Scheme Design And Experimental Verification Of Inter-Satellite Clock Noise Transfer In Taiji Program

The Taiji program is a space gravitational wave detection mission proposed by the Chinese Academy of Sciences,which uses the method of laser differential interference to detect pm-level displacement fluctuations caused by gravitational waves between satellites,and the frequency band is between 1m Hz and 1Hz.Digital phasemeter by measuring the phase fluctuation caused by the distance change between spacecraft to invert the gravitational wave information.When a digital phasemeter is in the acquisition of data,ADC（Analog to Digital Converter）sampling jitter will introduce timing error,and ADC is driven by an Ultra-stable oscillator.The difference between the collected data caused by the difference between the two spacecraft Ultra-stable oscillators is called clock noise.In order to eliminate the measurement error caused by the desynchronization of the clock between satellites,the current scheme given by LISA（Laser Interferometer Space Antenna）is to use the sideband multi-frequency clock noise transmission method for inter-satellite clock noise measurement,and finally in the data post-processing with the help of TDI（Time Delay Interferometry）technology can eliminate clock noise.Therefore,the ground principle verification of inter-satellite clock noise transmission is an important basis for realizing clock synchronization between spacecraft.This article first analyzes the causes and properties of clock noise.In the case of random jitter of the sampling clock,according to the nature of random jitter,assuming that random jitter obeys the normal distribution with a mean of 0,the corresponding clock noise is Gaussian white noise,and a clock jitter model is established to analyze the influence of random jitter standard deviationσand signal frequency on the signal signal-to-noise ratio.The process of clock noise transfer is studied by simulating the clock transfer link and its component EOM（Electro-optic Modulator）.The simulation results show that after EOM phase modulation,the clock noise is concentrated near the frequency point of the sideband laser.The multi-frequency noise extracted from the sideband beat frequency of the interference signal is approximately proportional to the clock noise,and the scaling factor is equal to the multi-frequency coefficient,which can provide a basis for the principle of transmitting clock noise by the sideband laser.This paper discusses the requirements,principles,and methods of inter-satellite clock noise transmission of the Taiji program,and designs experiments for principle verification,the experiment in this paper mainly uses one phasemeter to evaluate the clock difference between the two signal generators.By building an electronics experiment to test the limit value of the clock noise of the two systems,determine the parameters such as modulation frequency,difference frequency,and modulation index,and further verify the principle of sideband frequency doubling transmission scheme in optical experiments.The experimental results show that for 200,000 data points,the local frequency jitter amplitude of the clock noise of the two sets of experiments is0.25m Hz,the overall frequency drift of the clock noise of the electronics experiment is between±0.2m Hz,and the overall frequency drift of the optical experiment is slightly larger than that of electronics,between±0.4m Hz,after the clock noise and multi-frequency noise are reduced in common mode,the frequency jitter amplitude of the common-mode noise is the same as the clock noise,which is also 0.25m Hz.However,the overall frequency drift of the common-mode noise in the optical experiment is±0.01m Hz and the corresponding phase offset is 0.3rad,which is better than the results of electronic experiments,indicating that the clock noise cancellation scheme and related parameters proposed in this paper are reasonable and feasible,which can provide a reference for the Taiji program.In the 0.05 Hz～1 Hz frequency band,the suppression effect of inter-satellite clock noise is better than 2π×10^-5rad/Hz^1/2,which meets the noise needs of the Taiji pathfinder and lays an experimental and theoretical foundation for the design of clock noise transmission in future Taiji program.