Precise Frequency Dissemination Using the 19-kHz Pilot Tone on Stereo FM Radio Stations

A continuous 19-kHz pilot tone is included as part of the modulation format of stereo FM broadcast radio stations. An experiment was performed which measures the stability of the received pilot in urban and rural environments. A mathematical analysis is presented of the phase stability of the received pilot as a function of multipath. It shows that phase changes will exist dependent upon a reflected radio signal´s phase lag, relative amplitude, and the modulation index. Data are presented which indicate: 1) phase versus time of the pilot at the National Bureau of Standards (NBS) laboratory using a directional yagi and a vertical whip; 2) Allan variance measurements of ¿versus ¿using a directional yagi; and 3) the day-to-day phase delay of the pilot at five urban locations and three rural locations using a vertical whip. With 5 min of averaging time, the delay of the pilot at five urban sites was reproducible to within 2 ¿s each day over a five-day period. Delay was reproducible to within 0.8 ¿s at three urban sites.