Improvement of the atomic hydrogen maser for chinese compass system

Four hydrogen masers have being put into operation at the earth station of Chinese positioning system for more than six years. We have made many physical and electronics improvements 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 180 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. Now another 13 H masers were prepared for the Compass-2 system. Performance evaluation showed the frequency stability is in the order of E-15 for time interval from 100 s to 1day and the phase noise (SSB) is -150 dBc/100 Hz. The environmental sensitivities are also given in this paper.