Wei, J.H. Baboi, N. Shi, L. JACoW Publishing Geneva, Switzerland 978-3-95450-201-1 10.18429/JACoW-IBIC2018-TUPB07 English 273-277 TUPB07 HOM cavity dipole electron higher-order-mode Contribution to a conference proceedings 2019 2019-01 https://doi.org/10.18429/JACoW-IBIC2018-TUPB07 http://jacow.org/ibic2018/papers/tupb07.pdf FLASH is a free-electron laser driven by a supercon-ducting linac at DESY in Hamburg. It generates high-brilliance XUV and soft X-ray pulses by SASE (Self Amplified Spontaneous Emission). Many accelerating cavities are equipped with HOMBPMs (Higher Order Mode based Beam Position Monitors) to align the beam and monitor the transverse beam position. However, these lose their position prediction ability over time. In this paper, we applied an efficient measurement and signal analysis with various data process methods including PLS (Partial Least Square) and SVD (Singular Value Decomposition) to determine the transverse beam position. By fitting the HOM signals with a genetic algorithm, we implemented a new HOMBPM calibration procedure and obtained reliable beam prediction positions over a long time. A stable RMS error of about 0.2 mm by using the spectra of signals and 0.15 mm by using the new method over two months has been observed.