Patent US 4,295,372

The output of the vertical accelerometer of a ships inertial navigation sem (SINS), in pulse form, is first counted and scaled to provide an output related directly to the total acceleration. The SINS computer provides the latitude, and north and east velocities of the ship, which are then converted into the theoretical gravity and the Coriolis error. These two factors are subtracted from the total scaled acceleration, and the resultant output is passed through a filter that corresponds to a particular weighting function (heavily weighted for the middle time) to remove the factor of ships heave acceleration. The final output is the free-air gravity anomaly in terms of real time.