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For most of the 20th century, the radio frequency 500 kHz (known as 600 meters or 500 kc for most of the century, before kilohertz replaced kilocycle) was the international calling and distress frequency for ships on the high seas. All communication was by Morse code. Standard abbreviations and "Q codes" were used to overcome language barriers. Except for distress (SOS) traffic, stations shifted to nearby "working frequencies" (425, 454, 468, 480, and 512 kHz) to exchange messages once contact was established. All large ships at sea had to monitor 500 kHz at all times, either with a licensed radio operator or with equipment that detected an automatic alarm signal. Shore stations throughout the world operated on this frequency to exchange messages with ships and to issue warning about weather and the like. At night, transmission ranges of 3 000-4 000 miles (4 830-6 440 kilometers) were typical. Daytime ranges were much shorter, 300-500 miles (480-800 kilometers).
Detailed procedures for use of 500 kHz were specified by the 1932 Madrid Radio Conference, but its use dates back to Marconi's first transatlantic radio transmission. All stations on 500kHz were required to maintain a silent period for three minutes, twice every hour, starting at 15 and 45 minutes past the hour. This allowed weak stations in distress to be heard. Once a distress call was heard, 500 kHz was reserved for working the station in distress and all routine communications shifted to 512 kHz.
The 500 kHz calling and distress frequency was largely replaced by the Global Maritime Distress Safety System (GMDSS) in the late 1990s. The U.S. Coast Guard no longer monitors this frequency. China, the last official user, is expected to stop by 2006. The nearby frequencies of 518 kHz and 490 kHz are used for the Navtex component of GMDSS. There have been proposals to allocate frequencies at or near 500 kHz to amateur radio use.
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