1995A&A...293..613G

From 5 GHz VLBI observations of the quasar 1928+738 and the BL Lac object 2007+777 on 9 October 1985, we have determined their relative position in J2000.0 to be: {DELTA}α= 37min 42.50343s±0.00009s {DELTA}δ= 3deg 54' 41.6778"±0.0002". The uncertainties quoted are each approximately one statistical standard deviation and correspond to about 0.3milliarcseconds in right ascension and 0.2milliarcseconds in declination. This result demonstrates the feasibility of using single-band observations for accurate phase-delay differential astrometry of sources separated by 5deg on the sky. The plasma contribution to the phase delay, the potentially limiting source of error in single-band observations, has been estimated from ionospheric total electron content measurements obtained by satellite ground stations and from differences of phase-delay and group-delay observables.