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.
KEYWORDS: Clocks, Field programmable gate arrays, Time metrology, Beam splitters, Error analysis, Signal detection, Rubidium, Software development, Metrology, Signal processing
In order to perform gravity measurement with compact and portable instrument at several ten mGal accuracy level, a digital fringe signal processing method was proposed for the measurement time interval in a ballistic free-fall absolute gravimeter. This method based on the theory of digital phase-shift which was used in the SOPC system on a FPGA DE2 Electric Board and NIOS-II processor produced by Altera company. This method has been successfully used for the measurement of interference fringe numbers and time interval in NIM-3 ballistic free-fall absolute gravimeter.
The mass of free-fall absolute gravimeter can produce vertical gravitational attraction to the free-falling test body during the measurement of acceleration due to gravity. The vertical gravitational attraction can cause an artificial deviation to the measured value of gravitational acceleration. This paper describes the operating principle of a free-fall absolute gravimeter and the method used to determine the reference height of a gravimeter. It also describes the physical structure of NIM-3A absolute gravimeter lately developed by National Institute of Metrology (China), and studies the correction of gravimeter for Self-attraction effect.
By raising measurement accuracy of absolute gravimeter, we need to find out the influence factor and conduct evaluation. After the 8th international key comparison of absolute gravimeter in 2009(ICAG-2009), people emphasize putting forward some correction key point, one of them is the diffraction correction. Due to divergence of laser beam, wave front is arc-shaped, laser beam of interferometer cannot go all the way with falling body’s drop direction. Because of this reason, the measurement result is less than original value. This is called diffraction effect. Here, this correction is called “Diffraction Correction”. For our absolute gravimeter NIM-3A, we research this effect and bring forward evaluation method and correction value. In this paper, we will conduct research and calculate. Consequently, we could receive the correction value of acceleration of gravity.
A free-fall absolute gravimeter was used to measure the gravity acceleration of a corner-cube released in high vacuum, and the gravity acceleration was determined by fitting the free-falling trajectories obtained through optical interferometry. During the measurement, the self-vibration of an absolute gravimeter caused ground vibration and the change in optical path length due to vibration of vacuum-air interface, which resulted in a measurement error. Numerical simulation was run by introducing vibration disturbance to the trajectories of free-fall. The effect of disturbance under different instrumental self-vibration conditions was analyzed. Simulation results indicated that the deviation of calculated gravity acceleration from the preset value and residuals amplitude after fitting depended on the amplitude and initial phase of the vibration disturbance. The deviation value and fitting residuals amplitude increased with the increasing of amplitude and there was a one-to-one correspondence between the two. The deviation of calculated gravity acceleration decreases by properly setting the initial phase difference of vibration disturbance with respect to the interference fringe signal.
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