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International time keeping relies inter alia on the comparison of time scales realized in about 70 institutes collaborating with the BIPM in the realization of Coordinated Universal Time. The operation of dedicated timing receivers for signals of GNSS (Global Navigation Satellite Systems) and making the data available publicly has become common practice. In order that the results of comparisons represent accurate time comparisons, the signal delay in the receiver installation (antenna, cable, receiver unit) need to be periodically calibrated. In compliance with Recommendation 2 of the Consultative Committee for Time and Frequency in 2009, which requested from RMOs the support of calibration activities of BIPM, ROA (Spain) initiated EURAMET project 1156, “GPS link calibrations in support of CCTF-K001.UTC“. The work includes conducting bilateral and multilateral campaigns of visits with one laboratory acting as the pilot. For contributing to this work, PTB built a mobile versatile GPS calibration station which is shown in Fig. 1 . When this became known, an unexpected request for a calibration arrived at PTB. In early 2011, the CERN Neutrinos to Gran Sasso project (CNGS) had announced a surprising result when measuring the time of flight of neutrinos generated at CERN (Centre Europeén de Recherche Nuclaire, Geneva) to the OPERA detector (Oscillation Project with Emulsion Tracking Apparatus)  in the Laboratori Nazionali del Gran Sasso (LNGS), Italy (see Fig. 2). The evaluation indicated that the propagation speed of the neutrinos v was (2.48 ± 0.28 (stat.) ±0.30 (syst.))x10-5 higher than the vacuum speed of light . Of course this result and its cause immediately exercised the science community.
The highly complex experiment includes the time tagging of the epoch of release of bunches of neutrinos at CERN with respect to a time reference point tCERN and the time tagging of the epoch of arrival at LNGS with respect to a time reference point tLNGS. This requires a highly accurate continuous comparison of the two time reference points, which are realized at both sites using commercial caesium atomic clocks and dedicated timing equipment. GPS time receivers had been installed for that purpose in 2008. Before doing so, they had got a relative calibration at METAS, Switzerland.
The CNGS team searched meticulously for a potential malfunction in as many instruments as conceivable in the complex experiment, and thus decided inter alia to ask for a repetition of the GPS link calibration. To this end, in August 2011 the mobile GPS calibration station was operated in sequence at the accelerator control facility in in Prevessin (near Geneva) and at LNGS, respectively, accompanied by Thorsten Feldmann of PTB, who had assembled the device and written its operating software as part of his PhD thesis. The calibration revealed a difference between the actual and the assumed differential receiver delays of only 2.3 ns, with a 1-σ uncertainty of about 1 ns. So the search for the cause of the about 60 ns early arrival of the neutrinos had to go one. In the meantime, two potential causes of error in the time tagging system at OPERA have been identified, and experiments are ongoing to verify that these explain the unexpected results of 2011 . In the meantime also other time of flight experiments confirmed that the neutrino propagation speed does not significantly deviate from c.
Fig. 1 PTB’s GPS calibration set-up
Fig. 2 The OPERA detector in the underground laboratory of LNGS  (Source: CERN 2010).
 T. Feldmann, A. Bauch, D. Piester, M. Rost, E. Goldberg, S. Mitchell, B. Fonville, 2010, “Advanced GPS-based Time Link Calibration with PTB´s new GPS Calibration Setup,” Proc. 42nd PTTI, November 15-18, 2010, Reston VA, USA, 509-526
 The Opera collaboration, R. Acquafredda et al., “The OPERA experiment in the CERN to Gran Sasso neutrino beam”, JINST 4 (2009) P04018.
 The Opera collaboration, T. Adam et al., “Measurement of the neutrino velocity with the Opera detector in the CNGS beam”, arxiv.org/abs/1109.4897
 Javier Serrano, CERN, private communication May 2012.