Determination of the Scale Factor of LCR-PET1006 Gravimeter and Comparison with Brussels T03 Cryogenic Gravimeter.

A new series of registrations of the gravity tides has been achieved at the Royal Observatory of Belgium with the LaCoste & Romberg Portable Earth-Tides PET1006 which has been calibrated in two different ways. This instrument is equipped with a step motor mounted directly on the axis of the micrometer. It becomes possible to do very precise micrometric calibration to determine the ratio between the micrometer screw rotations and the signal output modulations. The gravimeter was installed on the calibration platform which induces well known sinusoidal inertial accelerations at different periods controlled by a quartz clock. In parallel with the signal produced by the gravimeter, we register a signal coming from a displacement sensor which records the motion of the platform. This sensor has a scale factor referenced to a LASER interferometer. For periods in the range of 200 to 300 seconds, the phase lag between the gravimeter and the displacement sensor is quasi stable around 15 sec. It corresponds to the transfer function of the RC filter implemented in the Maximum Voltage Retroaction electronics. So a correction could be applied to the amplitude of the gravimeter signals, for compensating the frequency dependant attenuation. This is a key point for achieving a precise calibration with the inertial platform method. We shall also correct the scale factor of the micrometric screw of the PET 1006 gravimeter. With this new results, it becomes possible by recording simultaneously the tidal signal with different gravimeters, to derive tidal factors for the Brussels station and thus to determine the scale factor of the T03 cryogenic gravimeter.