Fully use the redundancy in TWSTFT and GNSS Time and Frequency Transfer

The major techniques used for the international UTC/TAI time and frequency transfers are the two independent space techniques: TWSTFT (two-way satellite time and frequency transfer) and GNSS (global navigation satellite system: GPS, GALILEO, GLONASS etc.). Comprised of multi- techniques, this system is highly redundant, of which not a single technique has dominant advantage with respect to the others. There are three kinds of redundancy: 1) TWSTFT and GNSS are redundant each other; 2) in GNSS, only GPS is used at present; 3) TWSTFT is measured as a network but used as single links. However the present strategy for UTC/TAI time and frequency transfer is single-technique-single-link. The rich redundancy is not used. How to process these redundancies is a challenge to accurate time and frequency transfer. About a third of the UTC time laboratories operate two or more techniques and they contribute four fifth of the total atomic clocks and all the primary frequency standards. It is easy to understand that if all the redundancy would be fully used for the UTC/TAI generation, significant gain will be achieved in the viewpoints of accuracy, precision and robustness. Earlier studies have proposed the TWSTFT network transfer and the combined TWSTFT and GNSS transfer. This paper discusses the theoretical and numerical problems of how fully use the whole redundancy, by gathering together the above two methods and this leads to a multi-techniques-network time and frequency transfer.