Since early in the 20th century, the radio frequency of 500 kilohertz (500 kHz) has been an international calling and distress frequency for Morse code maritime communication. The US Coast Guard and comparable agencies of other nations used to maintain 24 hour watches on this frequency, staffed by highly skilled radio operators. Many SOS calls and medical emergencies at sea were handled here until the late '80s. However, because of the near disappearance of the commercial use of Morse code, this frequency is now rarely used. In particular, emergency traffic on 500 kHz has been almost completely replaced by the Global Maritime Distress Safety System (GMDSS) — beginning in the late 1990s, most nations ended monitoring of transmissions on 500 kHz, and China, the last official user, is expecte