Interferometric gravimeter apparatus and method

A gravity value is measured using two light beams which each reflect from both a freefall test mass and a stationary reference test mass which is inertially supported by a long period isolation device. The optical path lengths of the light beams change equally and oppositely in response to gravity and equally in response to disturbances, resulting in cancellation of the undesirable effects of the disturbances by common mode rejection and in a desirable increase in the number of gravity induced measurement fringes, when the two light beams are combined interferometrically.