Use of two traveling GPS receivers for a relative calibration campaign among European laboratories

We report about a GPS receiver relative calibration campaign, which took place between five European National Metrology Institutes or Designated Institutes: LNE-SYRTE in Observatoire de Paris (Paris, France), where the reference receiver of the campaign was located, ROA (San Fernando, Spain), SP (Borås, Sweden), PTB (Braunschweig, Germany) and INRIM (Torino, Italy). We used as traveling equipment two main units, both connected to a single antenna, and we kept track of the offset between both traveling units in all the visited sites. An external validation of the resulting hardware delays is provided against the time scale differences derived from the UTC - UTC(k) data published by BIPM in its monthly Circular T. Thanks to a very good stability of the traveling equipment, we obtained expanded uncertainty estimates within 2.0 ns (k = 2) for the hardware delays.