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Pilot tone)
In telecommunication, 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 is used to indicate that there is stereophonic information on a subcarrier at 38 kHz (19×2, the second harmonic of the pilot). If no pilot tone is present, then the 38 kHz (more often 39 kHz) subcarrier is not stereophonic information, and is used for other purposes. A guard band of ±4kHz (15-23 kHz) is used to protect the pilot tone from interference from the baseband audio signal (50 Hz-15 kHz), and from the lower sideband of the stereo subcarrier (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.
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