Synchronization of Clocks by Very-Long-Baseline Interferometry

Two hydrogen-maser clocks, one at the Haystack Observatory in Massachusetts and one at the National Radio Astronomy Observatory in West Virginia, were synchronized by means of very-long-baseline interferometry (VLBI) observations of several extragalactic radio sources on March 28, and again on September 23, 1977. Observations were made sequentially in eight 360-kHz bands distributed between about 8.4 and 8.5 GHz with spacings designed to enable the group-delay difference between the signals received at the two observatories from a given source to be estimated unambiguously, within an uncertainty of less than 1 ns set by receiver noise. The epoch and the rate differences between the observatories' clocks for each experiment were estimated by analysis of observations that spanned several hours. The application of corrections for the contributions to the delays of the antennas, feeds, receiver systems, and recorders, yielded absolute determinations of the clock epoch differences. During each experiment, portable cesium clocks were flown from the U.S. Naval Observatory in Washington, DC, to the observatories and back. The traveling-clock data, analyzed in each case after the VLBI synchronization had been completed, confirmed the VLBI results to within 18 and 14 us for the first and second experiments, respectively.