GNGTS 2017 - 36° Convegno Nazionale

26 GNGTS 2017 S essione 1.1 tions and anthropic disturbances. At present, it exhibits a very high duty cycle (>95%). Fig. 2 shows the record of two months. The spikes in the record correspond to earthquake events. The analysis of later data is in progress. Both laser cavities of G and Gingerino lie in the horizontal plane and measure rotation around the vertical axis. To full exploit rota- tional information, it is however necessary to reconstruct the full 3-dimensional angular velocity vector. Indeed, a single RLG cannot distinguish between a rotational speed (chan- ging Ω) around its axis and an angular tilt of the axis (changing  ϑ ). ROMY, a large tridi- mensional array of four triangular RLG built near Munich for seismic study thanks to an ERC grant, is presently in commissioning. It is expected to achieve a very high sensitivity in the bandwidth up to a few hundred seconds useful for seismic application. In setting up a RLG array is it necessary to carefully consider several requirements, for example it should be noted that a RLG needs to work properly of a bias rotation that is given by Earth. Then, not every RLG axis orientations are suitable; in particular the orientation near orthogonal to Earth axis are forbidden. Exploiting the expertise that we have gained by the operation of Gingerino and of the other RLG GP-2 that is operating in the INFN Pisa laboratories, we have now developing the project for the construction of a new RLG array (Di Virgilio et al. , 2017). This array will include a RLG oriented with its axis parallel to the Earth rotational axis, a second one lying on the ho- rizontal plane, a third one with the axis outside the meridian plane. A part the orientation, the RLG should have identical geometry. A fourth RLG could be added to increase by redundancy the accuracy. The target is to allow a very precise identification of the direction of the signals, using a new design much more cost effective, achieving a noise equivalent level beneath 10 –11 rad/s/√Hz up to 1 day of integration time and extending the observation to three dimensions. The very large bandwidth of the sensitivity, extended from acoustic frequencies down virtual- ly to the DC, can produce information on a lot of phenomenologies. The comparison of the data produced by the existing RLG is Europe (the G ring of Wettzell and the array ROMY, 4 RLG) will provide unique information of geophysics and geodesy. The array we are studying and proposing will give also important geodetic information. The first RLG, oriented parallel to the Ear- th rotational axis, will directly test the Earth rotation rate Ω T . In this configuration in (1) is ϑ =0 and the Sagnac frequency is at its ma- ximum value. Then, possible slight misalign- ment will affect the accuracy of Ω T only at the second order, and an accuracy better of 1 ppb can be achieve by controlling the orientation at 3·10 -5 rad. Such an accuracy on Ω T value results competitive with the present measure- ments of the length of the day (LOD), which is made through VLBI (Very Large Baseline Fig. 1 - Noise power spectrum of Gingerino. Fig. 2 - Record of two months of the Sagnac frequency of Gingerino, from 2017 May 4th, up to June 6th. The spikes in the record correspond to earthquake events. The biggest one is the Mw 3.8 Pizzoli (AQ) earthquake on June 9th.