DETERMINATION OF THE RELATIVISTIC RED SHIFT BY NIST IN BOULDER.

In a recently completed project, involving a NIST researcher, the relativistic red shift correction due to gravity was estimated with a relative uncertainty of 2 x [10.sup.-17]. An accurate estimate of the relativistic red shift is an important factor in the list of systematic frequency shifts that must be determined in order to evaluate the performance of NIST-F1, the NIST cesium-fountain frequency standard; in fact, the relativistic red shift is the largest frequency bias for this standard. Moreover, the importance of this shift will become even greater in the future as frequency standards of increasing accuracy are developed.

The researchers used three different methods to arrive at their estimate--(1) a 1998 global gravitational model produced by NASA and the National Imagery and Mapping Agency, (2) a 1999 regional, high-resolution geoid model, and (3) recent measurements made by the National Geodetic Survey of a reference marker on the NIST site. Through a critical analysis of these three methods, the researchers estimate that the frequency correction for NIST-F1 in its current location is -1805.4 X [10.sup.-16], with an estimated relative uncertainty of 0.2 X [10.sup.-16]. This is well below the NIST-Fi uncertainty of 1.5 X [10.sup.-15], so at this time the red-shift correction is not of particular concern. It is worth noting that when the division moves this standard to a different floor of the building, which should happen in about a year, the correction will have to be changed.