Calibration of GNSS stations for UTC

Abstract Global navigation satellite system (GNSS) time transfer has been used for four decades in the generation of universal time coordinated (UTC), starting with GPS in the 1980s. Calibration of GNSS receivers is then essential to ensure that time links used in UTC are accurate and remain stable with time. In this paper we study the different calibration schemes based on relative calibration of GNSS receivers that have been operated to fulfill these goals. We specially focus on the current calibration scheme in operation since 2014 and based on a collaboration between the BIPM and the Regional Metrology Organizations. We show that the achieved accuracy and long-term stability of calibration results, thus of UTC time links, are well in line with the stated uncertainties. We study an extended set of absolute calibration results and show that they complete and validate the BIPM scheme, and that relative calibration is still the essential tool to ensure accurate time links for UTC.