APPLICATION OF HYDROGEN-MASER TECHNOLOGY TO THE SEARCH FOR GRAVITATIONAL RADIATION

Abstract : The Deep Space Network (DSN) is currently able to track distant spacecraft with a worldwide network of 70 m stations located near Goldstone, California; Tidbinbilla, Australia and Madrid, Spain. Hydrogen-maser timekeeping at all three sites makes possible a search for gravitational radiation at useful levels of sensitivity. We report on 10.5 days of data from the December 1988 opportunity with the Pioneer 10 spacecraft at a distance of 44.2 astronomical units (22052 s). A microwave link near 2.3 GHz was established with the spacecraft using one station of the DSN 011 the uplink and another on the downlink. The utilization of hydrogen masers at each station, plus the spacecraft transponder, effectively provided a one-arm interferometer for detecting gravitational radiation with a phase-coherent pathlength of 12.25 hr. Starting with integrated cycle count of the Doppler shift as raw data, we sequentially differenced the data at a sample interval of 60 s and thereby generated a Doppler frequency record. Next, we removed trends caused by the relative motions of the spacecraft and the DSN stations in inertial space. The resulting residual frequency record was dominated by refraction of the 2.5 GHz signal in the Earth's atmosphere and ionosphere. By applying a regression analysis to a stratified atmospheric model, we reduced these refraction effects by more than an order of magnitude. We show plots of the detrended Doppler residuals at the 60 second sample interval. The overall RMS residual is on the order of 2 mHz (10(-12) in fractioxlal frequency), but the major contributor to the noise at this level is the high-frequency component caused by the poor signal to noise ratio in the received signal from the distant Pioneer spacecraft. We illustrate this property by displaying the power spectral density of the reduced data. As expected, the high-frequency end of the spectrum is typical of thermal noise in the microwave receiver.