View source for 500 kHz

{{Short description|Maritime radio frequency at 500 kHz}}
{{about|the maritime band|the amateur radio band|630-meter band}}
{{redirect|auto alarm|automobile alarm|car alarm}}
{{use dmy dates|date=November 2022}}
From early in the 20th&nbsp;century, the radio frequency of '''500&nbsp;kilohertz''' (500&nbsp;kHz) was an [[international distress frequency|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&nbsp;meters''', or, using the earlier frequency unit name, 500&nbsp;[[Cycle per second|kilocycles]] (per second) or 500&nbsp;kc.

Maritime authorities of many nations, including the [[Maritime and Coastguard Agency]] and the [[United States Coast Guard]], once maintained 24&nbsp;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 However, as the use of Morse code over radio is now obsolete in commercial shipping, 500&nbsp;kHz is obsolete as a Morse distress frequency. Beginning in the late 1990s, most nations ended monitoring of transmissions on 500&nbsp;kHz and emergency traffic on 500&nbsp;kHz has been replaced by the [[Global Maritime Distress Safety System]] (GMDSS).

== Current status ==
The 500&nbsp;kHz frequency has now been allocated to the maritime Navigational Data or ''[[NAVDAT]]'' broadcast system.

The nearby frequencies of 518&nbsp;kHz and 490&nbsp;kHz are used for the [[Navtex|NAVTEX]] component of [[Global Maritime Distress Safety System|GMDSS]]. Proposals to allocate frequencies at or near 500&nbsp;kHz to [[amateur radio]] use resulted in the international allocation of 472–479&nbsp;kHz to the [[630-meter band|630-meter amateur radio band]], now implemented in many countries.

== Initial adoption ==
International standards for the use of 500&nbsp;kHz first appeared in the first [[International Radiotelegraph Convention (1906)|International Radiotelegraph Convention]] in [[Berlin]], which was signed 3&nbsp;November 1906, and became effective 1&nbsp;July 1908.

The second service regulation affixed to this Convention designated 500&nbsp;kHz as one of the standard frequencies to be employed by shore stations, specifying that
: "Two wave lengths, one of 300&nbsp;meters [1&nbsp;MHz] and the other of 600&nbsp;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&nbsp;MHz.<ref name=BeauchampEngineers2001>{{cite book |author=Beauchamp, Ken |title=History of Telegraphy |date=January 2001 |publisher=Institution of Electrical Engineers |isbn=978-0-85296-792-8 |pages=256 ff |url=https://books.google.com/books?id=k3XCkncd83AC&pg=PA256}}</ref><ref name=Huurdeman2003>{{cite book |author=Huurdeman, Anton A. |date=31 July 2003 |title=The Worldwide History of Telecommunications |publisher=John Wiley & Sons|isbn=978-0-471-20505-0 |pages=358 ff |url=https://books.google.com/books?id=SnjGRDVIUL4C&pg=PA358}}</ref>

== Expanded policies ==
International standards for the use of 500&nbsp;kHz were expanded by the second [[International Radiotelegraph Convention (1912)|International Radiotelegraph Convention]], which was held in London after the [[sinking of the RMS Titanic|sinking of the RMS ''Titanic'']]. This meeting produced an agreement which was signed on 5&nbsp;July 1912, and became effective 1&nbsp;July 1913.

The Service Regulations, affixed to the 1912 convention, established 500&nbsp;kHz as the primary frequency for seagoing communication, and the standard ship frequency was changed from 1,000&nbsp;kHz to 500&nbsp;kHz, to match the coastal station standard. Communication was generally conducted in [[Morse code]], initially using [[spark-gap transmitter]]s. Most two-way radio contacts were to be initiated on this frequency, although once established, the participating stations could shift to another frequency to avoid the congestion on 500&nbsp;kHz. To facilitate communication between operators speaking different languages, standardized abbreviations were used, including a set of "[[Q code]]s" specified by the 1912 Service Regulations.

[[File:SS Jeremiah O'Brien LF radio.agr.jpg|thumb|500&nbsp;kHz transmitter and receiver position on [[SS Jeremiah O'Brien]], a World War&nbsp;II [[Liberty ship]]]]
Article&nbsp;XXI of the Service Regulations required that whenever an {{overline|{{sc|[[SOS]]}}}} distress call was heard, all transmissions unrelated to the emergency had to immediately cease until the emergency was declared over.

There was a potential problem if a ship transmitted a distress call: The use of 500&nbsp;kHz as a common frequency often led to heavy congestion, especially around major ports and shipping lanes, and it was possible the distress message would be drowned out by the bedlam of ongoing commercial traffic. To help address this problem, the Service Regulation's Article&nbsp;XXXII specified that
: Coastal stations engaged in the transmission of long radiograms shall suspend the transmission at the end of each period of 15&nbsp;minutes, and remain silent for a period of three minutes before resuming the transmission. Coastal and shipboard stations working under the conditions specified in Article&nbsp;XXXV, par.&nbsp;2, shall suspend work at the end of each period of 15&nbsp;minutes and listen in with a wave length of 600&nbsp;meters during a period of 3&nbsp;minutes before resuming the transmission.

During distress working all non-distress traffic was banned from 500&nbsp;kHz and adjacent coast stations then monitored 512&nbsp;kHz as an additional calling frequency for ordinary traffic.

== Later policies ==
The silent and monitoring periods were soon expanded and standardized. For example, Regulation&nbsp;44, from the 27&nbsp;July 1914, edition of ''Radio Communication Laws of the United States'', stated: "The international standard wave length is ''600''&nbsp;meters, and the operators of all ''coast stations'' are required, during the hours the station is in operation, to 'listen in' at intervals of not more than 15&nbsp;minutes and for a period not less than 2&nbsp;minutes, with the receiving apparatus tuned to receive this wave length, for the purpose of determining if any distress signals or messages are being sent and to determine if the transmitting operations of the 'listening station' are causing interference with other radio communication."

International refinements for the use of 500&nbsp;kHz were specified in later agreements, including the 1932 Madrid Radio Conference. In later years, except for distress traffic, stations shifted to nearby "working frequencies" to exchange messages once contact was established: 425, 454, 468, 480, and 512&nbsp;kHz were used by ships while the coast stations had their own individual working frequencies.

Twice each hour, all stations operating on 500&nbsp;kHz were required to maintain a strictly enforced three-minute silent period, starting at 15 and 45&nbsp;minutes past the hour.
[[File:Radioroom.png|thumb|right|200px|Ship's radio room clock with silence periods marked]]
As a visual memory aid, a typical clock in a ship's radio room would have the silence periods marked by shading the [[Circular sector|sectors]] between h+15ᵐ to h+18ᵐ and h+45ᵐ to h+48ᵐ in RED. Similar sectors between h+00ᵐ to h+03ᵐ and h+30ᵐ to h+33ᵐ are marked in GREEN which is the corresponding silence period for the [[2182&nbsp;kHz]] voice communications distress signals.

In addition, during this silent period all coastal and ship stations were required to monitor the frequency, listening for any distress signals.<ref>{{cite magazine |author= |title={{grey|[no title cited]}} |year=1954 |magazine=Ships and the Sea |volume=4 |issue= |pages= |publisher=Kalmbach Publishing Co. |url=https://books.google.com/books?id=i6wPAAAAIAAJ}}</ref>{{full citation |date=November 2022|reason=cited item is a magazine, author, issue, & page nr. needed}} All large ships at sea had to monitor 500&nbsp;kHz at all times, either with a licensed radio operator or with equipment (called an auto-alarm) that  detected an automatically sent distress signal consisting of long dashes.
[[File:Autoalarm-Signal, 500 kHz.oga|thumb|Simulated auto-alarm signal.]]
Shore stations throughout the world operated on this frequency to exchange messages with ships and to issue warning about weather and other navigational warnings.

At night, transmission ranges of 3,000–4,000 miles (4,500–6,500&nbsp;kilometers) were typical. Daytime ranges were much shorter, on the order of 300–1,500&nbsp;miles (500–2,500&nbsp;kilometers). ''Terman's Radio Engineering Handbook'' (1948) shows the maximum distance for 1&nbsp;kW over salt water to be 1,500 miles, and this distance was routinely covered by ships at sea, where signals from ships and nearby coastal stations would cause congestion, covering up distant and weaker signals. During the silence, a distress signal could more easily be heard at great distances.

== NAVDAT ==

Following the adoption of [[Global Maritime Distress Safety System|GMDSS]] in 1999 and the subsequent obsolescence of 500&nbsp;kHz as a Morse distress frequency, the 2019 [[World Radiocommunication Conference]] (WRC-19) allocated {{nobr| 500 ± 5 kHz }} to the maritime NAVDAT service.<ref>{{Cite web |title=Final Acts |series=ITU World Radiocommunication Conference 2019 (WRC-19) |url=https://www.itu.int/dms_pub/itu-r/opb/act/R-ACT-WRC.14-2019-PDF-E.pdf |url-status=live |archive-url=https://web.archive.org/web/20200408180702/https://www.itu.int/dms_pub/itu-r/opb/act/R-ACT-WRC.14-2019-PDF-E.pdf |archive-date=2020-04-08 }}</ref>

NAVDAT is intended for the broadcast of data from shore-to-ship and may thus be compared to [[NAVTEX]]. However, NAVDAT uses [[Quadrature amplitude modulation|QAM modulation]] (in comparison to the [[SITOR]] used by NAVTEX) and is therefore capable of much higher data throughput. This allows NAVDAT broadcasts to carry images and other data as well as plain text, further allowing this data to be presented directly on an [[Electronic Chart Display and Information System|Electronic Chart Display and Information System (ECDIS)]].<ref>{{Cite web |date=January 2019 |title=Characteristics of a digital system, named Navigational Data for broadcasting maritime safety and security related information from shore-to-ship in the 500&nbsp;kHz band |id=ITU Recommendation ITU-R M.2010-1 |publisher=[[International Telecommunication Union]] |place=Geneva, CH |url=https://www.itu.int/dms_pubrec/itu-r/rec/m/R-REC-M.2010-1-201901-I!!PDF-E.pdf |url-status=live|archive-url=https://web.archive.org/web/20220107100155/https://www.itu.int/dms_pubrec/itu-r/rec/m/R-REC-M.2010-1-201901-I!!PDF-E.pdf |archive-date=2022-01-07 }}</ref> This presents a significant improvement over the text-only NAVTEX system.

As of February&nbsp;2023, no maritime authorities have begun [[NAVDAT]] broadcasts.

==Amateur radio==
{{see also|630-meter band}}
Maritime traffic currently displaced from the 500&nbsp;kHz band in most countries, and with the [[International Telecommunication Union|ITU]] 472–479&nbsp;kHz amateur allocation, most countries no longer using it have allocated frequencies near 500&nbsp;kHz to [[amateur radio]] use on a secondary basis, although the primary allocation of the band remains with the maritime mobile service.

Full details of these allocations can be found under the article on the [[630-meter band|630 metre]] amateur radio band.

== See also ==
* [[2182&nbsp;kHz]]{{Snd}} the international distress frequency for ''voice'' maritime communication
* [[630-meter band]]
* [[Navtex]]{{Snd}} a maritime weather and safety text broadcast on 518&nbsp;kHz and 490&nbsp;kHz
* [[Aircraft emergency frequency]]
* [[Call for help]]{{Snd}} for emergency frequencies in current use
* [[Distress signal]]
* [[GMDSS]]
* [[Mayday]]
* [[Medium frequency#Propagation|Medium frequency radio propagation]]
* [[Radio Act of 1912]]
* [[Radio propagation]]
* [[SOS]]

==References==
{{reflist|25em}}

==External links==
* {{cite book |title=Berlin International Radiotelegraphic Convention |year=1907 |publisher=Govt. print. off. |orig-date=1906 |url=https://babel.hathitrust.org/cgi/pt?id=hvd.hnfct7;view=1up;seq=5}}
* {{cite book |title=Radio Laws and Regulations of the United States |year=1914 |publisher=Govt. print. off. |orig-date=27 July 1914 |url=https://babel.hathitrust.org/cgi/pt?id=uc1.b3420424;view=1up;seq=3}} —  (Includes the 1912 London Radiotelegraphic Convention)
* {{cite web |title=Excellent first hand account of distress frequency communications |url=http://jproc.ca/radiostor/cw500pt1.html}}
* {{cite web |title=International Telecommunication Union |url=http://www.itu.int/home/index.html}}
* {{cite web |title=Marine and Coastguard Agency |url=http://www.mcga.gov.uk/c4mca/mcga-mld-page.htm?textobjid=43E2ED43ADD24D4A |access-date=3 March 2007 |archive-date=27 September 2007 |archive-url=https://web.archive.org/web/20070927003212/http://www.mcga.gov.uk/c4mca/mcga-mld-page.htm?textobjid=43E2ED43ADD24D4A |url-status=dead }}
* {{cite web |title=Merchant Shipping Regulations |url=http://www.opsi.gov.uk/si/si1992/Uksi_19920003_en_4.htm}}

{{Two-way radio}}
{{Telecommunications}}

[[Category:Maritime communication]]
[[Category:History of radio]]
[[Category:Emergency communication]]
[[Category:Morse code]]