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.
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.