The Post-post-Newtonian Amendments of Radar Echo Delay Test in Reissner-Nordström Field

Starting from the differential equation of motion of photons on equatorial plane and in spherically symmetric field of gravitation and adopting the post-Newtonian approximate method, the post-post-Newtonian amendments of radar echo delay test in the Reissner-Nordström metric field are investigated. Via calculations the post-Newtionian and post-post-Newtonian amendments are obtained. When the electric charge Q is equal to zero, these results are applicable to electrically neutral celestial bodies, and the amendments of post-Newtonian parts are consistent with those provided by classical literature. The approximate expansion used in this paper can also be adopted to probe the amendments of still higher orders. The high-order amendments for radar echo have important significance to the ongoing high-precision spatial gravitation tests.