The 8th International Comparison of Absolute Gravimeters (ICAG2009) took place at the headquarters of the International Bureau of Weights and Measures (BIPM) from September to October 2009. It was the first ICAG organized as a key comparison in the framework of the CIPM Mutual Recognition Arrangement of the International Committee for Weights and Measures (CIPM MRA) (CIPM 1999). ICAG2009 was composed of a Key Comparison (KC) as defined by the CIPM MRA, organized by the Consultative Committee for Mass and Related Quantities (CCM) and designated as CCM.G-K1. Participating gravimeters and their operators came from national metrology institutes (NMIs) or their designated institutes (DIs) as defined by the CIPM MRA. A Pilot Study (PS) was run in parallel in order to include gravimeters and their operators from other institutes which, while not signatories of the CIPM MRA, nevertheless play important roles in international gravimetry measurements. The aim of the CIPM MRA is to have international acceptance of the measurement capabilities of the participating institutes in various fields of metrology. The results of CCM.G-K1 thus constitute an accurate and consistent gravity reference traceable to the SI (International System of Units), which can be used as the global basis for geodetic, geophysical and metrological observations of gravity. The measurements performed afterwards by the KC participants can be referred to the international metrological reference, i.e. they are SI-traceable. The ICAG2009 was complemented by a number of associated measurements: the Relative Gravity Campaign (RGC2009), high-precision levelling and an accurate gravity survey in support of the BIPM watt balance project. The major measurements took place at the BIPM between July and October 2009. Altogether 24 institutes with 22 absolute gravimeters (one of the 22 AGs was ultimately withdrawn) and nine relative gravimeters participated in the ICAG/RGC campaign. This paper is focused on the absolute gravity campaign. We review the history of the ICAGs and present the organization, data processing and the final results of the ICAG2009. After almost thirty years of hosting eight successive ICAGs, the CIPM decided to transfer the responsibility for piloting the future ICAGs to NMIs, although maintaining a supervisory role through its Consultative Committee for Mass and Related Quantities.

S U M M A R YWe have analysed the Ny-Ålesund very long baseline interferometry (VLBI) data over the period 1994 August to 2004 May, and we obtain secular displacement rates relative to a NNR-NUVEL-1A reference frame of 0.2 ± 0.5 mm yr −1 , −1.7 ± 0.5 mm yr −1 and 4.8 ± 1.1 mm yr −1 for the north, east and vertical directions, respectively. The corresponding global positioning system (GPS) station displacement rates relative to the same reference frame for the north, east, and vertical directions are 0.2 ± 0.6 mm yr −1 , −2.3 ± 0.6 mm yr −1 , and 6.4 ± 1.5 mm yr −1 at NYA1 and -−0.1 ± 0.5 mm yr −1 , −1.6 ± 0.5 mm yr −1 , and 6.9 ± 0.9 mm yr −1 at NALL, where these GPS rates were derived from the ITRF2000 velocity solution of Heflin. From the comparison at 25 globally distributed collocated sites, we found that the difference in uplift rate between VLBI and GPS at Ny-Ålesund is mainly due to a GPS reference frame scale rate error corresponding to 1.6 mm yr −1 in the GPS vertical rates. The uplift rate was estimated to be 5.2 ± 0.3 mm yr −1 from the analysis of the tide gauge data at Ny-Ålesund. Hence the uplift rates obtained from three different kinds of data are very consistent each other. The absolute gravity (AG) measurements at Ny-Ålesund, which were carried out four times (period: 1998-2002) by three different FG5 absolute gravimeters, lead to a decreasing secular rate of −2.5 ± 0.9 μGal yr −1 (1 μGal = 10 −8 m s −2 ). In this analysis, the actual data obtained from a superconducting gravimeter at Ny-Ålesund were used in the corrections for the gravity tide (including the ocean tide effect) and for the air pressure effect. We have estimated three geophysical contributions to examine the observed rates: (1) the effect of the sea-level (SL) change on a timescale of a few decades, (2) the effect of the present-day ice melting (PDIM) in Svalbard and (3) the sensitivity of the computed post-glacial rebound (PGR) effects to different choices of the models of past ice history and Earth's viscosity parameters. Our analysis indicates that the effect of SL change can be neglected as the main source of the discrepancy. On the other hand, the effect of PDIM cannot be ignored in explaining the mutual relation between the observed horizontal and vertical rates and the predicted ones. A large melting rate of the order of −75 cm yr 11 (i.e. roughly 1.6 times larger than the mean rate derived from glaciology over Svalbard) would explain the observed uplift but only half of the gravity changes. Our comparison results clearly point out the importance of both the estimation accuracy of the elastic deformations and better observation accuracy to constrain the size of PGR effects in the northwestern Svalbard more tightly.

The Sixth International Comparison of Absolute Gravimeters was held from 5 June to 28 August 2001 at the Bureau International des Poids et Mesures (BIPM), Sèvres. Seventeen absolute gravimeters were used to make measurements at five sites of the BIPM gravity network. The vertical gravity gradients at the sites and the ties between them were also measured using seventeen relative gravimeters. For the first time the ties were also measured using absolute gravimeters. Various methods of processing the absolute and relative data were tested to calculate the results. The final results of ICAG-2001 are presented. The acceleration due to gravity at a height of 0.90 m is given as (980 925 701.2 ± 5.5) µGal* and (980 928 018.8 ± 5.5) µGal for sites A and B, respectively,

In November 2013 an International Key Comparison, CCM.G-K2, was organized in the Underground Laboratory for Geodynamics in Walferdange. The comparison has assembled 25 participants coming from 19 countries and four different continents. The comparison was divided into two parts: the key comparison that included 10 NMIs or DIs, and the pilot study including all participants. The global result given by the pilot study confirms that all instruments are absolutely coherent to each other. The results obtained for the key comparison confirm a good agreement between the NMI instruments. 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 CCM, according to the provisions of the CIPM Mutual Recognition Arrangement (CIPM MRA).

Since the 1st International Comparison of Absolute Gravimeters (ICAG) and accompanying Relative Gravity Campaign (RGC) held at the BIPM in 1981, repeated ICAG-RGCs have been organized every four years. A total of 19 absolute gravimeters (AG) and 15 relative gravimeters (RG) participated in the 7th ICAG-RGC, which took place in 2005. Co-located absolute and relative gravity measurements as well as precision levelling measurements were carried out.The final version of the absolute g values of the 7th ICAG has been officially released recently. This paper is the final report of the 7th RGC and replaces the preliminary results published earlier. It covers the organization of the RGC and the data processing, analyses RG behaviour, computes g, δg and OAG (offset of AG) and discusses their uncertainties. In preparation for the BIPM key comparison ICAG-2009, a standard data-processing procedure has been developed and installed in the BIPM ICAG-RGC software package, GraviSoft. This was used for the final data processing.