb orbital errors produced by the gravity error, in which a set of empirical acceleration parameters are estimated along with the initial state vector. The 3-step strategy of data simulation with errors, orbit determination and orbital comparison is adopted. The tool used is a space data analysis software package GEODYN developed at Goddard Space Flight Center, NASA. We conclude that adoption of USB ranging and Doppler data in combination with VLBI delay and delay-rate data will improve the accuracy of orbit.Increasing observation accuracy(delay)and reducing the error of the measuring ship will improve the accuracy of orbit determination during Translunar orbit phase,and the reduced dynamic method and improvement of lunar gravity accuracy will benefit the accuracy of orbit determination during Misson Orbit phase.