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

John D. Anderson, John W. Armstrong, Eurlice L. Lau

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 unita (22052 s). A microwave link near 2.3 GHz was established with the spacecraft using one station of the DSN on 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 br. Starting with integrated cycle count of the Doppler shift as raw data, we sequentially differenced the data at a sample interval of 60s 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 2.3 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 fractional 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. On the other hand, the low frequency end is dominated by refraction of the signal by interplanetary plasma. The lowest noise is achieved for a period range from 2008 to 2000s where the advantage of hydrogen-maser timekeeping is most apparent. In this region, the local mean amplitude of the noise is on the order of 10-14 in fractional frequency. This implies that any sinusoidal gravitational waves in the bandwidth are limited to a strain amplitude of 4 x 10-14.
Published in: Proceedings of the 21th Annual Precise Time and Time Interval Systems and Applications Meeting
November 28 - 30, 1989
Sheraton Hotel
Redondo Beach, California
Pages: 259 - 268
Cite this article: Anderson, John D., Armstrong, John W., Lau, Eurlice L., "APPLICATION OF HYDROGEN-MASER TECHNOLOGY TO THE SEARCH FOR GRAVITATIONAL RADIATION," Proceedings of the 21th Annual Precise Time and Time Interval Systems and Applications Meeting, Redondo Beach, California, November 1989, pp. 259-268.
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