JP 3-50 National Search and Rescue Manual Vol I - US Navy

other craft or radio stations:
1. A radiotelegraph alarm signal, a series of twelve, 4-second
dashes with 1-second spacing, transmitted on 500 kHz for
actuating automatic alarm devices on ships and coastal marine
radio stations not maintaining a continuous listening watch on
500 kHz.
2. A radiotelephone alarm signal, a warbling two-tone signal on
1300 Hz and 2200 Hz, alternating 4 times per second and
continued for 30 seconds to 1 minute. It is transmitted on
2182 kHz or 156.8 MHz (channel 16) in alert or emergency
situations to attract attention and actuate automatic alarm
devices.
3. An All Ships Selective Call signal, actuating the receiving
selectors on all ships that are equipped with Digital
Selective Calling (DSC), regardless of code number. It is a
continuous sequential transmission of 11 unique audio
frequencies. The total duration of an "All Ships Call" signal
should be at least 5 seconds.
4. A Navigational Warning signal, a one-tone signal at 2200 Hz,
interrupted so that the duration and space are 250
milliseconds each. The signal is transmitted by coastal
stations continuously for a period of 15 seconds before
navigational warnings on radiotelephone in the mediumfrequency
maritime bands. The signal attracts the attention
of a watch or actuates an automatic device to activate a
loudspeaker.
B. Distress signals indicate that a craft or person is threatened
by imminent danger and requires immediate assistance. A voice distress
signal is preceded by the word "Mayday". Units hearing the distress
signal should cease transmitting and listen for at least 3 minutes
before resuming communications. Appendix C lists distress signals.
C. Urgency signals indicate that a calling station
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