Pilot signal
（重定向自Pilot tone）
![Spectrum of an FM broadcast signal. The pilot tone is the orange vertical line on the right of the spectrogram.](/Images/godic/202502/02/Frequency_Spectrum_FM-Radio4237.gif")
In telecommunications, a pilot signal is a signal, usually a single frequency, transmitted over a communications system for supervisory, control, equalization, continuity, synchronization, or reference purposes.
In FM stereo broadcasting, a pilot tone of 19 kHz indicates that there is stereophonic information at 38 kHz (19×2, the second harmonic of the pilot). The receiver doubles the frequency of the pilot tone and uses it as a phase reference to demodulate the stereo information.