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Long-length fiber Bragg grating (FBG) distributed sensors are multiplexed, and measurement of Bragg wavelength distribution along the gratings are demonstrated. This sensing system adopts synthesis of optical coherence function (SOCF) as a reflectometry technique to measure the distribution of the local reflection spectra inside the long-length FBGs. Simulations have been carried out to determine appropriate FBG reflectivity for the multiplexing. In the proposed system, which is a heterodyne interferometer, heterodyne beat frequency for each FBG gets different from those for the others, when the measurement speed becomes higher. By introducing a way for compensating the difference, three long-length distributed FBG sensors, which are originally uniform and have the same Bragg wavelength, are successfully multiplexed with 1 kHz sampling rate.