Study On Several Important Technical Issues In Time-keeping

Time-keeping is the foundation and core work of standard time service. With the development of science and technology, time and frequency is playing an important role in many fields. The precision time has found widely applications in defense modernization and the national economy development. Many scientific research work need high precision time, as well as measurement and calibration field. Timing, modern communication, navigation and position, and computer auto-control all depend on precision time scale and accurate time synchronization. Standard time service system has been the important technique foundation. The National Time Service Center (NTSC), Chinese Academy of Sciences, is responsible for the establishing and maintaining and dissemination of national standard time signal. As time-keeping is the core content of time service, therefore, the study on the important technique issues in time-keeping is of great importance.There are three main parts in time-keeping system. Firstly, what kind of frequency standards should be used to create and maintain a stable time scale, nowadays, most of the time-keeping laboratories use commercial atomic frequency standards include Hydrogen maser, cesium atomic clock and rubidium atomic clock. Secondly, how a time scale is established and how to define the international standard time reference. At present, the local atomic time scale, TA(k), is established by different methods. The UTC system has been the international reference time scale for 30 years, however, there has been an argue in the international time society on if a new reference time system should be introduced or not, or, the UTC system will be used continuously.Local time comparing and long distance time synchronization is the third part in time-keeping. Local time comparing is to measure time error among clocks, the main technique used at present is time interval measurement and phase comparing measurement. Long distance time signal synchronization is used to link clocks and different time scales all over the world to the international atomic time scale TAI.Due to its very good short-term frequency stability, hydrogen maser has been given more and more attention in time service. However, the key problem is how to use the hydrogen clock and make use of its great potential. The property of hydrogen maser used in time-keeping and hydrogen combining cesium clock used in time-keeping are presented in this paper. The research has got valuable result. The research has got conclusion that while the hydrogen clock is used with cesium clock to calculate local time scale, the data of hydrogen clocks should been pre-processed if the hydrogen clock has big and regular long-term frequency drift. That is to predict the speed variation according to its last time TA(k)-HM line speed variation. If the pre-processed hydrogen clock data are used to calculate local time scale, the stability of the local atomic time scale will be improved.Additionally, if a hydrogen clock has very good short-term stability and regular long-term frequency drift, then it can be used as the master clock, such a real-time master clock system will be better than if cesium clock is used, and a stable and accurate UTC(k) will be realized by frequency estimating and controlling.Home-made hydrogen masers are measured and the result are analyzed in this paper. From the result data we can conclude that the performance of the two hydrogen maser made by Shanghai Astronomical Observatory has been improved greatly, the short-term frequency fluctuate is big than that of Sigma-tau maser made in USA, approximately equal to the fluctuate of a cesium clock. Therefore, we can say that the home-made hydrogen maser has potential in time-keeping, but its long-term stability should be improved continuously.The research of double mixed time different measurement system used in NTSC's time-keeping system is described in this paper. The practical measuring data are presented. Comparing result data of hydrogen masers and cesium clocks are also presented in detail.The solution of Chinese new time-keeping and atomic time scale system is introduced in this paper. The construction of NTSC's standard time system and real-time UTC(NTSC) physical master clock system and its application was presented. The calculation of TA(NTSC) and the result of stability of 2006 are presented. Finally, the realization of Chinese Jointed Atomic Time Scale (JATC) system was introduced.