Abstract
Two Way Satellite Time and Frequency Transfer (TWSTFT) enables us to perform time transfer precisely, for atmospheric and geometric delays between the earth station and the satellite are almost canceled because of the path reciprocity. However, the delays in the uplink path and the downlink path of the earth station cannot cancel due to the different signal paths. The delay difference makes the absolute value of the time difference unknown, while the fluctuation in the delay difference degrades the stability of the time transfer.
We have developed a delay-difference calibration system using a multi-channel TWSTFT modem. Because the signals for the calibration are processed with the received time-transfer signals from other stations, not only a short-term variation but also a secular change in the delay difference can be measured. It is found that the diurnal variation of the delay difference becomes a few hundred pico-second even if the low noise amplifier works in the temperature-controlled box. As for the time transfer result between two hydrogen masers, its stability was improved by the correction of the delay difference. We confirmed the calibration system was important to improve the precision of TWSTFT.
