Recent advances in VLBI astrometry

Abstract

We provide a summary of recent work we have performed to extend phase referencing to hitherto unreachable regimes. These include very high frequencies (mm and sub-mm) where the short coherence times have prevented source switching; low frequencies, where direction dependent effects prevent source phase transfer; and decomposing the atmosphere into the Ionospheric and Tropospheric components and then correcting for these piece-wise. Results from these three approaches, Source Frequency Phase Referencing, Multi-View Phase Referencing and Multi-Frequency Phase Referencing, are presented in the context of the contribution of the atmosphere, which is to be corrected.