The curved wavefronts of a Gaussian beam used in absolute gravimeters introduce a systematic error in the measured gravity value. Diffraction correction is defined to denote the correction of the error. This paper presents an evaluation of the diffraction correction for some absolute gravimeters participated in the 10th International Comparison of Absolute Gravimeters. An automatic M2-measurement instrument is introduced to achieve an efficient and accurate measurement of the beam waist diameter. On the basis, the diffraction corrections are calculated and estimated. The results indicate that the waist diameter of two laser beams deviates from the typical value. After optimized, the diffraction corrections of all the estimated AGs are less than 2.4 μGal. The accuracy of the correction is better than 0.1 μGal. This work is of great significance for improving the accuracy of the gravity measurements in the comparison.
The 10th International Comparison of Absolute Gravimeters (ICAG-2017) was held in Changping campus of National Institute of Metrology (NIM), China in October 2017. The observation of gravity variations using relative gravimeters plays an important role in absolute gravimeter comparison and the link of gravity reference value after comparison. We carried out a continuous observation of gPhone gravimeter-119 simultaneously alongside a superconductive gravimeter iGrav-012 for several months. The calibration factor of gPhone-119 is determined to be 0.99355±0.00004 with a precision of 0.004%. When the observation time exceeds 33000 minutes, the calibration values and uncertainties tend to be stable and the precision is better than 0.01%. The non-tidal gravity changes during ICAG-2017 recorded by gPhone-119 are analyzed. The tendency of gravity variations is roughly consistent with that recorded by iGrav-012. The result indicates that the peak-to-peak value of gravity changes is less than 1.5 μGal during the period of ICAG-2017.
CG-6 is a new generation of full-automatic relative gravimeter produced by Scintrex Company, Canada. It can be used to measure the vertical gravity gradient. This paper mainly addresses dynamic precision of CG-6 relative gravimeters in the vertical gravity gradient measurements during the Comparison of Absolute Gravimeters. This paper analyzes the repeatability and consistency of 4 CG-6 gravimeters in dynamic test. We process the static and dynamic experimental data of 4 CG-6 gravimeters. Results show that the dynamic precision of CG-6 gravimeters in vertical gravity gradient measurements is better than 3μGal1. The static drift rates are all less than 3μGal ∙ h-1. One of the CG-6 gravimeters has been used to monitor the NIM (National Institute of Metrology, China) local gravity network, especially in measuring the vertical gravity gradients. This allows for an evaluation of the overall dynamic performance of CG-6 gravimeters and their stability concerning highly precise determination of vertical gravity gradients for the micro-gravity network.
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