NIST PERFORMS COMPARISON OF FREQUENCY STANDARDS.

A post-processed time scale, involving an ensemble of five hydrogen masers, has been developed by a NIST scientist in Boulder to serve as a reference for comparing primary frequency standards. During the last 2 years, this time scale has been used to evaluate the relative frequencies of NIST-7, an optically pumped cesium-beam standard, and NIST-F1, the cesium-fountain frequency standard. The comparisons indicate that the frequencies of NIST-7 and NIST-F1 have remained in agreement within their measured uncertainties over the last two years, thus adding confidence to the methods used for evaluating the uncertainties of the standards. Having completed this overlap of operation with good results, NIST-7 will now be taken out of operation.

It is particularly important to note that this scale allows comparison of frequency standards not operated at exactly the same time. The stability of this time scale, called AT1E, is less than [sigma]y([tau]) = 1 X [10.sup.-15] for averaging times from 1 d to 100 d, with a minimum near 3 X [10.sup.-16] at about 20 d. When combined with frequency comparison data obtained through various satellite-based methods, this time scale also provides a means for absolute comparisons of the frequencies of NIST primary frequency standards with those of other countries. Measurements show remarkably good agreement among the frequency standards of NIST, France, and Germany. Considering that these standards are of quite different designs, it is clear that the measurement of frequency at these levels is in very good shape. The more recent comparisons of NIST and German cesium-fountain frequency standards have been accomplished using a two-way time-transfer link between these two labs. This link supports comparisons at a relati ve uncertainty of less than I X [10.sup.-15], more than a factor of two better than other comparisons, which are made using th common-view GPS method. The German and NIST fountain standards were found to agree within their uncertainties.