Comparison of Caesium fountain clocks in Europe and Asia

A remote comparison campaign of Caesium fountain clocks from four National Metrology Institutes in Europe and Asia was carried out in May 2013. Six fountains at Physikalisch-Technische Bundesanstalt (PTB) Germany, Scientific Research Institute of Physical-Technical and Radio Technical Measurements (VNIIFTRI-SU) Russia, National Physical Laboratory of India (NPLI) India and National Institute of Metrology (NIM) China were compared by Two-Way Satellite Time and Frequency Transfer (TWSTFT) and GPS Carrier Phase (GPS CP) techniques. The frequency differences and the comparison uncertainties between the compared fountain pairs were evaluated. The comparison uncertainty (uc) results from combining the uncertainties of the two compared fountain clocks and the uncertainty introduced by the comparison link. All the frequency differences between the fountains agreed within the 1-sigma uncertainty in the low 10-15 level. We present in this paper the measurements and the evaluation of the campaign in details.