High Precision Astrometry over Large Angular Scales with Closure Constraints: the Triplet 1803+784/1928+738/2007+777

The technique of differential astrometry using the phase-delay VLBI observable promises fractional precisions of ≃2 × 10 –9 in the determination of the separation of sources 5° or 6° apart on the sky (Guirado et al. 1995a; Lara et al. 1996). In our present research we seek further improvement in this technique through using triplets of radio sources, which provide a closure constraint in the determination of relative angular positions. This constraint not only eases the resolution of the phase-cycle ambiguities (a major problem in the least-squares approach to astrometry with phase delays), but it also strongly constrains the space of allowable parameter values.