VLBI DIFFERENTIAL ASTROMETRY OF THE RADIO SOURCE-1928+738 AND SOURCE-2007+777 AT 5-GHZ

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 alpha = 37(m) 42(s).50343 +/-0(S).00009 Delta delta = 3 degrees 54' 41''.6778 +/- 0''.0002 The uncertainties quoted are each approximately one statistical standard deviation and correspond to about 0.3 milliarcseconds in right ascension and 0.2 milliarcseconds in declination. This result demonstrates the feasibility of using single-band observations for accurate phase-delay differential astrometry of sources separated by 5 degrees 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.