Patent ID: 7181967

Claim:
A gravity gradient instrument, comprising: a first, second, third and fourth accelerometer equally spaced around the circumference of a circle, with the accelerometer sensitive axes tangential to the circle, and arranged in opposing pairs with the first accelerometer opposite the second accelerometer and the third accelerometer opposite the fourth accelerometer; in use the accelerometers are spun around an axis normal to the circle and passing through the circle's centre; a summing amplifier which receives outputs of the accelerometers and combines them in a manner to cancel common mode output signals, to produce an instrument output; and one or more feedback loops extending from the instrument output to one or more of the accelerometers to compensate for errors; wherein the feedback loop or loops include one or more additional inputs in addition to the instrument output, the additional inputs including signals representing one or more of: components of attitude, velocity and acceleration, the physical environment and flight conditions, taken alone or in combination.