Absolute calibration of GNSS time transfer systems at CNES

Nowadays, the GNSS time transfer is the most widespread technique for remote atomic clock comparison used for the International Atomic Time (TAI) calculation. In order to insure the TAI links accuracy and long-term stability, calibration of the GNSS reception chain is a must. Since 2005, French Space Agency (CNES) has developed an absolute calibration technique based on the Naval Research Laboratory (NRL) approach. The receiver is calibrated using GNSS signals generated by a satellite simulator while the antenna calibration is done using a specific anechoic chamber with Vector Network Analyzer (VNA) signals. The results of this technique have shown that accuracy around 1 nanosecond for the whole chain composed by a receiver, an antenna, and a cable can be achieved. The purpose of this paper is to present several methods to validate such low an accuracy.