High Precision Differential Astrometry

While conventional imaging in VLBI provides information only about the relative position between different features of a given source, phase-referenced observations can provide precise positional information with respect to an external reference. The use of differential techniques relies on the fact that most of the instrumental and propagation effects are common for radio sources sufficiently close together in the sky. These effects cancel by subtracting the visibility phases of the reference source from that of the neighboring objects. Differential VLBI astrometry yields relative positions of radio sources with sub-milliarcsecond accuracy. These high precisions can be applied to solve various astrophysical problems. In this paper I will review the progress made recently in this field, both in technique and science.