Precision Orbit Determination Of Lunar Probe With VLBI, USB And Space VLBI

As the closest natural planet to the earth, the moon was always the favored target in human deep space explorations. Nearly half number of deep space missions were the lunar explorations. The lunar probe was launched for lunar exploration, so the orbit determination of lunar probe played an important role in the mission. The accuracy of orbit determination was not only critical to the implementation of the mission, but also the premise of the analysis of scientific data for lunar exploration.The probe orbit determination accuracy related to the perturbation model, the observation model, the kind of tracking technology, etc. First of all, the perturbation model accuracy determined the orbit determination accuracy, and the non-spherical gravitation was the maximum perturbation for the probe on the lunar orbit, so the lunar gravity model accuracy affected the lunar orbit determination accuracy. Secondly, because the terrestrial tracking station and the lunar probe located in the different space-time reference frame, it was necessary to transformed them to a same reference coordinate system. The coordinate transformation usually relied on the coordinate transformation parameters, of which accuracy influenced the accuracy of transformation, the accuracy of observation model, and the orbit determination accuracy eventually. Finally, multi-kinds of technologies were usually used to track and measure the probe in lunar explorations. For example, the unified S-Band (USB) ranging/Doppler system and the Very Long Baseline Interferometry (VLBI) system as the ground tracking system jointly supported the lunar orbit capture in Chinese lunar exploration project, and the VLBI prominently improved the precision of orbit determination of Chang'e (CE) probes. As the extension of the ground VLBI, the Space VLBI (SVLBI) moved an antenna to the artificial satellite, which improved the measuring accuracy significantly with its extra-long baseline length. The observation to the extragalactic sources also riched the orbit information of probe which contributed to improve the orbit determination accuracy.Based on the above analysis and tracking data of Chinese Chang'e-2 (CE-2) probe, this paper carried out the work as follows:(1) We reviewed the development and present situation of lunar probe orbit determination and analyzed the influence of lunar gravity model and coordinate transformation paramters on the orbit determination. The importance of VLBI in lunar exploration was emphasized and the Space VLBI was introduced to lunar probe orbit determination.(2) The theoretical foundation of lunar probe orbit determination was introduced in detail. We also introduced the perturbation models of lunar probe in space and technologies for probe tracking and measuring. The ranging model, the Doppler measuring model and the VLBI observation model were listed. The space-time reference frame in orbit determination of lunar probe was described. The transformation between the different coordinate systems, the transformation between different time systems were also introduced.(3) Effects of different lunar gravity models on CE-2 lunar orbit determination performance were investigated and assessed using VLBI tracking data. The overlap orbit analysis and orbit prediction were used to evaluate the orbit precision, and the best lunar gravity model for orbit determination was conclusioned. The effects of different degrees and orders of spherical harmonics on the orbit determination were also analyzed, and the least degree and order for precision orbit determination of two types of CE-2 orbits were conclusioned.(4) The VLBI model for CE-2 observation which was suitable for estimating the probe initial states and Earth Rotation Parameters (ERP) simultaneously was brought up. The partial derivative with respect to ERP were derived. The orbit determination for transfer orbit and lunar orbit with CE-2 VLBI data was analyzed respectively. Results showed that the orbit determination precision was obviously improved. The estimate values of ERP were also compared to the precise values of ERP released by International Earth Rotation Service (IERS). Results showed that there were obvious improvement in ERP accuracy compared to ERP predicted values.(5) According to the reference orbit of CE-2 probe and the station of Chinese VLBI Network (CVN), the SVLBI delay and delay-rate data were simulated to research in the orbit determination of CE-2. Because of joint utilization of USB and Ground VLBI (GVLBI), the USB ranging and Doppler data were also simulated. The orbit determination results with only Space VLBI, USB and VLBI respectively were compared and analyzed. Combinations of USB and SVLBI, USB and GVLBI, SVLBI and GVLBI, USB, SVLBI and GVLBI, were used to determine the CE-2 lunar orbit respectively. It is conclusioned that using all three types of data can get the best orbit determination precision, and the addition of SVLBI data can beneficial to the CE-2 orbit determination precision and the ERP precision.(6) According to the situation that current usable software for lunar probe orbit determination was generally not related to the calculating the ERP and the orbit parameters simultaneously. A software with independent intellectual property right has been established. The software which successfully solves the ERP parameters and determine the orbit can be simulate and process the USB, Ground VLBI, Space VLBI and other types of tracking data for probe. Through a lot of numerical analysis and test, the reliability of the software has been verified.