Principle and Accuracy Problems of Gravitational Redshift Experiments in Space

In this paper, the problems about the principle and accuracy in the experiments of space gravitational redshift are analyzed. Firstly, according to Einstein Equivalent Principle (inertial force is equivalent to gravity), the main part of Earth's gravity inside the spacecraft is cancelled out by inertial force, so that the microgravity inside the spacecraft is much smaller than the gravity at its orbit. For this reason, the gravitational potential of the space borne clock should be taken as an effective gravitational potential corresponding to the microgravity. However, in Ref.[4] (GP-A experiment) and Ref.[5] (Radio astron satellite program), the gravitational potential of the clock is simply taken as the gravitational potential at the orbit. Secondly, testing the relativistic redshift requires a comparison between theoretical prediction and experimental measurement. The accuracy of this test is determined by the larger error. Therefore, if improving just the experimental precision but not the accuracy of the Earth Gravitational Model, then the test accuracy cannot be improved.