500 kHz

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From early in the 20th century, the radio frequency of 500 kilohertz (500 kHz) was an international calling and distress frequency for Morse code maritime communication. For much of its early history, this frequency was referred to by its equivalent wavelength, 600 meters, or, using the earlier frequency unit name, 500 kilocycles (per second) or 500 kc. Maritime authorities of many nations, including the Maritime and Coastguard Agency and the United States Coast Guard, once maintained 24 hour watches on this frequency, staffed by skilled radio operators. Many SOS calls and medical emergencies at sea were handled via this frequency. However, as the use of Morse code over radio is uncommon in commercial shipping, 500 kHz is no longer used as a Morse code distress frequency. Beginning in the late 1990s, most nations ended official monitoring of transmissions on 500 kHz and most emergency traffic on 500 kHz has been replaced by the Global Maritime Distress Safety System (GMDSS).