Absolute Calibration of GPS Time Transfer Systems and Extension to Galileo

Nowadays, remote atomic clock comparison using the GNSS time transfer is the most widespread technique 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. With the arrival of Galileo, it is very important to extend the absolute calibration to Galileo receivers. This paper presents several methods to validate the absolute technique on one side and shows our first preliminary Galileo calibration results on the other side.