Aims: Relativistic bending in the vicinity of a massive body is characterized only by the post-Newtonian parameter gamma within the standard parameterized post-Newtonian formalism, which is unity in General Relativity. To estimate this parameter, we use very long baseline interferometry (VLBI) to measure the gravitational deflection of radio waves by Solar System bodies emitted by distant compact radio sources.
Methods: We analyze geodetic VLBI observations recorded since 1979. We compare estimates of gamma and errors obtained with various analysis schemes, including global estimations over several time spans and with various Sun elongation cut-off angles, and with analysis of radio source coordinate time series.
Results: We arrive at the conclusion that the relativistic parameter gamma cannot be estimated at better than 2×10^-4. The main factor of limitation is the uncertainty in determining of (global or session-wise) radio source coordinates. A sum of various instrumental and modeling errors and analysis strategy defects, which cannot be decorrelated and corrected yet, is at the origin of the limitating noise.