Hydrogen Maser Support of VLBI for the NASA Crustal Dynamics Project

Abstract

For the NASA Crustal Dynamics Project VLBI network, signals from quasars are recorded simultaneously at widely separated antennas. It is well known that hydrogen maser frequency standards provide the stable frequency reference used to precisely measure the difference in arrival time of the radio signals at the different antennas, enabling the determination of precise distances between the antennas. This paper reviews the practical requirements for maser support of VLBI for the Crustal Dynamics Project and describes the means used to meet these requirements for a network of eight fixed and three mobile stations which participate in approximately 200 VLBI experiments per year at locations in North America and the Pacific.The masers must provide a stable reference signal with frequency stability better than 1E-14 for tau 1 s of 100 to 1000 seconds. The masers used in the Project have a stability of approximately 2E-15 for these tau ! s.The extensive schedule of observations demands exceptional reliability of the network of hydrogen masers. Each maser must have a long mean time between failures and short down times for preventive maintenance and any component failures. The NR masers were developed to fulfill those needs, and BFEC has established an extensive system of communications, data acquisition and analysis, design, and logistics support to ensure this performance.The NR maser is equipped with a microprocessor which actively monitors 64 voltage, current, temperature and signal levels. Failures are often anticipated and prevented by thorough monitoring and analysis of important performance parameters over time. For data acquisition and analysis, BFEC has established a comprehensive network of communications with the masers. At the stations, data is retrieved by a small computer through a maser RS-232 port and then relayed to BFEC through electronic mail or direct phone lines. In the mobile VLBI data systems, a lap-top computer is used for data retrieval and then taken to a nearby telephone to access electronic mail.With data for each maser taken an average of twice a week, a large data base has been compiled over the past four years. With such an extensive history on eleven NR masers, many failures can be predicted 505 M.