Precise frequency transfer experiments using VLBI and other techniques

We carried out inter-comparison experiments between VLBI, GPS, TWSTFT and DMTD in order to demonstrate that VLBI can measure the correct time difference between Koganei and Kashima, Japan (about 109 km) on August and October of 2010. The frequency stability of VLBI reached up to 5×10^-14 at an averaging time of several hundreds seconds and the value surpassed the frequency stability of GPS. In the experiments, we also evaluated the accuracy of each technique by inserting an artificial delay change into the path of the reference signal cable using a coaxial phase shifter. The results reveal that the artificial delay changes measured by VLBI and DMTD show good agreement (within 10 picoseconds). In general the VLBI results match the DMTD results better than GPS does. From these experiments, we confirmed the capability of the geodetic VLBI technique for time transfer application.