During the construction of immersed tube tunnels,the immersed tube is in a floating state after the installation of the measurement tower,and it is difficult to acquire the position of the installed positioning equipment on the measuring tower and the geometric relationship with the immersed tube due to its swaying state.A"back-calibration"measurement method is proposed,which includes the calibration of the measurement and control system and the correction of positioning accuracy.The calibration of the measurement and control system encompasses the initial calibration of the tube section and the measuring tower.The correction of positioning accuracy includes establishing the construction coordinate system,data processing,accuracy comparison,and correction.By installing a GNSS receiver on the top feature points of the tube to collect its coordinates in real-time and compare them with the real-time calculation results of the measurement tower's control system,an analysis of the root mean square error before and after back-calibration is conducted through engineering cases.The significant reduction in root mean square error after back-calibration proves the effectiveness of the back-calibration method in improving measurement accuracy,thereby ensuring the accuracy and reliability of the immersed tube installation process.
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