The Possibility of Checking the Equivalence Principle in a Null   Gravitational Redshift by a Two-Resonator Laser System
  A scheme of an optical detector is proposed for checking Einsteins equivalence principle (EEP) in a null gravitational redshift experiment and for testing methods for calculating the length of a resonator in a weak variable gravitational field by recording the variations of the difference frequency of resonators caused by lunisolar variations of the geopotential in a double or a two-resonator laser system. 
R. A. Daishev, Z. G. Murzakhanov, A. F. Skochilov