Absolute calibration of GNSS time transfer systems: NRL and CNES techniques comparison

The GNSS reception chains (timing receiver + antenna + antenna cable) are used to perform time comparisons of remote atomic clocks. This time transfer system represents the basis of the time laboratories contributions for the realization of the Temps Atomique International (TAI). The calibration of these chains are necessary to ensure accuracy and long term stability of time links used in TAI but also in precise time station (PTS) dedicated to GALILEO. Currently, the most widely used approach to determine the electrical-delay of these devices is the differential method developed by the BIPM (Bureau des Poids et Mesures). Another solution is the absolute calibration of each reception chain element in using artificial signals. This method was first defined and put into operation by NRL (Naval Research Laboratory) and the University of Colorado. In 2005, CNES (Centre National d´Etudes Spatiales) decided to develop this technique with a similar approach. This paper will compare the results of NRL and CNES absolute calibrations performed on two different time transfer systems. First of all, it will describe the calibration method used by each laboratory and the differences which characterize them. An uncertainty budget will be presented for the results of both calibration techniques. Then, both time transfer systems, composed of different receivers and antennas models, will be described briefly. Finally, the absolute calibrations results performed using the NRL and the CNES methods will be compared.