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

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