Mamoru Sekido, National Institute of Information and Communications Technology (NICT), Japan; Marco Pizzocaro, Istituto Nazionale di Ricerca Metrologica (INRIM), Italy; Kazuhiro Takefuji, NICT & Japan Aerospace Exploration Agency (JAXA), Japan; Hideki Ujihara, Hidekazu Hachisu, Nils Nemitz, Masanori Tsutsumi, NICT, Japan; Tetsuro Kondo, NICT, Japan and Chinese Academy of Sciences, Shanghai Astronomical Observatory, China; Eiji Kawai, Ryuichi Ichikawa, Kunitaka Namba, Yoshihiro Okamoto, Rumi Takahashi, Junichi Komuro, NICT, Japan; Cecilia Clivati, Istituto Nazionale di Ricerca Metrologica (INRIM), Italy; Filippo Bregolin, INRIM, Italy and Toptica Photonics AG, Germany; Piero Barbieri, Alberto Mura, Elena Cantoni, Giancarlo Cerretto, Filippo Levi, INRIM, Italy; Giuseppe Maccaferri, Mauro Roma, Claudio Bortolotti, Monia Negusini, Istituto Nazionale di Astrofisica (INAF), Istituto di Radioastronomia (IRA), Italy; Roberto Ricci, INRIM and IRA, Italy; Giampaolo Zacchiroli, Juri Roda, IRA, Italy; Julia Leute, Bureau International des Poids et Mesures (BIPM), and LNE-SYRTE, Observatoire de Paris, Université PSL, CNRS, Sorbonne Université, France; Gérard Petit, BIPM, France; Federico Perini, IRA, Italy; Davide Calonico, INRIM, Italy; and Tetsuya Ido, NICT, Japan

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For the aim to enable precise frequency transfer over earth radius scale distance, NICT developed a broadband VLBI system observing 3-14 GHz radio frequency range, and that was implemented in a pair of transportable 2.4m diameter VLBI stations and Kashima 34m diameter VLBI station. The small diameter VLBI stations installed in Medicina observatory of INAF (Italy) and NICT headquarters in Koganei (Japan) were used compare the atomic frequency standards. The poor sensitivity of VLBI observation with the small antenna pair was compensated by participation of the large diameter antenna for boosting the signal to noise ratio (SNR) of the VLBI observation. Owing to the broad observing frequency range, a few pico seconds precision of group delay is achieved on intercontinental baseline even with small diameter antenna. Advanced digital data acquisition with highspeed 16 GHz sampling enabled stable group delay measurement. A series of frequency transfer experiments was conducted during Oct. 2018 – Feb. 2019 by using the broadband VLBI system between ytterbium (Yb) optical lattice clock of INRiM (Italy) and strontium (Sr) lattice clock of NICT(Japan) separated by 9000km. The deviation of fractional frequency ratio between Yb and Sr was obtained by VLBI observation as y(Yb/Sr)=2.5(2.8)x10-16. The uncertainty in this experiment is evaluated not to be limited by the VLBI link itself, and further improvement is expected. Prospect of VLBI frequency link and uncertainty budget in VLBI observation is discussed in this report.