In this paper we present the preliminary results of phase-referenced VLBA observations of two pulsars PSRB1937+21 and PSRB0329+54. Since pulsar observations usually require a lower observing frequency than that used for other astrometric applications, ionospheric delays are a significant source of systematic error. For the observations presented here we used total-electron-content (TEC) measurements derived from dual-frequency GPS observations (Wilson, Mannucci & Edwards, 1995) to calibrate ionospheric delays.
Using this calibration technique were able to obtain positions for these two pulsars which we believe to be accurate to approximately 2 mas in each coordinate. With this level of accuracy we expect astrometric pulsar observations to be able to address a variety of interesting issues ranging from reference frame alignment and fundamental astrometry to the origin of pulsar velocities.