Research On Frequency Steering Method Based On Cesium Fountain Clock

Precision time is widely used in many aspects of national defense modernization and national economic construction.Coordinated Universal Time(UTC)is an internationally recognized standard time.UTC(k)is the local physical realization of UTC,and a reference for the time service system.Its main function is to provide standard time and frequency signals.Atomic clock frequency steering is an important part in the generation and maintenance of UTC(k),an ideal reference atomic time is the basis for frequency steering.Compared with UTC,the atomic time should have the characteristics of accuracy,real-time,stability and reliability.The frequency steering algorithm is used to calculate the steering value,and the frequency steering device is used to steer the output signal of the atomic clock,so that the performance of the signal after steering is as consistent as possible with the reference atomic time,which ensures the generation of precision time and frequency signals.Therefore,the choice of atomic clock frequency control method is directly related to the performance of UTC(k)signal.The cesium fountain clock is the most accurate device for reproducing the time interval unit "second",almost no frequency drift,and has good long-term stability.It is usually used as a calibration reference for the International Atomic Time(TAI)and has excellent accuracy.Studying the method of applying the advantages of the cesium fountain clock to the frequency steering of the atomic clock will help to further improve the stability and accuracy of UTC(k).This thesis has done in-depth research in several aspects,mainly including: the frequency tight steering technology of hydrogen atomic clock taking the cesium fountain clock as reference,the frequency steering method based on multiple primary frequency standards,and the frequency correction method based on the optimal control theory and the combined atomic time,finally the research results were analyzed and verified by the measured data.The main research contents and innovations are summarized as follows:(1)The performance analysis and evaluation of a single atomic clock and different types of atomic clocks are the basis for the calculation of atomic time,which can help to take advantage of different atomic clocks.For the primary frequency standards,taking the cesium fountain clock as an example,the data collection and preprocessing methods are given,the typical characteristics of stability are analyzed,and the self-evaluation of accuracy method and the rate evaluation method relative to TAI are studied.For the time keeping atomic clocks,the performance monitoring method of the atomic clocks are studied in combination with its internal physical state parameters,the stability characteristics of hydrogen and cesium clocks were analyzed.Three “predictability” quantitative evaluation methods were studied,and the advantages and disadvantages of different methods are compared..(2)For most of the cesium fountain clocks that do not have 1PPS signal and standard frequency signal output,the frequency tight steering technology of hydrogen atomic clock taking the cesium fountain clock as a reference has been carried out,which achieve the purpose of real-time physical reproduction of the cesium fountain clock.Through the analysis of the influence of different steering frequency and steering strength on the performance of the signal after steering,the rules that should be followed in choosing the steering strength and steering frequency are given.A dynamic adaptive steering strength algorithm is proposed to improve the short-term stability of the signal after steering under the condition of meeting the maximum Time difference threshold from the reference cesium fountain clock.(3)The frequency steering method based on multiple primary frequency standards is studied,including the atomic time algorithm for the primary frequency standards,and the atomic time prediction algorithm based on different sliding window lengths when the primary frequency standards are interrupted simultaneously.Using the atomic time calculated by multiple primary frequency standards as a reference to steer the frequency of the hydrogen atomic clock.The results show that the signal after steer combines the advantages of good short-term stability of the hydrogen clock and the good long-term stability of the reference atomic time.Within 60 days,the time difference between the steered signal and the reference atomic time is kept within ± 0.2 ns.For atomic time prediction,properly increasing the length of the sliding window can reduce the influence of noise on the prediction model.Using the predicted atomic time with different window length as reference to control the frequency of hydrogen atomic clock,the results show that the difference between the signals after steered and the real atomic time mainly comes from the prediction error of the atomic time.(4)In order to further improve the stability and accuracy of the reference atomic time,a combined atomic time algorithm based on Vondrak-Cepek is proposed,which integrates the accuracy of the cesium fountain clock and the short-term stability advantages of the hydrogen atomic clock.The algorithm is based on the existing atomic time,through further combined processing of existing atomic time and high-performance atomic clock,which can achieve the purpose of improving the performance of the combined atomic time.Based on the measured data,first use the hydrogen atomic clock to calculate the atomic time,and secondly use the Vondrak-Cepek combined atomic time algorithm to combine the cesium fountain clock and the hydrogen clock atomic time.The result shows that the combined atomic time integrates the short-term stability of the hydrogen clock atomic time and the accuracy of the cesium fountain clock.The hourly stability is 3.25E-15,and the RMS difference from UTC is 1.2ns.(5)A frequency correction method based on optimal control theory and the combined atomic time is proposed.The optimal control theory is the linear quadratic Gaussian optimal control algorithm(LQG algorithm),which is used to calculate the frequency steering value of the atomic clock.Compared with the traditional least square estimation method,the LQG algorithm has higher reliability and can also improve the short-term stability of the steered signal.Firstly,based on the simulation data of different types of noise and atomic clock anomalies,the effectiveness of the LQG algorithm is verified.Secondly,based on the measured data,taking the combined atomic time as the reference,and using the LQG algorithm to steer the frequency of the hydrogen atomic clock,the results show that the short-term stability of the signal after steer is better than the reference atomic time,and the time difference from the reference atomic time is kept within ± 0.25 ns,and the 5-day stability of the steered signal reaches 1.95E-15,which is 45.8% higher than UTC(NTSC)5-day stability.