A method of simulating a PLL circuit is provided, which makes it possible to accurately judge the frequency of the feedback clock signal to conduct simulation even if the frequency of the output clock signal is changed according to the necessity. This multi-peak reference grating is suitable for use in swept-wavelength interrogation systems, such as those utilizing Bragg grating sensors. The position measuring unit measures a position of the moving vehicle. The feedback signals from chips of different distances or heights are converted into different voltages and sent to a signal processing circuit.