Viking relativity experiment: Verification of signal retardation by solar gravity

Analysis of 14 months of data obtained from radio ranging to the Viking spacecraft verified, to an estimated accuracy of 0.1%, the prediction of the general theory of relativity that the round-trip times of light signals traveling between the Earth and Mars are increased by the direct effect of solar gravity. The correspondig value for the metric parameter ..gamma.. is 1.000 +- 0.002, where the quoted uncertainty, twice the formal standard deviation, allows for possible systematic errors.