PROGRESS REPORT OF CNES ACTIVITIES REGARDING THE ABSOLUTE CALIBRATION METHOD

摘要

Global Navigation Satellite System (GNSS) time and frequency transfer is among the most useful tools for comparisons of remote clocks. It represents the basis of the time laboratories contributions for the realization of Temps Atomique International (TAI). The GNSS reception chains (antenna, antenna cable, geodetic receiver), used currently to perform the time comparisons, must be calibrated periodically to ensure their accuracy and their long-term stability. The most widely approach used to determine the electrical delay and the time stability of these chains is the differential method developed by the BIPM. Since 2005, CNES (French Space Agency) has developed the absolute calibration, using artificial signals, which opens the possibility to calibrate independently each element of the reception chain with a low uncertainty. The time-delay uncertainties of receiver and cable are respectively about of 0.40 ns and 0.050 ns for k=1. Recently, our efforts have mainly focused on the behavior of several receivers (Ashtech Z12-T, Septentrio PolaRx2, and Dicom GTR50) and a GNSS signal simulator (Spirent 4760) according to the temperature and the hygrometry variations. The first results of this study have been presented during the last EFTF meeting. At the present time, we have a good comprehension of the behavior of investigated receivers and the simulator according to the environment where they are located. Since 2009, the absolute calibration investigations have mainly concerned the last element of the reception chain, the GNSS antenna. Through this paper, we give a progress report of the CNES activities relating to the absolute calibration. First of all, it displays the variations of the internal electrical delay of some devices (several receivers and a simulator) according to the temperature, the hygrometry, and the aging. The antenna calibration procedure is then described.