Dual beam VLBI techniques for precision astrometry of VERA project

The National Astronomical Observatory has just started the VERA project, VLBI Exploration of Radio Astrometry. The VERA aims at obtaining very high accurate position of galactic maser sources by referencing the position to nearby quasars. The maser position is to be measured with the accuracy in an order of 10 micro are-seconds that means the distance to the source beyond 10-kilo parsec could be measured by monitoring the annual parallax motion. The measuring range of the VERA is about 100 times greater than that of the HIPPARCOS. To achieve such high positional accuracy, the VERA first introduced a dual beam telescope. Simultaneous observations of a maser object and a quasar closely separated makes possible to remove a large phase fluctuation induced in the paths of each direction. The closer separation, however, decreases possibility of finding a bright quasar near the target. The insufficient strength of the quasar may increase thermal phase fluctuation. The optimal compromising is necessary to be studied. In this article the authors show a brief outline of the VERA observing system and present results of the error analysis in considering statistical property of the atmospheric phase fluctuation and discuss the time required to complete the measurements of 500 maser sources in our galaxy.