Abstract: A gravimeter is disclosed that comprises a base, a reference device coupled to the base and configured to move along a first axis, a falling device configured to free fall from a first position to a second position on a second axis that is parallel to the first axis, a measurement module coupled to the reference device and configured to provide a first signal of the displacement of the reference device relative to the base and provide a second signal of the displacement of the falling device relative to the reference device. A processing unit accepts the first and second signals and computes a displacement of the falling device in inertial space by processing the first and second signals and subtracting the processed first signal from the processed second signal.

Abstract: A gravimeter is disclosed that comprises a base, a reference device coupled to the base and configured to move along a first axis, a falling device configured to free fall from a first position to a second position on a second axis that is parallel to the first axis, a measurement module coupled to the reference device and configured to provide a first signal of the displacement of the reference device relative to the base and provide a second signal of the displacement of the falling device relative to the reference device. A processing unit accepts the first and second signals and computes a displacement of the falling device in inertial space by processing the first and second signals and subtracting the processed first signal from the processed second signal.

Abstract: A closed loop sensor that utilizes a piezoelectric structure. In one embodiment, drive/non-control electrodes apply drive voltages to the piezoelectric structure and drive/control electrodes apply drive/control voltages to the piezoelectric structure to cause drive mode displacement of the piezoelectric structure and cancel motion induced pickup mode and quadrature displacements of the piezoelectric structure. In another embodiment, pickup/control electrodes detect a pickup signal from the piezoelectric structure corresponding to the motion induced pickup mode and quadrature displacements and apply a control signal to the piezoelectric structure so as to cancel these displacements. In still another embodiment of the invention, an optical sensing device optically senses the motion induced pickup mode and quadrature displacements and control electrodes apply a control signal to the piezoelectric structure so as to cancel them.

Abstract: Digital FM demodulator and method in which zero-crossings of a frequency modulated input signal are detected, a binary pulse is provided for each of the zero-crossings, and the binary pulse stream is filtered digitally to provide enhanced resolution of the incremental phase (frequency) of the input signal over a period which is asynchronous with the input signal. In the disclosed embodiments, the digital filtering is done with comb decimation filters, with a higher order filter being employed where greater resolution is desired.

Abstract: A method and apparatus are disclosed for closed-loop phase modulation, intensity demodulation, control, and estimation of the non-reciprocal phase shift induced in a rotation sensing interferometer with a reciprocal phase modulator near one end of a closed light path. A bias modulation signal and a feedback modulation signal are generated, and the two signals are summed to form a summed signal which is applied to the phase modulator. The step transition frequency of the bias modulation signal is chosen so as not to fall on even harmonics of the proper frequency f.sub.p =1/2.tau..sub.o. The feedback modulation signal can consist of any periodic amplitude-modulated square wave with spectral frequency components at harmonics of the bias modulation step transition frequency. The spectral power of the feedback modulation signal must be distributed across frequencies other than even harmonics of the proper frequency f.sub.p.