Research On Time Comparison Method Based On VLBI

Time is the physical quantity with the highest measurement accuracy and precision among all physical quantities.That is why many physical quantities are measured by time.It is believed that measurement science can be advanced by improving the measurement accuracy and precision of time.One method of time measurement is remote time comparison,used to measure the relative deviation between remote clocks.Remote time comparison is the basis of remote time synchronization,remote time transfer,and the international atomic time.VLBI is one of the most accurate techniques in astrometry and space geodesy.VLBI uses the time and frequency signal of the hydrogen atomic clock output as the reference signal to observe radio signals of the extragalactic radio sources(e.g.,quasars).We measured the time delay and delay change rate of radio signals arriving at two VLBI stations based on the principle of interferometry.Then the direction of radio sources and baseline vectors between stations can be measured accurately,and the clock difference between stations can be accurately estimated.With this level of accuracy in estimating the clock difference between stations,VLBI is likely to become a new and high-precision remote time comparison method different from the current remote time comparison methods such as two-way satellite time and frequency transfer and the GNSS common view.The purpose of developing time comparison method based on VLBI is to create a remote time comparison method with much higher accuracy and precision to promote the advancement of measurement science.This thesis systematically studies the VLBI time comparison method,major error sources and related characteristics.It also looks into the system delay calibration methods for the VLBI time comparison.The main contents and conclusions are as follows.(1)This thesis studies the basic principle of the VLBI time comparison method in astrometry and space geodesy,analyzes the influence of clock difference on the VLBI measurement results,explores the analytical principles and implementation process of VLBI estimation clock difference,and analyzes the basic characteristics of VLBI time comparison.In order to obtain the instantaneous clock difference of atomic clocks between stations,a VLBI time comparison method without clock difference constraint was proposed,and a mathematical model was established.(2)This thesis studies the characteristics of VLBI time comparison error sources and how the error affects the results and analyzed the correlations among sources of errors.VLBI time comparison and GNSS common view time comparison share similar principle,error sources,and clock difference measurement.Both of them are indirect time comparison method.In this study,the processing models of tropospheric delay in VLBI and GNSS common view method are analyzed and compared.As the method of fixed zenith tropospheric delay in the GNSS common view method is a simple approximate method,the residual error of zenith troposphere can directly affect the results of common view time comparison.Astrometric and geodesic VLBI takes the zenith tropospheric delay as the parameter and measures it together with the clock difference parameter,avoiding the effect of zenith tropospheric delay on the baseline measurement results.However,according to the analysis of the VLBI observation equation,there is a certain correlation between the zenith tropospheric delay and the clock difference,which has a certain smoothing effect on the clock difference estimation.(3)Combined with the observation system of VLBI station,this study analyzed the composition and measurement of the VLBI measurement system delay.As the constant term system delay such as digital processing terminal delay is the main system delay of VLBI time comparison and cannot be directly measured,it is considered that we can use the relative system delay calibration methods of zero baseline Interferometry and short baseline Interferometry.In view of the time-varying characteristics of system delay in the mobile calibration station when moving,the thesis puts forward a method of system delay calibration for VLBI time comparison with double mobile calibration stations,and demonstrates its feasibility.(4)It is the first time to use Chinese VLBI network observation data to conduct many remote time comparison experiments.The precision of clock difference estimation between stations can reach 36 picoseconds,better than other remote time comparison techniques.By using satellite common view and VLBI colocation observation,the time comparison results of VLBI and satellite common view are compared.After analyzing the characteristics of the major error sources,the thesis found that the VLBI time comparison results have good consistency with the satellite common view time comparison results,but the comparison precision of VLBI time comparison is much better than satellite common view.