Atomic hydrogen maser group for Chinese positioning system

Three hydrogen masers were put into operation at the earth station of Chinese positioning system four years ago. We have made many physical and electronics improvement on these hydrogen masers for improving the long-term performance. The hydrogen maser´s size is 110 cm(high)*54 cm(width)*73 cm(depth). It weighs 250 kg. By performance comparing with each other, we may choose the best one as a frequency reference for this system. A kind of cavity auto-tuning system was applied for improving of the long-term performance. Performance evaluation showed the frequency stability is in the order of E-15 for time interval from 80s to 1 day and the phase noise (SSB) is -150 dBc/100 Hz. The environmental sensitivities are also given in this paper.