Redundancy in the TAI TWSTFT time transfer network

TWSTF (TW) is a precise time transfer technique and has being used to generate the TAI since 1999. From the beginning with only 3 measured points per week over a few baselines to now 12-24 points per day and measured as a worldwide network, the TW observable becomes highly redundant. However single baseline is still the only geometry in TAI time transfer practice. As shown in Fig. I-1, among the 35 measured links carried out by 9 European-American TW laboratories, only the 8 links directly with PTB (double lines) are used for the TAI computation. The rest 27 links are in fact wasted. Generally, for a network of N points, there are N(N-1)/2 measured links. The actual European-American TW network is consisted of 12 TW laboratories. This situation that TW links are measured as a network but still used as single baselines is becoming more and more critical with the recent increase of the number of the TW laboratories, especially in Asia-Pacific areas. This is only the geometry redundancy. The TAI network is redundant also in multi-techniques.