Abstract: A gravity survey and aircraft guidance system comprises a pendulum clock mounted in an aircraft in level flight, having a pendulum bob with a signal sender mounted thereon. A signal receiver mounted in proxmity to the pendulum bob generates an electrical pulse at each pendulum swing. The pendulum swings in a plane having a component parallel to the east-west direction of spin of the Earth. The pendulum has a nominal oscillation frequency at a first aircraft speed with respect to the Earth's surface. The pendulum will have a second oscillation frequency at a second aircraft speed with respect to the Earth's surface due to the change in the Earth's spin-induced centrifugal force on the pendulum. Alternately, the pendulum will have a second oscillation frequency when the aircraft flies over a mass anomaly in the Earth's surface. A frequency discriminator connected to the signal receiver, generates a correction signal in response to changes in the oscillation frequency of the pendulum.