Progress in Measurements of the Gravitational Bending of Radio Waves Using the VLBA

We have used the Very Long Baseline Array (VLBA) at 43, 23 and 15 GHz to measure the solar gravitational deflection of radio waves among four radio sources during an 18-day period in October 2005. Using phase-referenced radio interferometry to fit the measured phase delay to the propagation equation of the parameterized post-Newtonian (PPN) formalism, we have determined the deflection parameter gamma = 0.9998 +/- 0.0003$ (68% confidence level), in agreement with General Relativity. The results come mainly from 43 GHz observations where the refraction effects of the solar corona were negligible beyond 3 degrees from the sun. The purpose of this experiment is three-fold: to improve on the previous results in the gravitational bending experiments near the solar limb; to examine and evaluate the accuracy limits of terrestrial VLBI techniques; and to determine the prospects and outcomes of future experiments. Our conclusion is that a series of improved designed experiments with the VLBA could increase the presented accuracy by at least a factor of 4.