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 Altogether 24 institutes with 22 absolute gravimeters (one of the 22 AGs was ultimately withdrawn) and nine relative gravimeters participated in the ICAG/RGC campaign.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.

19 Lantmäteriet-the Swedish mapping, cadastral and land registration, Sweden

The second international comparison of absolute gravimeters was held in Walferdange, Grand Duchy of Luxembourg, in November 2007, in which twenty absolute gravimeters took part. A short description of the data processing and adjustments will be presented here and will be followed by the presentation of the results. Two different methods were applied to estimate the relative offsets between the gravimeters. We show that the results are equivalent as the uncertainties of both adjustments overlap. The absolute gravity meters agree with one another with a standard deviation of 2 μgal (1 gal = 1 cm/s 2 ). In 1999, a laboratory ( Fig. 5.1) dedicated to the comparison of absolute gravimeters was built within the WULG. The laboratory lies 100 m below the surface at a distance of 300 m from the entrance of the mine. The WULG is environmentally stable (i.e., constant temperature and humidity within the lab), and is extremely well isolated from anthropogenic noise. It has the power and space requirements to be able to accommodate up 16 instruments operating simultaneously. IntroductionMultiple absolute gravimeter comparisons are regularly carried out. Being absolute instruments, these gravimeters cannot really be calibrated. Only some of their components (such as the atomic clock and the laser) can be calibrated by comparison with known standards. The only way one currently has to verify their good working order is via a simultaneous comparison with other absolute gravimeters of the same and/or if possible even of a different model, to detect possible systematic errors.During a comparison, we cannot estimate how accurate the meters are: in fact, as we have no way to know the true value of g, we can only investigate the relative offsets between instruments. This means that all instruments can suffer from the same unknown and undetectable systematic error. However, differences larger than the uncertainty of the measurements, is usually indicative of a possible systematic error.For the second comparison in Walferdange, a few new procedures have been introduced. First, some of the participants accepted to take part in a

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