International distress frequency
An international distress frequency is a radio frequency that is designated for emergency communication by international agreement.
Use of some distress frequencies is permitted for calling other stations to establish contact, whereupon the stations move to another frequency. Such channels are known as distress, safety and calling frequencies.
Satellite processing from all 121.5 or 243 MHz locators has been discontinued. Since February 1, 2009, the U.S. Coast Guard only monitors distress signals from emergency position indicating radio beacons that broadcast using digital 406 MHz signals. Digital 406 MHz models became the only ones approved for use in both commercial and recreational watercraft worldwide on January 1, 2007. 
International distress frequencies, currently in use are :
- 500 kHz is still monitored but as of 2010 has ceased to be used as the primary distress calling frequency at sea
- 2182 kHz for medium range maritime voice use. But please note the US Coast Guard has said "beginning August 1st, 2013 the Coast Guard would no longer monitor 2182 kHz" (http://www.uscg.mil/d13/cfvs/Distress.asp)
- Several HF maritime voice frequencies exist for long-distance distress calls:
- 121.5 MHz is the civilian aircraft emergency frequency or International Air Distress frequency. It is used by civilian distress radiobeacons; however, the Cospas-Sarsat system no longer monitors the frequency.
- 243 MHz for NATO military aircraft emergency frequencies
- 406 MHz to 406.1 MHz is used by the Cospas-Sarsat international satellite-based search and rescue (SAR) distress alert detection and information distribution system
Digital Selective Calling frequencies
Several maritime frequencies are used for Digital Selective Calling (DSC), and they are also monitored for DSC distress signals:
- 2,187.5 kHz
- 4,207.5 kHz
- 6,312 kHz
- 8,414.5 kHz
- 12,577 kHz
- 16,804.5 kHz
- 156.525 MHz Marine VHF radio Channel 70
Amateur radio frequencies
- Emergency Centre of Activity (ECOA) frequencies informally established by the International Amateur Radio Union regional organizations (frequencies shown in kHz):
|Region 1||Region 2||Region 3|
|3760||3750 or 3985||3600|
|7110||7060, 7240 or 7290||7110|
- Emergency/Disaster Relief Interoperation Voice Channels of the amateur radio Global ALE High Frequency Network: 3791.0 kHz USB, 7185.5 kHz USB, 10145.5 kHz USB, 14346.0 kHz USB, 18117.5 kHz USB, 21432.5 kHz USB, 24932.0 kHz USB, 28312.5 kHz USB. 
- Bartlett, Tim (2006). VHF Handbook. Southampton: The Royal Yachting Association. p. 28, 31. ISBN 978-1-905104-03-1.
- "HF Distress and Safety Watchkeeping Schedule" (HTML). U.S. Coast Guard Navigation Center. U.S. Coast Guard. Retrieved Oct 12, 2011.