GRAVITY GRADIOMETRY. SOMETHING NEW IN INERTIAL NAVIGATION.

Gravity gradiometer development programs are now underway at Bell Aerospace, the Draper Laboratory and Hughes Aircraft. Bell Aerospace is developing a gravity gradiometer based on the Bell model VII accelerometer. Four low noise versions of the model VII are mounted on a rotating base. By demodulating the summed and differenced accelerometer outputs at twice the rotation frequency it is possible to obtain the gradient signals. The Draper Laboratory is exploring a spherical gravity gradiometer drawing on the technology of floated inertial components. It consists chiefly of a hollow beryllium sphere, with two radially opposite proof masses, suspended near neutral buoyancy in fluid contained by a spherical housing. Spatial changes in gravity act on the proof masses to cause torques on the float. An electrostatic suspension system controls the float in all six degrees of freedom and measures torques produced by gravity gradients. Hughes Aircraft is developing a rotating gradiometer consisting of two perpendicular arms with proof masses at each end. Gravity gradients produce torques about a torsional pivot connecting the arms. Rotating the gradiometer at half the natural frequency of the differential mode of the structure causes the gravity gradient torques to excite this mode at its natural frequency and provides a large mechanical amplification of the gradient signal. Gravity gradiometry promises many applications, both civilian and military. It will permit airborne gravity surveying for oil and minerals as well as geodetic work. For present and future fixed-base ballistic missiles, gradiometry should provide a cost-effective means of collecting large amounts of gravity data for formulation of launch-region gravity models, More important, however, will be the impact of gravity gradiometry on strategic systems employing long-range ballistic missiles launched from a moving base.