From early in the early 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 However, as the use of Morse code over radio is now obsolete in commercial shipping, 500 kHz is obsolete as a Morse distress frequency. Beginning in the late 1990s, most nations ended monitoring of transmissions on 500 kHz and emergency traffic on 500 kHz has been replaced by the Global Maritime Distress Safety System (GMDSS). Current status The 500 kHz frequency has now been allocated to the maritime Navigational Data or NAVDAT broadcast system. The nearby frequencies of 518 kHz and 490 kHz are used for the NAVTEX component of GMDSS. Proposals to allocate frequencies at or near 500 kHz to amateur radio use resulted in the international allocation of 472–479 kHz to the 630-meter amateur radio band, now implemented in many countries. Initial adoption International standards for the use of 500 kHz first appeared in the first International Radiotelegraph Convention in Berlin, which was signed 3 November 1906, and became effective 1 July 1908. The second service regulation affixed to this Convention designated 500 kHz as one of the standard frequencies to be employed by shore stations, specifying that "Two wave lengths, one of 300 meters [1 MHz] and the other of 600 meters, are authorized for general public service. Every coastal station opened to such service shall use one or the other of these two wave lengths." These regulations also specified that ship stations normally used 1 MHz. Expanded policies International standards for the use of 500 kHz were expanded by the second International Radiotelegraph Convention, which was held in London after the sinking of the RMS Titanic. This meeting produced an agreement which was signed on 5 July 1912, and became effective 1 July 1913. The Service Regulations, affixed to the 1912 convention, established 500 kHz as the primary frequency for seagoing communication, and the standard