Analysis and filtering of the effect of tides on the hydrostatic levelling systems at CERN

Abstract

To meet alignment tolerances that are becoming tighter and tighter (±10 μm for the Compact Linear Collider (CLIC) project), the surveyors in the Survey Section at European Organization for Nuclear Research (CERN) must master the tilt effects exerted on their hydrostatic levelling system (HLS) networks. These effects are many and have varied consequences, although the majority of them tilt the ground and also the water surface present inside HLS (sensors in a homogeneous way. In order to model all inclinations together as a block, we have adjusted, at each time t, the line through the seven sensors in the Transfer Tunnel 1 (TT1) experiment. After removal of this signal, the residual amplitudes of the readings are less than the HLS alignment tolerances of the proposed CLIC main accelerator. In addition, the residual signals have lost their semidiurnal and diurnal periodic components, proving that any local effects in the TT1 facility cannot be detected with the accuracy of our systems. Further progress has to be made however, to master the effect of temperature on the HLS. The periods remaining in the residual HLS signal proves the presence of uncorrected thermal effects.