The accuracy evaluation of NIST-7

We have performed evaluations of the major systematic errors in NIST-7 with an overall uncertainty of less than a part in 10/sup 14/. The complete evaluation process has been separated into two parts. With a computer-controlled, digital servo system and some new measurement techniques, we now perform core evaluations (second-order Zeeman and Doppler shifts, cavity pulling and phase shift, line overlap and some electronic shifts) with an overall uncertainty of less than one part in 10/sup 14/ in just a few days of measurements. The complete evaluation of all small and subtle effects in both the physics and electronics requires a few hundred days of data. But, these small effects are not variable at the 10/sup -14/ level and their infrequent evaluation does not detract from the operational accuracy of the standard.<
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