Transcontinental and Intercontinental Portable Clock Time Comparison

Because of the relatively low transportation costs of the rubidium portable clock, a schedule of relatively frequent comparisons with two of the main counterparts of NBS (Boulder, CO) have been executed: nearly monthly comparisons with the U. S. Naval Observatory (USNO) in Washington, DC, and trips every quarter year to the International Time Bureau (BIH) in Paris, France. The measurement results of the USNO and BIH comparisons are analyzed as a time series using a least squares quadratic fit. The measurements (time readings) yield a standard deviation of about 100 ns and indicate time scale drifts of about 1 × 10-13 per year. Comparison with Loran-C data demonstrates the superior time comparison ability of portable clock trips, exceeding that of Loran-C by up to one order of magnitude.