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Absolute gravimeters are precise instruments for directly measuring the gravity acceleration. It can be used as a measurement standard for periodic calibration of the relative gravimeters. At present, the measurement accuracy of absolute gravimeter has reached the order of micro-gal (1 μGal = 1×10−8 m/s2), which is mainly limited by vibration noise. Vibration isolation and vibration correction technology are two commonly used methods to deal with vibration noise. Compared with vibration isolation, vibration correction has a simpler structure and better dynamic performance, which use a vibration sensor to detect the vibration noise. With increasing application demands of absolute gravimeters in field gravity measurements and marine/airborne gravity measurements, vibration correction technology is preferable to deal with vibration noise in noisy and complex measurement environment. In this paper, the development of vibration correction technology in absolute gravity measurement is reviewed in detail, focusing on two aspects of algorithm optimization and hardware improvement of the vibration correction system. As for algorithm optimization, the key is to determine the transfer function between the real movement and the sensor output. Most of the works improve the model to approach the real transfer function, including introducing more parameters to the model and adopting filtering technology. And other works focus on the performance of the vibration sensor, such as applying sensors with high sensitivity and high accuracy. Moreover, a type of vibration sensor with a built-in retroreflector which is specified for vibration correction system, is proposed by some groups. As for future applications in dynamic absolute gravity measurement, the dynamic performance of the vibration sensor is taken into account. Accelerometers with a more extensive measuring range and wider bandwidth are applied to replace seismometers in several dynamic measurement attempts at present. Therefore, achieving high accuracy in a large measurement range will be the main difficulty to overcome for dynamic applications. This paper finally looks forward to the future development trend of vibration correction technology. The trade-off between accuracy, dynamic performance and time cost will be a future problem. The good news is that domestic research on vibration correction technology developed early and rapidly. After decades of effort, the existing technique has reached the advanced international level, whether in optical laser interferometry or atomic interferometry. This no doubt provides powerful technical support for promoting domestic dynamic absolute gravity measurements in the future.

Gravitational acceleration metrology is the primary means to ensure the international mutual recognition and uniformity of gravity measurement activities, and guarantees the traceability and accuracy of gravity measurement results. Absolute gravimeters are taken as the main tool and carrier for the traceability and transmission of the gravity acceleration value. It can be used as a standard for periodic calibration of the relative gravimeters, as a "ruler" for land, ocean, aviation, and satellite gravity measurement. The National Institute of Metrology (China) has held the First International Comparison in Asia Pacific Regional and the 10th International Comparison of Absolute Gravimeters (ICAG-2017). Based on the former experiences, this paper introduces the preparation, organization and evaluation method of all international comparison measurements. Firstly, the site selection of comparison, observation of vertical gravity gradient, and tidal model before the comparison is introduced. During the comparison, the measurements for the diffraction beam correction and the arrangement of measurement sites were demonstrated. Then, the raw comparison measurements were transferred to the same heights and time, and performed based on the given observation model and constraint equation. After pre-processing, the evaluation of the instrument uncertainty was verified through a compliance test, the degree of instrument equivalence (DoE) and short-term reproducibility were calculated. By analyzing previous comparison results, this paper discusses the significance of international comparison in improving the accuracy of observation data in geophysical research and application, and puts forward some problems faced in the current stage.

The first Asia–Pacific Comparison of Absolute Gravimeters was held in National Institute of Metrology China from January 2016 to April 2016. Ten absolute gravimeters from Asian-Pacific region participated in the comparison. The comparison is linked to the 9th International Key Comparison of Absolute Gravimeters in 2013. Two data processing methods are present. The first one is to determine the comparison reference values only by the measurements of the two gravimeters from National Metrology Institutes and Designated Institutes. The second one is to obtain the comparison reference values by including the measurements of all the gravimeters which participated in the comparison. The results are presented in this paper, including the comparison reference values and the degrees of equivalence for the participating absolute gravimeters. The linking converters for the three comparisons were compared as well. Clearly, including more gravimeters that participated in the preceding ICAG would achieve better linking result and global equivalence. [ABSTRACT FROM AUTHOR]

The gravity is used to solve geodesy's primary tasks, such as determining geoid and defining the height and gravimetric reference networks of different scales, from national to global. Knowledge of gravity is of great importance for both metrology and geodetic metrology. In addition to the historical overview of absolute gravimeters, this paper presents the theoretical basis of the most commonly used method for determining the absolute value of gravity. The principle of operation of the absolute gravimeter FG5 and the importance of international comparison of absolute gravimeters are briefly presented. An overview of the gravimetric reference systems is given, emphasizing the establishment of the International Reference Gravimetric System. The previous works concerning the absolute determination of acceleration due to Earth's gravity field in Serbia are presented. Finally, the importance of determining the absolute value of gravity from the geodetic and metrological perspective is pointed out. Both national and international significance of determining absolute gravity in defining gravimetric reference systems and the importance of absolute gravity in monitoring global phenomena are emphasized.

We revisit a number of details that arise when doing joint AG-SG (absolute gravimeter-superconducting gravimeter) calibrations, focusing on the scale factor determination and the AG mean value that derives from the offset. When fitting SG data to AG data, the choice of which time span to use for the SG data can make a difference, as well as the inclusion of a trend that might be present in the fitting. The SG time delay has only a small effect. We review a number of options discussed recently in the literature on whether drops or sets provide the most accurate scale factor, and how to reject drops and sets to get the most consistent result. Two effects are clearly indicated by our tests, one being to smooth the raw SG 1 s (or similar sampling interval) data for times that coincide with AG drops, the other being a second pass in processing to reject residual outliers after the initial fit. Although drops can usefully provide smaller SG calibration errors compared to using set data, set values are more robust to data problems but one has to use the standard error to avoid large uncertainties. When combining scale factor determinations for the same SG at the same station, the expected gradual reduction of the error with each new experiment is consistent with the method of conflation. This is valid even when the SG data acquisition system is changed, or different AG’s are used. We also find a relationship between the AG mean values obtained from SG to AG fits with the traditional short-term AG (‘site’) measurements usually done with shorter datasets. This involves different zero levels and corrections in the AG versus SG processing. Without using the Micro-g FG5 software it is possible to use the SG-derived corrections for tides, barometric pressure, and polar motion to convert an AG-SG calibration experiment into a site measurement (and vice versa). Finally, we provide a simple method for AG users who do not have the FG5-software to find an internal FG5 parameter that allows us to convert AG values between different transfer heights when there is a change in gradient. [ABSTRACT FROM AUTHOR]

Over the past two decades, superconducting gravimeters (SGs) have been a key tool to investigate a number of geophysical processes leading to time-variable gravity changes. As SGs are relative meters, even though they are the most sensitive and stable devices currently available, they need to be accurately calibrated. Each branch of Earth sciences that benefits from high-precision gravity monitoring demands calibration of gravity sensors to accuracy of better than 0.1%. This research deals with a calibration experiment performed at the Strasbourg (France) SG site by means of two FG5 (#206 and #211) absolute gravimeters (AGs) and new-generation spring meters (Scintrex Ltd. Autograv CG-3M and CG5 and Microg-LaCoste gPhone). Our goal is to try to use the newest generation of spring mechanical gravimeters (MGs) for calibrating SGs. We discuss the results in terms of precision and accuracy of the SG calibration by means of different metrological and methodological approaches. With the FG5 #211 we derive scale factors for the SG-GWR C026 located in Strasbourg in agreement with those routinely obtained since 1997 by means of the FG5 #206. This confirms that the estimation of the scale factors is independent of the AG sensor. From a moving-window regression analysis between the synthetic body tides and both the SG and MG gravity records we detect significant fluctuations of the SG scale factors over time due to the instability of the instrumental sensitivity of the MGs. Our main results demonstrate that, owing to the time variability of their sensitivity, the used spring meters, even if well calibrated, cannot be used as a stable reference for SGs. As a result, MGs are not suitable to replace AGs for SG calibration, and we conclude that currently the method using parallel recording with absolute gravity meters is still the most feasible calibration approach for SGs. [ABSTRACT FROM AUTHOR]