The relativistic redshift with 3×10−17 uncertainty at NIST, Boulder, Colorado, USA

We have estimated the relativistic redshift correction due to gravity, necessary to reference to the geoid the measurements of the new frequency standards at the National Institute of Standards and Technology (NIST) in Boulder, Colorado, USA, using a new local survey and various methods and models. We referenced the frequency offsets computed from different methods to the same geoid surface, one defined with respect to the current best estimate of an ideal mean-Earth ellipsoid. The new fractional frequency results are (1) −1797.61×10−16, based on the global gravitational model EGM96; (2a) −1798.72×10−16, based on the regional, high-resolution geoid model G96SSS; (2b) −1798.49×10−16, based on the regional, high-resolution geoid model G99SSS; and (3) −1798.91×10−16, based on the value for the geopotential number provided in the National Geodetic Survey's data sheet for the NIST reference marker. The minus sign implies that clocks run faster in the laboratory in Boulder than a standard clock located on the geoid. The values from (2b) and (3) are expected to be the most accurate and are also independent. Based on these results, we estimate the frequency shift at the reference point at NIST to be −1798.7×10−16, with an estimated standard uncertainty of ±0.3×10−16.