Time transfer and TAI

The American Global Positioning System (GPS) has served the principal needs of national timing laboratories for regular comparisons of remote atomic clocks for the last two decades. Single-channel GPS C/A-code common-view time transfer is, however, barely sufficient for comparison of today´s atomic clocks within a few days, and certainly not sufficient for comparison of clocks currently being designed. For this reason the timing community is engaged in the development of new approaches to time and frequency comparisons, including techniques based on multi-channel GPS and GLONASS C/A-code measurements, GLONASS P-code measurements, GPS carrier-phase measurements, temperature-stabilized antennas, standardization of receiver software, and two-way satellite time and frequency transfer through telecommunication satellites. This paper reviews the above-mentioned techniques and assesses their potential impact on TAI