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Präzise Schweremessungen des BKG - eine nationale und internationale Referenz für das System Erde
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The increasing importance of terrestrial gravimetry in monitoring global change processes, in providing a reference for satellite measurements and in applications in metrology necessitates a stable reference system reflecting the measurement accuracy achievable by modern gravimeters. Therefore, over the last decade, the International Association of Geodesy (IAG) has developed a system to achieve accurate, homogeneous, long-term global recording of Earth’s gravity, while taking advantage of the potential of today’s absolute gravity measurements. The current status of the International Gravity Reference System and Frame is presented as worked out by the IAG Joint Working Group 2.1.1 “Establishment of a global absolute gravity reference system” during the period 2015–2019. Here, the system is defined by the instantaneous acceleration of free-fall, expressed in the International System of Units (SI) and a set of conventional corrections for the time-independent components of gravity effects. The frame as the systems realization includes a set of conventional temporal gravity corrections which represent a uniform set of minimum requirements. Measurements with absolute gravimeters, the traceability of which is ensured by comparisons and monitoring at reference stations, provide the basis of the frame. A global set of such stations providing absolute gravity values at the microgal level is the backbone of the frame. Core stations with at least one available space geodetic technique will provide a link to the terrestrial reference frame. Expanded facilities enabling instrumental verification as well as repeated regional and additional comparisons will complement key comparisons at the level of the International Committee for Weights and Measures (CIPM) and ensure a common reference and the traceability to the SI. To make the gravity reference system accessible to any user and to replace the previous IGSN71 network, an infrastructure based on absolute gravity observations needs to be built up. This requires the support of national agencies, which are encouraged to establish compatible first order gravity networks and to provide information about existing absolute gravity observations.
The regional key comparison of absolute gravimeters, EURAMET.M.G-K3 and the simultaneously organized additional comparison, was held in Germany at the Geodetic Observatory Wettzell of the German Federal Agency for Cartography and Geodesy in the spring of 2018.
Here we present the list of the participants who actually performed measurements during the comparison, the data submitted by the operators as well as the results of the determination of the gravity as a function of height at the comparison sites. The measurement strategy is briefly discussed and the results of the data harmonization is documented. Finally, the results of the constrained least squares adjustment are presented including the degrees of equivalence of each gravimeter and the key comparison reference values.
Main text
To reach the main text of this paper, click on Final Report . Note that this text is that which appears in Appendix B of the BIPM key comparison database kcdb.bipm.org/ .
The final report has been peer-reviewed and approved for publication by the CCM, according to the provisions of the CIPM Mutual Recognition Arrangement (CIPM MRA).
In continental plate interiors, ground surface movements are at the limit of the noise level and close to or below the accuracy of current geodetic techniques. Absolute gravity measurements are valuable to quantify slow vertical movements, as this instrument is drift free and, unlike GPS, independent of the terrestrial reference frame. Repeated absolute gravity (AG) measurements have been performed in Oostende (Belgian coastline) and at eight stations along a southwest-northeast profile across the Belgian Ardennes and the Roer Valley Graben (Germany), in order to estimate the tectonic deformation in the area. The AG measurements, repeated once or twice a year, can resolve elusive gravity changes with a precision better than 3.7 nm/s2/yr (95% confidence interval) after 11 years, even in difficult conditions. After 8-15 years (depending on the station), we find that the gravity rates of change lie in the [-3.1, 8.1] nm/s2/yr interval and result from a combination of anthropogenic, climatic, tectonic, and glacial isostatic adjustment (GIA) effects. After correcting for the GIA, the inferred gravity rates and consequently, the vertical land movements, reduce to zero within the uncertainty level at all stations except Jlich (because of man-induced subsidence) and Sohier (possibly, an artifact because of the shortness of the time series at that station).
Temporal gravimeter observations, used in geodesy and geophysics to study the Earth's gravity field variations, are influenced by local water storage changes (WSC). At the Geodetic Observatory Wettzell (Germany), WSC in the snow pack, top soil, unsaturated saprolite and fractured aquifer are all important terms of the local water budget. In this study, lysimeter measurements are used for the first time to estimate the hydrological influence on temporal gravimeter observations. Lysimeter data are used to estimate WSC at the field scale in combination with complementary observations and a hydrological 1-D model. From these estimated WSC, we calculate the hydrological gravity response. The results are compared to other methods used in the past to correct temporal gravity observations for the local hydrological influence. Lysimeter measurements significantly improve the independent estimation of WSC and thus provide a better way of reducing the local hydrological effect from gravimeter measurements. We find that the gravity residuals are caused to a larger extent by local WSC than previously stated. At sites where temporal gravity observations are used to study geophysical processes beyond local hydrology, the installation of a lysimeter is recommended.
We demonstrate a scheme for realizing a compact cold atom gravimeter. The use
of a hollow pyramidal configuration allows to achieve all functions: trapping,
interferometer and detection with a unique laser beam leading to a drastic
reduction in complexity and volume. In particular, we demonstrate a relative
sensitivity to acceleration of gravity (g) of 1.7 x 10-7 at one second, with a
moderate laser power of 50 mW. This simple geometry combined to such a high
sensitivity opens wide perspectives for practical applications (P. Bouyer and
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The National Institute of Metrology, China, successfully organized the 10th International Comparison of Absolute Gravimeters in its Changping Campus in 2017 (ICAG-2017). The ICAG-2017 is registered as the CCM.G-K2.2017 Key Comparison (KC) and Pilot Study (PS) in the frame of CIPM Mutual Recognition Arrangement of the International Committee for Weights and Measures (CIPM-MRA). KC instrument belongs to an institute of National Metrology Institutes (NMIs) or its Designated Institute (DIs). PS instrument belongs to an institute that is not listed as NMI or DI. The comparison allows the determination of the degree of equivalence of national standards for free-fall acceleration measurement and ensures the consistency of the participating results with their declared uncertainties. This is the first time that such a comparison has been organized outside of the European continent and establishes a new absolute gravimeter international comparison site in China. With the participation of 13 KC gravimeters, it is also the largest comparison up to now. In parallel, a pilot study was also conducted which allowed the participation of 17 PS gravimeters including several atomic gravimeters. For the KC part of ICAG-2017, the results have already been published on “Metrologia” (Volume 57,07,002). This paper presents the results of the PS part of ICAG-2017 in which 15 of the 17 gravimeters are in equivalence.
The importance of global satellite navigation systems for everyday life and for numerous surveying tasks is well known to every user. Wherever positioning or navigation tasks are involved, GNSS devices for the use of GPS, GLONASS, Galileo or Beidou are used, which provide coordinates in a reference frame of a global geodetic reference system. What is less known is the elementary activities of the Federal Agency for Cartography and Geodesy (BKG) in global geodesy. The activities are integrated into international cooperation projects, which are primarily coordinated by the International Association of Geodesy (IAG) and its components. - Published in: VDV-Magazin : Geodäsie und Geoinformatik (ISSN 1863-1320) (2020) No. 5, p. 378-388. Online version: https://www.bkg.bund.de/SharedDocs/Downloads/BKG/DE/Downloads-Wettzell/Artikel-2020.html
In a context of global change and increasing anthropic pressure on the environment, monitoring the Earth system and its evolution has become one of the key missions of geosciences. Geodesy is the geoscience that measures the geometric shape of the Earth, its orientation in space, and gravity field.Time-variable gravity, because of its high accuracy, can be used to build an enhanced picture and understanding of the changing Earth. Ground-based gravimetry can determine the change in gravity related to the Earth rotation fluctuation, to celestial-body and Earth attractions, to the mass in the direct vicinity of the instruments, and vertical displacement of the instrument itself on the ground.
In this paper, we review the geophysical questions that can be addressed byground gravimeters used to monitor time-variable gravity. This is done in relation to the instrumental characteristics, noise sources and good practices. We also discuss the next challenges to be met by ground gravimetry,the place that terrestrial gravimetry should hold in the Earth observation system, and perspectives and recommendations about the future of ground gravity instrumentation.
The A-10 field absolute gravimeter produced by the company Micro-g LaCoste Inc. was found to comply well with the producer specification of an uncertainty of 100 nm s(-2) for the determination of gravity acceleration. Repeated observational checks at a known reference station and careful calibration of instrumental standards demonstrated that the gravity measurements quality could further be enhanced. This opens new applications for precise gravimetry like the establishment of reference networks for monitoring global change processes where uncertainties of a few 10 nm s(-2) are of high value. The results and experiences from two extensive field campaigns using an A-10 gravimeter are presented.
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