VOYAGER-2 TEST OF THE RADAR TIME-DELAY EFFECT

By analyzing radio range measurements generated during the solar conjunction of the Voyager 2 spacecraft in 1985, we have been able to test accurately the relativistic time-delay effect with a spacecraft at a distance of 3 x 10(9) km from the Earth. Because the range measurements were generated simultaneously at two frequencies on the downlink, it was possible to derive calibrations for the delay resulting from intervening solar plasma. In addition, a record of attitude thruster firings was used to model small accelerations of the spacecraft that could have produced a range error. It was thereby possible to determine the parameterized post-Newtonian (PPN) parameter-gamma to an accuracy of 3% with only a sparse amount of range data. We discuss how this result, and the results of other time-delay tests, could be of interest for constraining a possible spatial anisotropy in the relativistic time-delay which can occur in some theories of gravity.