For much of the 20th century, 500 kHz was the primary international distress frequency.
Its use has been phased out in favor of the Global Maritime Distress Safety System.
Such channels are known as distress, safety and calling frequencies.
Since February 1, 2009, the U.S. Coast Guard only monitors distress signals from emergency position indicating radio beacons (EPIRBs) that broadcast using digital 406 MHz signals.
[2] Digital 406 MHz models became the only ones approved for use in both commercial and recreational watercraft worldwide on January 1, 2007.