Accurate Gravimetry at the BIPM Watt Balance Site

Accurate gravity measurements were made to support the International Bureau of Weights and Measures (BIPM) watt balance (WB) project in association with the eighth International Comparison of Absolute Gravimeters (ICAG-2009) and the accompanying Relative Gravity Campaign (RGC-2009) at the BIPM in 2009. The goal of WB project is to determine the Planck constant h for the realization of a new definition of the kilogram. The accurate value of free fall acceleration g is crucial for the precise determination of h. During the formal ICAG-2009 and the RGC-2009, four absolute and six relative gravimeters took part in the WB gravity campaign. The results can therefore be converted to the international reference of the ICAG results, i.e., they are SI-traceable. The WB gravity network is a regular 3D grid over the site in the WB laboratory that serves to evaluate gravity acceleration at the test mass centre of the future WB setup. The local Earth tide parameters were determined by analyzing a 6 months record of a gPhone spring-type gravimeter. These parameters, together with the atmospheric and polar motion corrections, enable precise prediction of the instantaneous values of the acceleration of free fall required by the WB experiment with an accuracy reaching 5 mu Gal. In addition, repeated precise levelling has been carried out to monitor the stability of the WB pillar