Research On Laser Time Transfer And Payload Technology For Chinese Space Station

A large-scale,long-term manned space station will be built by China before 2022,named CSS(Chinese Space Station)with the orbit of around 350 to 450 km,and will conduct studies on various technical and scientific issues.Among them,based on the space station’s high-precision time-frequency experimental system,the world’s first space optical clock will be built,and the laser time transfer(LTT)technology is obtained to compare and evaluate the clocks between ground and space segments requiring time stability of less than 0.1 ps @ 300 s and 1 ps @ 1 day.In this paper,the research and design of laser time transfer system and the payload technology onboard CSS is studied and is applied to measure the time differences between space optical clock of CSS and ground clock.Because it serves the space optical clock and the stability requirement of time-frequency transmission system and payload is the highest in the world at present,the design of the LTT system is extremely challenging.The system needs to be specifically designed to meet the high time stability.Such problems include the varying detection delay along with the temperature changing,the time walk caused by varying photon number by satellite distance and atmospheric thickness,the contradiction between wide field of view and narrowband spectral filtering,which will affect the measurement precision and time stability or even fail the observation.In this paper,the design of the whole LTT system and the payload for CSS is performed and the research of key technologies are carried out to improve stability.The main research work of this paper is as following:1)According to the orbit characteristics of the CSS,the field of view,the number of photons received from the satellite and the earth are analyzed,the error and stability of the measuring system are decomposed and discussed.The high precision photon detection,the high frequency measurement and timing technology are proposed to design the LTT system,the requirement of the onboard payload and ground station are also put forward.2)Aiming at the detection delay drift caused by the change of ambient temperature of the system,the post-temperature drift compensation technology is researched.The detection delay is obtained,and the temperature drift compensation model is established.With this compensation method,a detection system delay of 240 ps over 12 ℃ was removed.The corresponding TDEV long-term stability was increased by a 0.5–1 order of magnitude.3)In response to the time walk problem of Single Photon Avalanche Diode(SPAD)caused by the variation of the detected photon number,the multi-beam detection technology based on fiber optic couplers is carried out,so that within a certain range of light intensity changing,the SPAD can always work in a single photon measurement state,to avoid the decrease in stability caused by changing in detection accuracy and time delay for fluctuations in laser intensity.The experiment shows to multi-photons the detection precision has reduced to 17 ps from 32 ps,and the time walk reaches less than 5 ps from 13 ps.4)To meet the orbit and high time stability requirement of the LTT system onboard CSS,a large field of view and uniform optical system design and research are carried out.The non-imaging system of polytetrafluorethylene(PTFE)scattering plate is innovative adopted,so that the payload could not only cover the field of view up to 128° but also perform spectral filtering to reduce the background noise from the earth.As the distance from the ground station to the CSS changes very fast,as well as the atmospheric thickness,the photons density can change 20 times,which will cause detection delay drift by the time walk effect of Single Photon Avalanche Diode(SPAD)detector.To reach a high stability,a sub-region attenuation plate is designed to control the photon capacity of the detector in the whole measurement arc down to a much lower range of 4 times.5)The LTT payload onboard CSS is also designed and carried out.The precision and the temperature drift of the detector is improved by the temperature-controlled high voltage regulator and avalanche voltage trimmer circuit.High precision time measurement unit and channel calibration are used to achieve high precision and low drift event timing.Through the design of small sized reflector array,the reflector ranging accuracy and error are improved.The final test of the payload shows that the precision of the LTT payload is better than 30 ps,and the time stability is about 0.09 ps @ 300 s and 0.6 ps @ 1 day,which meets the design requirements of the laser time transfer system of the space station.The above research is of great significance in reducing the measurement delay drift and improving the stability of the laser time transfer system.The research and design in this paper have been applied to the development of the LTT payload and to the ground system.At present,the performance of the LTT payload has met the requirement of the high-precision time-frequency experimental system of CSS,and reaches the international advanced level.