Testing General Relativity with Radar

Abstract

We describe a test of the influence of solar gravity on the echo times of radar waves reflected from planets. By deriving a theoretical expression solely in terms of measurable quantities, we demonstrate for a simple model of planetary motion that radar determinations of interplanetary echo times will allow the predictions of general relativity to be distinguished from those of Newtonian theory. The basic idea involves estimating the unknown parameters of the planetary orbits from time-delay measurements made when the target planet is near the earth and using the measurements made when the path of the radar wave passes near the sun to distinguish the effect of solar gravity on delay.