  On 18 November 1993, we observed the QSO 1150+812 and the BL Lac object
1803+784, nearly 15\deg apart on the sky, with a VLBI array, simultaneously
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 = 6h 7m 33.18469 arcsec +- 0.00020 arcsec \Delta\delta = -2deg 30'
25''.13557 +- 0''.00075 These standard errors contain estimated contributions
from the propagation medium, the effects of source opacity, and the possible
misidentification of the reference point in some of the maps. We obtained
comparably accurate and consistent relative positions of the radio sources with
GPS-based ionosphere corrections, thus demonstrating that dual-frequency
observations are not required for state-of-the-art accuracy in VLBI astrometry.
These results also demonstrate the feasibility of using single-frequency
measurements for sources separated by 15$^\circ$ on the sky, and open the
avenue for the application of this technique on a full-sky scale.
