The mechanical component of the angular accelerometer. The sensitive axis is along the center, vertical line.
The mechanical component of the translational accelerometer. The sensitive axis is along the center, vertical line, and the accelerometer is symmetric for a 180˚ rotation about the sensitive axis.
The mechanical assembly of the three-axis cross-component SGG. The design gives a compact instrument that fits within a 26 cm diameter sphere.
A schematic of an SGG sensing circuit (circuit a shown, circuit b likewise) indicating the location of the inductive elements and the resistors used to store persistent currents I 1 and I 2. The SQUID sensor is indicated by a circle with x's representing the Josephson junctions.
A schematic of the mode-splitting circuitry used to substantially increase the stiffness of the accelerometer pair to common-mode excitations.
A schematic view of the liquid helium test cryostat. The actual dimensions of the cryostat are 254 cm high by 64 cm diameter.
The noise floors for the gradient output (lower) and the z-acceleration output after balance. The cancellation at the 0.03 125 Hz drive frequency and its harmonics (caused by stiction in the center air bearing) indicate a good common-mode rejection throughout the signal band.
The gradient output (noise limited) versus time. The standard deviation for the 1 h trace is 0.53 E.
The spectral density of the gradiometer noise floor. The increase in noise density as the frequency decreases is a combination of electronics noise and coupling to temperature gradients within the instrument.
A sketch of the gravitational gradient calibration arrangement using Pb bricks on a continuously rotating turntable.
Gravitational gradient calibration signal from rotating Pb bricks showing both the measured and calculated signals.
A schematic view of the refrigerated cryostat. Heterodyning from vibration at the many harmonics of the pulse frequency was a problem in this cryostat.
The gravitational gradient calibration signal in the pulse-tube cryostat. The positions are approximately the same as in Fig. 10.
Important design parameters of the cross-component SGG.
Important design parameters of the translational accelerometers.
Measured (calculated) mode frequencies for various, but matched, currents in the mode-splitting circuits.
SGG parameters and measured errors for three different mode-splitting currents.
Estimated error signals in the SGG on an airborne platform.
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