Digital selective calling (DSC) is a standard for transmitting predefined digital messages via the medium-frequency (MF), high-frequency (HF) and very-high-frequency (VHF) maritime radio systems. It is a core part of the Global Maritime Distress Safety System (GMDSS). [1]
DSC was developed to replace a voice call in older procedures. Because a DSC signal uses a stable signal with a narrow bandwidth and the receiver has no squelch, it has a slightly longer range than analog signals, [1] with up to twenty-five percent longer range and significantly faster. [2] DSC senders are programmed with the ship's Maritime Mobile Service Identity (MMSI) and may be connected to the ship's Global Positioning System (GPS), which allows the apparatus to know who it is, what time it is and where it is. This allows a distress signal to be sent very quickly. [1]
Often, ships use separate VHF DSC and MF/HF DSC controllers. For VHF, DSC has its own dedicated receiver for monitoring Channel 70, but uses the main VHF transceiver for transmission. However, for the user, the controller is often a single unit. [1] MF/HF DSC devices monitor multiple bands for distress, urgency and safety sécurité transmissions in the 2, 4, 6, 8, 12 and 16 MHz marine bands. At minimum, controllers will monitor 2187.5 kHz and 8414.5 kHz and one more band. [3] However for automated monitoring a second, receive-only antenna is often needed (especially on non-commercial leisure boats) since a separate tuner is used apart from the main one; [4] [5] this is separate from programming radios to monitor user-defined DSC frequencies (which would use the main antenna). [6] [7]
When sending a distress signal, the DSC device will at minimum include the ship's MMSI number. It may also include the coordinates if radio is connected to GPS system and, if necessary, the channel for the following radiotelephony or radiotelex messages. [3] The distress can be sent either as a single-frequency or multi-frequency attempt. In the former, a distress signal is sent on one band and the system will wait up to four minutes for a DSC acknowledgment from a coast station. If none is received, it will repeat the distress alert up to five times. In a multi-frequency attempt, the distress signal is sent on the MF and all the HF distress frequencies in turn. As this requires retuning the antenna for each sending, without waiting for an acknowledgment, a multi-frequency attempt should only be done if there are only a few minutes until the ship's batteries are under water. As the distress message can only be sent on one of the bands, many ships and coast stations may be listening to a band without the message, and will after five minutes relay the distress signal to a coast station. [8]
Distress calls can be both non designated and designated. The latter allows one of ten predefined designations to be sent along with the distress signal. These are "abandoning ship", "fire or explosion", "flooding", "collision", "grounding", "listing", "sinking", "disabled and adrift", "piracy or attack" and "man overboard". [8] To avoid false distress alerts, distress buttons normally have protective covers, often with a spring-loaded cover so two hands need to be used simultaneously. Alternatively, some devices have two-button systems. [9] Operators are required to cancel falsely sent distress alerts with a transmission on the channel designated by the distress signal. [10]
A coast station which receives a DSC distress alert will wait 2.75 minutes prior to sending an acknowledgment to allow other vessels in the nearby area to receive the alert. This helps shore stations narrow down where a vessel without GPS is located. The sending device will then both stop repeating the alert, and tune to the designated channel for the distress message to be sent. Ships receiving a distress alert who are outside coast station range or do not receive an acknowledgment, are required to relay the distress alert by any means to land. [9]
The priorities for communication are, Distress, Urgency, Safety and Routine. A Distress DSC call is called an Alert. Urgency, Safety and Routine are called Announcements.
Class A VHFs, used on commercial ships, have the ability to send distress, distress relay, all ships urgency, all ships safety, individual, group, geographic area and telephone alerts/announcements on DSC channel 70 (Digital channel reserved for DSC only.). Class D VHFs, used for most leisure vessels, can send distress, all ships urgency, all ships safety and individual alerts/announcements on VHF channels on DSC channel 70. On both class A and D you can be directed to Ch 06, 08, 72, 77 or other simplex channel for the follow-up RT (radio telephony [voice]) call. Class D has only one antenna and thus can only watch Channel 70 when not transmitting. For routine alerts, which are used to establish communication with another station on a working channel, the receiver acknowledges to confirm that communication can be done on the appropriate channel. [11]
While there are reserved frequencies for distress HF DSC calls, there is no prohibition against broadcasting non-distress, "routine" calls on other DSC-designated frequencies, which are defined in ITU M.541 as: [12]
There is a general consensus for routine calls to use 2177.0, 4208.0, 6312.5, 8415.0, 12577.5, and 16805.0 kHz (the first frequency listed above in each band). [6] [7] [13]
The DSC is a synchronous system using characters composed from a ten-bit error detecting code. The bits are encoded using frequency-shift keying. For High Frequency and Medium Frequency two tones 170 Hz apart either side of the allocated frequency with 100 Baud symbol rate are used. For VHF the two tones used are 1300 and 2100 Hz with a symbol rate of 1200 Baud. Each character is transmitted twice with a time delay. The detailed specification is published in the International Telecommunication Union recommendation ITU-R M.493, revision 15 published in 2019 being the most recent. [14]
In telecommunication, a maritime broadcast communications net is a communications net that is used for international distress calling, including international lifeboat, lifecraft, and survival-craft high frequency (HF); aeronautical emergency very high frequency (VHF); survival ultra high frequency (UHF); international calling and safety very high frequency (VHF); combined scene-of-search-and-rescue; and other similar and related purposes.
Very high frequency (VHF) is the ITU designation for the range of radio frequency electromagnetic waves from 30 to 300 megahertz (MHz), with corresponding wavelengths of ten meters to one meter. Frequencies immediately below VHF are denoted high frequency (HF), and the next higher frequencies are known as ultra high frequency (UHF).
Sécurité is a procedure word used in the maritime radio service that warns the crew that the following message is important safety information. The most common use of this is by coast radio stations before the broadcast of navigational warnings and meteorological information.
SOS is a Morse code distress signal, used internationally, originally established for maritime use. In formal notation SOS is written with an overscore line, to indicate that the Morse code equivalents for the individual letters of "SOS" are transmitted as an unbroken sequence of three dots / three dashes / three dots, with no spaces between the letters. In International Morse Code three dots form the letter "S" and three dashes make the letter "O", so "S O S" became a common way to remember the order of the dots and dashes. IWB, VZE, 3B, and V7 form equivalent sequences, but traditionally SOS is the easiest to remember.
Medium frequency (MF) is the ITU designation for radio frequencies (RF) in the range of 300 kilohertz (kHz) to 3 megahertz (MHz). Part of this band is the medium wave (MW) AM broadcast band. The MF band is also known as the hectometer band as the wavelengths range from ten to one hectometers. Frequencies immediately below MF are denoted as low frequency (LF), while the first band of higher frequencies is known as high frequency (HF). MF is mostly used for AM radio broadcasting, navigational radio beacons, maritime ship-to-shore communication, and transoceanic air traffic control.
High frequency (HF) is the ITU designation for the range of radio frequency electromagnetic waves between 3 and 30 megahertz (MHz). It is also known as the decameter band or decameter wave as its wavelengths range from one to ten decameters. Frequencies immediately below HF are denoted medium frequency (MF), while the next band of higher frequencies is known as the very high frequency (VHF) band. The HF band is a major part of the shortwave band of frequencies, so communication at these frequencies is often called shortwave radio. Because radio waves in this band can be reflected back to Earth by the ionosphere layer in the atmosphere – a method known as "skip" or "skywave" propagation – these frequencies are suitable for long-distance communication across intercontinental distances and for mountainous terrains which prevent line-of-sight communications. The band is used by international shortwave broadcasting stations (3.95–25.82 MHz), aviation communication, government time stations, weather stations, amateur radio and citizens band services, among other uses.
A distress signal, also known as a distress call, is an internationally recognized means for obtaining help. Distress signals are communicated by transmitting radio signals, displaying a visually observable item or illumination, or making a sound audible from a distance.
Marine VHF radio is a worldwide system of two way radio transceivers on ships and watercraft used for bidirectional voice communication from ship-to-ship, ship-to-shore, and in certain circumstances ship-to-aircraft. It uses FM channels in the very high frequency (VHF) radio band in the frequency range between 156 and 174 MHz, inclusive, designated by the International Telecommunication Union as the VHF maritime mobile band. In some countries additional channels are used, such as the L and F channels for leisure and fishing vessels in the Nordic countries. Transmitter power is limited to 25 watts, giving them a range of about 100 kilometres.
The Global Maritime Distress and Safety System (GMDSS) is a worldwide system for automated emergency signal communication for ships at sea developed by the United Nations' International Maritime Organization (IMO) as part of the SOLAS Convention.
The radio spectrum is the part of the electromagnetic spectrum with frequencies from 3 Hz to 3,000 GHz (3 THz). Electromagnetic waves in this frequency range, called radio waves, are widely used in modern technology, particularly in telecommunication. To prevent interference between different users, the generation and transmission of radio waves is strictly regulated by national laws, coordinated by an international body, the International Telecommunication Union (ITU).
NAVTEX, sometimes styled Navtex or NavTex, is an international automated medium frequency direct-printing service for delivery of navigational and meteorological warnings and forecasts, as well as urgent maritime safety information (MSI) to ships.
Amateur radio frequency allocation is done by national telecommunication authorities. Globally, the International Telecommunication Union (ITU) oversees how much radio spectrum is set aside for amateur radio transmissions. Individual amateur stations are free to use any frequency within authorized frequency ranges; authorized bands may vary by the class of the station license.
2182 kHz is a radio frequency designed exclusively for distress calls and related calling operations in the maritime service.
Channel 16 VHF is a marine VHF radio frequency designated as an international distress frequency. Primarily intended for distress, urgency and safety priority calls, the frequency may also carry routine calls used to establish communication before switching to another working channel.
An international distress frequency is a radio frequency that is designated for emergency communication by international agreement.
Survival radios are carried by pilots and search and rescue teams to facilitate rescue in an emergency. They are generally designed to transmit on international distress frequencies. Maritime systems have been standardized under the Global Maritime Distress Safety System. Civil and military organisation's utilized different frequencies to communicate and no infringement on either sector would take place. For emergencies involving civilian aircraft, the radio frequency used is VHF 121.5 MHz and for military aircraft incidents, the frequency used is UHF 243 MHz.
A personal radio service is any system that allows individual to operate radio transmitters and receivers for personal purposes with minimal or no special license or individual authorization. Personal radio services exist around the world and typically use light-weight walkie talkie portable radios. The power output, antenna size, and technical characteristics of the equipment are set by regulations in each country. Many regions have standardized personal radio service rules to allow travelers from one country to use their equipment in another country. Examples of standardized services include PMR446 and FM Citizens Band Radio (CB) in the EU and several other countries/regions. 26–27 MHz CB radio is the oldest personal radio service and is used in nearly every country worldwide, with many countries and regions copying the United States 40-channel frequency plan. In many countries, CB radio is less popular due to the availability of other personal radio services that offer shorter antennas and better protection from noise and interference.
During World War II, the German Army relied on an diverse array of communications to maintain contact with its mobile forces and in particular with its armoured forces. Most of this equipment received the generic prefix FuG for Funkgerät, meaning "radio device". Occasionally the shorted Fu designation were used and there were exceptions to both these systems. Number ranges were not unique across the services so sometimes different equipment used by different services had the same FuG prefix. This article is a list and a description of the radio equipment.