Improvements of IMGC absolute gravimeter G. D’Agostino, S. Desogus, G. Durando, A. Germak, C. Origlia IMGC, Istituto di Metrologia “G. Colonnetti”, strada delle Cacce 73, I-10135 Torino, Italy Abstract. International comparisons, periodically organized by the Bureau International des Poids et Mesures (BIPM) and in the future jointly with the Working Group on Gravimetry (WGG) of the Consultative Committee for Mass and Related Quantities (CCM) of the Comité International des Poids et Mesures (CIPM), show significant differences between absolute gravity measurements. Many sources of errors have been considered to evaluate each uncertainty budget, in spite of that, in some cases, the observed differences from an averaged value are statistically larger than expected by these evaluations; some causes of uncertainty cannot be well quantified because they are based on theoretical hypothesis. In this framework the Istituto di Metrologia “G. Colonnetti” (IMGC) is developing a second new gravimeter which will replace the old one, with the aim of decrease the uncertainty budget. It is derived from the original design, but it has been radically modified for what concerns the optical, mechanical and electronic system. The present paper underlines the improvements achieved with this second generation gravimeter. Keywords. Acceleration due to gravity, absolute gravity measurement, absolute gravimeter. 1 Introduction The principle adopted for the majority of absolute gravimeters is based on the reconstruction of the orbit of a test body thrown up and/or dropped in vacuum (F. Alasia et al. (1982), Feng Yong-Yuan et al. (1982), M. Zumberge (1982), T. M. Niebauer et al. (1995), J. E. Faller et al. (1988)). A corner- cube prism, that constitutes the moving mirror of an optical interferometer (Michelson interferometer and derivatives), is included on the test body. The optical parts composing the interferometer are one of the significant sources of error in these instruments (Hanada H. et al. (1988), Hanada H. et al. (1996)). Their influence is mainly random and normally produces increased dispersion in the results, but sometime it could be polarized. Although they are taken into consideration in the uncertainty evaluation as Type A factors (Guide to the Expression of Uncertainty in Measurement (1995)), reducing their effect can only benefit absolute gravity measurements in terms of repeatability. These factors, in some instruments, can have a systematic effect due to the specific design and operating condition. By evaluating a priori the errors introduced by the optical interferometer, it can be avoided to introduce an unknown bias into the measurement. In such cases, reduction of errors arising from the interferometer would be an important advantage. 2 Instrument improvements The first prototype of the IMGC absolute rise- and-fall gravimeter was designed and constructed in 1976 (F. Alasia et al. (1982)) in cooperation with the BIPM. A lot of changes have been adopted in subsequent versions (Germak A. et al. (2002)). This paper discusses the most recent changes and the consequent improvement for a new absolute gravimeter now under construction, and concerning: a) size and weight of the instrument, b) electronic and software set-up, measurement automation and times, c) front-end interface, d) monolithic corner-cube, e) apparatus and procedure for balancing the corner-cube, f) interferometer (Jamin).