Calibration of radiometric data for relativity and solar plasma during a solar conjunction

The round trip propagation time of a radio signal passing close to the Sun is affected by the charged particle environment of the solar plasma and the increased path length associated with the curvature of space predictedby general relativity. It has been difficult to separate estimates of the individual contributions to the signal delay. In January 1997, the Near Earth Asteroid Rendezvous (NEAR) spacecraft passed through a deep solar conjunction. For a month before and after conjunction, the spacecraft was pointed at the Sun and attitude operations were suspended. Since the NEAR spacecraft is equipped with two way X-band Doppler and range and the spacecraft attitude was favorable, a high precision spacecraft ephemeris was obtained. Data from the solar conjunction period of the NEAR mission is used to solve explicitly for the total electron content of the solar plasma along the signal path by using the signal delay in the range data to calibrate the signal advance in the Doppler data. The result is a precise estimate of the general relativity delay, and of the total electron content along the signal path, which can be used to improve models of the plasma emanating from the Sun. The general relativity parameter y is determined to less than 0.5 percent, which is competitive with the most accurate current verifications of general relativity.