Development of VLBI time transfer system toward 0.1 nsec accuracy using transportable ground terminal for calibration

To improve the accuracy of time transfer in very long baseline interferometry (VLBI) technique, it is planned to perform a short baseline interferometry (SBI) experiment instead of zero baseline interferometry (ZBI) which was demonstrated in Japan-US baseline in 1986. A transportable VLBI receiving system which consists of 1.2 m antenna and delay calibration system is under development. The source flux available in SBI is weaker than ZBI because extended radio sources are resolved with distance. However, an improved VLBI data recorder with recording speeds up to 256 Mbps provides us higher sensitivity to observe weak sources in spite of small aperture antenna. Using the above system, it is easier to apply any VLBI stations for time transfer. It is expected that the new system will have 0.1 nsec level accuracy in time transfer. To confirm time transfer accuracy, a closure test will be made using more than three stations