Tropospheric Delay Calibrations for VERA

We present techniques for the tropospheric delay calibration, which is the key to increasing the accuracy of the phase-referencing astrometry with Very Long Baseline Interferometry (VLBI). We study three methods, and make a comparison of these methods to discuss the accuracy in calibration. Our results show that all three methods can calibrate the tropospheric zenith delay within accuracy of $\sim$ 2cm. We also present simulations of positional errors in VLBI Exploration of Radio Astrometry (VERA) at the presence of an error in the tropospheric zenith delay, showing that parallax measurements with accuracy of 10 $\mu$ as can be readily achieved for sources at high declination and with small separation angles between the target Galactic maser and extragalactic position references.