Digital Selective Calling
Encyclopedia
Digital Selective Calling or DSC is a standard for sending pre-defined 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).
, it has a slightly longer range than analog signal
s, with up 25 percent longer range and significantly faster. 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.
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. MF/HF DSC devices monitor multiple distress, urgency
and safety
bands in the 2, 4, 6, 8, 12 and 16 MHz bands. At minimum, controllers will monitor 2187.5 kHz and 8414.5 kHz and one more band.
if available and, if necessary, the channel for the following radiotelephony or radiotelex messages. 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.
Distress calls can be both non designated and designated. The latter allows one of ten pre-defined 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
". 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. Operators are required to cancel falsely sent distress alerts with a transmission on the channel designated by the distress signal.
A coast station which receives a DSC distress alert will immediately send an acknowledgment. The sending device with 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 of coast station range or do not receive a acknowledgment, are required to relay the distress alert by any means to land.
Medium frequency
Medium frequency refers to radio frequencies in the range of 300 kHz to 3 MHz. Part of this band is the medium wave AM broadcast band. The MF band is also known as the hectometer band or hectometer wave as the wavelengths range from ten down to one hectometers...
(MF), high frequency
High frequency
High frequency radio frequencies are between 3 and 30 MHz. Also known as the decameter band or decameter wave as the wavelengths range from one to ten decameters . Frequencies immediately below HF are denoted Medium-frequency , and the next higher frequencies are known as Very high frequency...
(HF) and very high frequency
Marine VHF radio
Marine VHF radio is installed on all large ships and most seagoing small craft. It is used for a wide variety of purposes, including summoning rescue services and communicating with harbours, locks, bridges and marinas, and operates in the VHF frequency range, between 156 to 174 MHz...
(VHF) maritime radio systems. It is a core part of the Global Maritime Distress Safety System
Global Maritime Distress Safety System
The Global Maritime Distress and Safety System is an internationally agreed-upon set of safety procedures, types of equipment, and communication protocols used to increase safety and make it easier to rescue distressed ships, boats and aircraft....
(GMDSS).
Workings
DSC was developed to replace a call in older procedures. Because a DSC signal uses a stable signal with a narrow bandwidth and the receiver has no squelchSquelch
In telecommunications, squelch is a circuit function that acts to suppress the audio output of a receiver in the absence of a sufficiently strong desired input signal.-Carrier squelch:...
, it has a slightly longer range than analog signal
Analog signal
An analog or analogue signal is any continuous signal for which the time varying feature of the signal is a representation of some other time varying quantity, i.e., analogous to another time varying signal. It differs from a digital signal in terms of small fluctuations in the signal which are...
s, with up 25 percent longer range and significantly faster. DSC senders are programmed with the ship's Maritime Mobile Service Identity
Maritime Mobile Service Identity
A Maritime Mobile Service Identity is a series of nine digits which are sent in digital form over a radio frequency channel in order to uniquely identify ship stations, ship earth stations, coast stations, coast earth stations, and group calls...
(MMSI) and may be connected to the ship's Global Positioning System
Global Positioning System
The Global Positioning System is a space-based global navigation satellite system that provides location and time information in all weather, anywhere on or near the Earth, where there is an unobstructed line of sight to four or more GPS satellites...
(GPS), which allows the apparatus to know who it is, what time it is and where it is. This allows a distress signal
Distress signal
A distress signal is an internationally recognized means for obtaining help. Distress signals take the form of or are commonly made by using radio signals, displaying a visually detected item or illumination, or making an audible sound, from a distance....
to be sent very quickly.
Often, ships use separate VHF DSC and MF/HF DSC controllers. For VHF, DSC has its own dedicated Receiver
Receiver
-Technology:* The listening device part of a telephone** The handset that sometimes contains the above device** The headset that can also contain the above device.* Audio-Visual receiver* Digital media receiver...
for monitoring Channel 70, but uses the main VHF transceiver for transmission
Transmission (telecommunications)
Transmission, in telecommunications, is the process of sending, propagating and receiving an analogue or digital information signal over a physical point-to-point or point-to-multipoint transmission medium, either wired, optical fiber or wireless...
. However, for the user, the controller is often a single unit. MF/HF DSC devices monitor multiple distress, urgency
Pan-pan
In radiotelephone communications, a call of three repetitions of pan-pan is used to signify that there is an urgency on board a boat, ship, aircraft or other vehicle but that, for the time being at least, there is no immediate danger to anyone's life or to the vessel itself. This is referred to as...
and safety
Securite
When a marine radio transmission begins with "Sécurité, sécurité, sécurité" , it means that what follows is important safety information. The most common use of this is by coast radio stations before the broadcast of navigational warnings and meteorological information.It is normal practice to...
bands in the 2, 4, 6, 8, 12 and 16 MHz bands. At minimum, controllers will monitor 2187.5 kHz and 8414.5 kHz and one more band.
Distress
When sending a distress signal, the DSC device will at minimum include the ship's MMSI number. It will also include the coordinatesGeographic coordinate system
A geographic coordinate system is a coordinate system that enables every location on the Earth to be specified by a set of numbers. The coordinates are often chosen such that one of the numbers represent vertical position, and two or three of the numbers represent horizontal position...
if available and, if necessary, the channel for the following radiotelephony or radiotelex messages. 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
Tuned filter
In signal processing, a tuned filter is a stage in the processing channel which accepts or rejects signals which are tuned for a specific type.-History:...
the antenna
Antenna (radio)
An antenna is an electrical device which converts electric currents into radio waves, and vice versa. It is usually used with a radio transmitter or radio receiver...
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.
Distress calls can be both non designated and designated. The latter allows one of ten pre-defined designations to be sent along with the distress signal. These are "abandoning ship", "fire or explosion", "flooding", "collision
Ship collision
Ship collision is the structural impact between two ships or one ship and a floating or still object such as an iceberg. Ship collisions are of particular importance in marine accidents...
", "grounding", "listing", "sinking", "disabled and adrift", "piracy
Piracy
Piracy is an act of robbery or criminal violence at sea. The term can include acts committed on land, in the air, or in other major bodies of water or on a shore. It does not normally include crimes committed against persons traveling on the same vessel as the perpetrator...
or attack" and "man overboard
Man overboard
Man overboard is a situation in which a person has fallen from a boat or ship into the water and is in need of rescue. Whoever sees the person's fall should shout "man overboard" to alert other crew members and attempt to maintain visual contact with the person in the water...
". 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. Operators are required to cancel falsely sent distress alerts with a transmission on the channel designated by the distress signal.
A coast station which receives a DSC distress alert will immediately send an acknowledgment. The sending device with 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 of coast station range or do not receive a acknowledgment, are required to relay the distress alert by any means to land.