Pilot signal
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In telecommunications, a pilot 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.
If no 19 kHz pilot tone is present, then any signals in the 38-39 kHz range are ignored by a stereo receiver. A guard band of ±4 kHz (15-23 kHz) protects the pilot tone from interference from the baseband audio signal (50 Hz-15 kHz) and from the lower sideband of the double sideband stereo information (23-53 kHz). The third harmonic of the pilot (19×3, or 57 kHz) is used for Radio Data System.
In AM stereo, the bandwidth is too narrow to accommodate subcarriers, so the modulation itself is changed, and the pilot tone is subsonic (below the normal hearing range, instead of above it).
In color television, the color burst placed between each video field is the pilot signal to indicate that there are color subcarriers present.
In the NTSC television system, a pilot tone of 15.734 kHz is used to indicate the presence of MTS stereo.
In some analog video formats (Frequency modulation is standard method for recording the luminance part of the signal, and is used to record a composite video signal in Direct colour systems), e.g. Video 2000 and some Hi-band formats a pilot tone is added to the signal to detect and correct timebase errors.
Note: Sometimes it is necessary to employ several independent pilot frequencies. Most radio relay systems use radio or continuity pilots of their own but transmit also the pilot frequencies belonging to the carrier frequency multiplex system.
Source (in part): Federal Standard 1037C and MIL-STD-188
