Patent ID: 7788974

Claim:
A system comprising: a gravity gradient sensor comprising a pair of transversely arranged masses for use in a gravity gradiometer for measuring components of a gravity gradient tensor; a calibration sensor for sensing at room temperature prior to cryogenic operation of the gravity gradiometer whether one of the pair of masses associated with the calibration sensor is balanced; an adjusting mechanism for adjusting the balance of the masses at room temperature prior to cryogenic operation of the gravity gradiometer; and wherein the calibration sensor comprises: a resonant circuit having an inductor and a capacitor which has a first plate formed by a surface of one of the masses and a second plate spaced from that surface of one of the masses; and an oscillator for receiving a signal from the resonant circuit and for producing an output signal indicative of the spacing between the first plate and the second plate whereby in use the calibration sensor produces a signal indicative of the balance of the one of the pair of masses associated with the calibration sensor.