SIMBAD references

On 18 November 1993, we observed the QSO 1150+812 and the BL Lac object 1803+784, nearly 15° apart on the sky, with a VLBI array, simultaneously recording at 8.4 and 2.3GHz. Using difference astrometry techniques, we determined the coordinates of 1803+784 relative to those of 1150+812 to be Δα=6^h7^m33.18469s±0.00020s Δδ=-2°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° on the sky, and open the avenue for the application of this technique on a full-sky scale.