The CCIR, CONSIDERING that although for conventional radiotelephone systems a level of -26 dB appears, theoretically, to be adequate, operational experience shows that significant improvements in operational time are secured with higher levels; that for radiotelephone systems employing a frequency-modulated control channel, further protection of the pilot carrier is necessary to ensure end-to-end circuit gain stability.
ITU-R F.454-1 SPANISH-1978 history
1978ITU-R F.454-1 FRENCH-1978 Pilot Carrier Level for HF Single-Sideband and Independent-Sideband Reduced-Carrier Systems - Section 3Aa - Technical Characteristics