Corresponding author: Hiroshi Takiguchi, e-mail: htaki@nict.go.jp VLBI MEASUREMENTS FOR FREQUENCY TRANSFER Hiroshi Takiguchi 1 , Yasuhiro Koyama 1 , Ryuichi Ichikawa 1 , Tadahiro Gotoh 1 , Atsutoshi Ishii 1,2,3 , Thomas Hobiger 1 , and Mizuhiko Hosokawa 1 1 National Institute of Information and Communications Technology, 893-1 Hirai, Kashima, Ibaraki, 314-8501, Japan 2 Geographical Survey Institute, 1 Kitasato, Tsukuba, Ibaraki, 305-0811, Japan 3 Advance Engineering Services Co., Ltd., 1-6-1 Takezono, Tsukuba, Ibaraki, 305-0032, Japan Abstract We compare the frequency transfer precision between VLBI and GPS carrier phase using IVS and IGS observation data in order to confirm the potential of VLBI time and frequency transfer. The results of the VLBI frequency transfer show that the stability follows a 1/tau law very closely. And that shows the stability has reached about 2 x 10 -11 at 1 sec. In this study, the results show that VLBI frequency transfer is more stable than GPS on the same baseline and same period. These results show that geodetic VLBI technique has the potential for precise frequency transfer. 1. Introduction Modern cold-atom-based frequency standards have already archived the uncertainty of 10 -15 at a few days. Moreover cold-atom-based optical clocks have the potential to realize the uncertainty on a 10 -16 to 10 -17 level after a few hours (Takamoto et al., 2005). On the other hand, time transfer precision of two-way satellite time and frequency transfer and GPS carrier phase experiments have reached the 10 -10 @1sec level (Ray and Senior, 2005 etc.). In order to compare such modern standards by these time transfer techniques, it is necessary to average over long periods. Since these techniques are not sufficient to compare next standards improvements of high precision time transfer techniques are strongly desired. Very Long Baseline Interferometry (VLBI) is one of the space geodetic techniques measures the arrival time delays between multiple stations utilizing radio signals from distant celestial radio sources. In the usual geodetic VLBI analysis, clock offsets and their rates of change at each station are estimated with respect to a selected reference station. The averaged formal error (1 sigma) of the clock offsets is typically about 20 picoseconds when analyzing geodetic VLBI experiments which are regularly conducted by the International VLBI Service for Geodesy and Astrometry (IVS). This precision is nearly one order better than other techniques like GPS or two-way satellite time transfer. It is feasible to use geodetic VLBI for comparison of primary frequency standards when radio telescopes are deployed at time and frequency laboratories. For this purpose, we have started to develop a compact and transportable VLBI system (Ishii et al.,