Towards global phase-delay VLBI astrometry: observations of QSO 1150+812 and BL 1803+784

On 18 November 1993, we observed the QSO 1150+812 and the BL Lac object 180 3+784, nearly 15 degrees apart on the sky, with a VLBI array, simultaneousl y recording at 8.4 and 2.3 GHz. Using difference astrometry techniques, we determined the coordinates of 1803+784 relative to those of 1150+812 to be Delta alpha = 6(h) 7(m) 33(s).18469 +/- 0.(s)00020 Delta delta = -2 degrees 30'25." 13557 +/- 0." 00075. These standard errors contain estimated contributions from the propagation medium, the effects of source opacity, and the possible misidentification o f the reference point in some of the maps. We obtained comparably accurate and consistent relative positions of the radio sources with GPS-based ionos phere corrections, thus demonstrating that dual-frequency observations are not required for state-of-the-art accuracy in VLBI astrometry. These result s also demonstrate the feasibility of using single-frequency measurements f or sources separated by 15 degrees on the sky, and open the avenue for the application of this technique on a full-sky scale.