47 U.S. Code § 320 - Stations liable to interfere with distress signals; designation and regulation
The Commission is authorized to designate from time to time radio stations the communications or signals of which, in its opinion, are liable to interfere with the transmission or reception of distress signals of ships. Such stations are required to keep a licensed radio operator listening in on the frequencies designated for signals of distress and radio communications relating thereto during the entire period the transmitter of such station is in operation.
Source(June 19, 1934, ch. 652, title III, § 320,48 Stat. 1090.)