The hydrogen maser and cesium clocks in time keeping at NTSC

Hydrogen masers and cesium clocks are two important kinds of precision frequency standard, and each of them has its own advantages in time keeping. It is important to investigate the way for effectively combining these two kinds of cocks in order to form a better atomic time scale. There are four hydrogen masers and eighteen Hp5071A cesium clocks at NTSC (National Time Service Center, the Chinese Academy of Sciences). Two of the hydrogen masers are MHM-2010 which is manufactured by symmetricom, and the other two are made in Shanghai Astronomy Observatory (SHAO), the Chinese Academy of Sciences. It is well known that the advantage of a Hydrogen maser is concentrated on its short term stability; however its frequency could drift when it runs for long term. The advantage of a cesium clock is its long term stability; however its short term stability is worse than that of a hydrogen maser. In addition, clocks, even though are of the same kind, their performance could be very different because of their noise and other factors. In this paper, the performance of atomic clocks at NTSC is studied through the analysis of their noises. In atomic time scale algorithm, the short term stability and the long term stability of each clock are considered separately, which makes the frequency of time scale more stable for both long and short terms. For calculating the weight of a group of clocks the difference between Allan deviation and standard deviation is discussed in detail.