The relativistic red shift with 2×10-17 uncertainty at NIST, Boulder, Colorado, USA

We have estimated the relativistic red shift correction due to gravity, necessary to reference to the geoid the measurements of the new cesium fountain frequency standard at the National Institute of Standards and Technology (NIST) in Boulder, Colorado, USA. The frequency correction due to the red shift is given by Δf/f=(W_p-W _o)/c², where c is the speed of light, W_p the gravity potential at the location of the cesium fountain and W, the gravity potential on the geoid. We have computed the geopotential number C=W_o-W_p in three ways: (1) Based on the global gravitational model EGM96. (2) Based on the regional, high-resolution geoid model G96SSS. (3) Based on the value provided in the National Geodetic Survey´s data sheet for the NIST reference marker. We have estimated the offsets between the reference surfaces associated with each of the above three values of C. After referencing the three C values to a geoid surface defined with respect to the current best estimate of an “ideal” mean-Earth ellipsoid, the three computations of C gave the following Δf/f results: (1) -1797.83×10^-16, (2) -1798.94×10^-16 and (3) -1798.91×10^-16. The minus sign implies that the cesium fountain runs faster in the laboratory in Boulder than a standard clock located on the geoid