Statistics – Applications
Scientific paper
Nov 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011acasn..52..539q&link_type=abstract
Acta Astronomica Sinica, vol. 52, no.6 p. 539-541
Statistics
Applications
Astrometry: Vlbi, Instrumentation: Detectors, Methods: Observation, Analytical
Scientific paper
In the background of the Chinese Lunar Exploration Project and the Yinghuo Project, through theoretical analysis, algorithm study, software development, data simulation, real data processing and so on, the positioning reductions of the European lunar satellite Smart-1 and Mars Express (MEX) satellite, as well as the Chinese Chang'e-1 (CE-1) and Chang'e-2 (CE-2) satellites are accomplished by using VLBI and USB tracking data in this dissertation. The progress is made in various aspects including the development of theoretical model, the construction of observation equation, the analysis of the condition of normal equation, the selection and determination of the constraint, the analysis of data simulation, the detection of outliers in observations, the maintenance of the stability of the solution of parameters, the development of the practical software system, the processing of the real tracking data and so on. The details of the research progress in this dissertation are written as follows: (1) The algorithm is analyzed concerning the positioning reduction of the deep spacecraft based on VLBI tracking data. Through data simulation, it is analyzed for the effects of the bias in predicted orbit, the white noises and systematic errors in VLBI delays, and USB ranges on the positioning reduction of spacecraft. Results show that it is preferable to suppress the dispersion of positioning data points by applying the constraint of geocentric distance of spacecraft when there are only VLBI tracking data. The positioning solution is a biased estimate via observations of three VLBI stations. For the case of four tracking stations, the uncertainty of the constraint should be in accordance with the bias in the predicted orbit. White noises in delays and ranges mainly result in dispersion of the sequence of positioning data points. If there is the systematic error of observations, the systematic offset of the positioning results is caused, and there are trend jumps in the shape of asymptotic line in the sequence of positioning points. When VLBI stations changed from three to four or vice versa, trend jumps could sometimes exist in the sequence of positioning points. The analysis could be as a reference to the follow-on Chinese Lunar Exploration Project and Yinghuo Project in the positioning reduction of spacecraft.
(2) The tracking data of the MEX satellite by the Chinese VLBI Network (CVN) on 2007 May 30 are processed. The results show that using the delays in precision of nanoseconds in the satellite positioning reduction is more effective than the delay rates in precision of picoseconds per second, and the contribution of the delay rates to the positioning is very limited. If the delays and their rates are jointly used in the positioning reduction, the correction to the adopted velocity should also be solved simultaneously with the position parameters. Otherwise the error in the priori velocity would directly influence the positioning precision. In order to improve the positioning precision of Martian satellite, it is very necessary for CVN to actively practice differential VLBI, same beam VLBI and so on. Then the systematic errors and the noise level of observations are further reduced.
(3) Through positioning reduction, the trajectory monitoring of pivotal arcs of the CE-1 satellite is accomplished, including the arcs of maneuvers in the approaching stage, lunar capturing stage, circumlunar stage and the stage of controlled landing on the Moon. Especially, based on the tracking observations of radio ranges and VLBI delays of the CE-1 satellite during the controlled landing on the Moon on 2009 March 1, the landing trajectory, the epoch of the landing, and the coordinates of the landing point are determined by positioning reduction. The three-dimensional positioning uncertainty is about 0.55 km. The trace determination of the rover on the lunar surface is made as planned in the follow-on Chinese lunar exploration project. To apply the constraint of geocentric distance is shown to be helpful to improve the positioning precision. It is worthy of paying close attention to the applications of the same beam VLBI technique, the digital lunar topographic model, and the detector-board observations between the lander and rover to the position/trace determination of the rover. (4) The trajectory evolution of the CE-2 satellite is precisely monitored in a realtime mode during pivotal arcs such as the maneuvers in the lunar approaching stage, near the perilune, in the adjustment of the circumlunar orbit, and in the experiment of the circumlunar orbit shift. Via positioning reduction, the successful capture of the CE-2 satellite by the Moon is quickly and accurately identified and concluded, and the precise monitoring of the 15 km orbit above the lunar surface is successfully realized. These laid the methods and software infrastructure, and accumulated practical experience in engineering and technology for the smooth and successful implementation of tasks in the follow-on Chinese lunar exploration projects. As shown in the following aspects, the researches in this dissertation are urgently expected to be improved and supplemented. (1) Do some further simulations on the check of software and algorithm as well as some extra comparisons with associated software (Geodyn, OCCAM, Calc/Solve) in order to disclose and improve shortcomings and limitations of the current software system. (2) Develop software modules for the correction of various observational errors related to the transmission medium, quasi-clock bias and gravitational retardation, in particular, the correction of the quasi-clock uncertainty based on observations of extragalactic radio sources with known coordinates, for the enhancement of the systematicness and integrity of the current software. (3) Improve the function of the existing software on the reduction of observations of differential, phase referencing and same beam VLBI with simulation and real data tests. (4) Investigate the application of Kalman filter to the positioning reduction of deep space probes and develop related software systems.
In summary, the progress in this dissertation is made in the positioning reduction of deep space probes tracked by VLBI concerning the algorithm study, software development, real observation processing and so on, while a further study is still urgent and arduous.
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