Calculation Of Fujian Network Clock Error Based On Noise Cross Correlation

The station time service includes both GPS timing and crystal punctuality,so the clock difference usually comes from the abnormal GPS timing signal reception and crystal timing error,and the manifestation is generally nonlinear jump and linear drift.The clock difference will adversely affect the quality monitoring of the observation data of the seismic network,especially the earthquake quick report and earthquake early warning based on the observation data of the network.In actual work,the influence of the clock difference needs to be eliminated.Therefore,it is of practical significance to evaluate and monitor the clock difference of the network.In this paper,based on the cross-correlation of background noise,the surface wave Green function is used to calculate the time delay,and then to study the clock difference of Fujian network.The purpose is to find a method suitable for monitoring the clock difference of Fujian network.The main work and conclusions are as follows:1.Introduce the research background and significance.Taking the M6.2 earthquake event in the southern Taiwan Strait on November 26,2018 as an example,the danger of clock difference to earthquake early warning is explained,and the clock difference is analyzed based on the arrival of the phase of the earthquake using the common method of judging the clock difference.Introduced the research status of clock difference calculation at home and abroad,and analyzed the common methods of clock difference determination and the limitations of previous work.In order to explore the method for calculating the clock difference of Fujian TV network,this research topic is proposed.2.Based on the background noise cross-correlation technology,this paper expounds the cross-correlation calculation and the theoretical support of using the background noise to study the clock difference,and selects the station pair for experimental calculation to illustrate the feasibility of the cross-correlation extraction delay.After obtaining the relative clock difference information using the time delay of the cross-correlation function,two methods are used to calculate the absolute clock difference.One is the method proposed by Sch?nfelder by solving the overdetermined equation system composed of the relative clock difference and the single absolute clock difference.That is,the equation method;the other is that Li Jun puts forward the center of the station to be sought,superimposes the cross-correlation function of all rays related to the station to be solved for time delay,and directly obtains the absolute clock difference of the station to be sought.Method,that is,the superposition method.3.Compare the calculation results of the equation method and the superposition method.Among them,in order to explore the calculation effect of the two methods under different clock feedback frequencies,this paper uses two cross-correlation methods,one half hour and one day.Combined with the travel time residuals of seismic events,the calculation results are compared: for the abnormal clock period,the superimposition method can control the relative error of the clock difference result to about 5%;for the normal clock period,the standard deviation of the superimposition method result is within 0.1s,which is less than the equation 0.2s of the law.It showsthat the superposition method has better accuracy and stability than the equation method at two different clock difference feedback frequencies.Therefore,the superimposition method can be used to realize the near real clock difference monitoring of the half-hour feedback frequency of Fujian Network.4.The two methods have different applicability.On the one hand,the superposition method requires that most stations in the network have no clock difference,so the superposition method is suitable for large networks such as the Fujian network,and the calculation effect is good,but for the network with a high proportion of clock difference stations,superposition The method is not feasible.It is illustrated by the extended application of clock difference calculation in small stations;and the calculation extended to OBS shows that the superposition method is not only applicable to land-based stations,but also to OBS clock difference estimation.On the other hand,the equation method has high requirements on the ray signal-to-noise ratio,that is,the ray length in the network is short,so it is suitable for calculating the clock difference of small networks.