Digital selective calling, or DSC, is a technology that's revolutionizing maritime communication. It's a digital system that allows ships to send and receive messages more efficiently.
The International Telecommunication Union (ITU) has been working to develop DSC standards for maritime communication since the 1990s. This effort has led to the creation of the Global Maritime Distress and Safety System (GMDSS), which relies on DSC for messaging.
DSC uses a digital signal to send messages, making it more reliable and faster than traditional radio communication. This is especially important in emergency situations, where every second counts.
In the event of an emergency, a ship can send a distress signal using DSC, which can be received by nearby vessels and coastal authorities. This helps to ensure a quick response and saves lives.
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What Is DSC?
DSC, or Digital Selective Calling, is a standard method used to transmit messages via VHF, MF, and HF.
It's primarily used for sending distress calls to other vessels and shore systems.
DSC allows you to directly call or receive a call, rather than having to listen to a speaker.
DSC can also support urgency, safety, and routine calls alongside distress.
A distress call made via DSC provides some information automatically when interfaced with other devices, which can massively improve response and rescue times.
The vessel's identity is sent, along with the MMSI, and if the VHF/DSC is interfaced with a GPS, it can also send the position of the vessel currently.
DSC has a narrow bandwidth and its own unique channel, channel 70, which all DSC-capable radios will monitor automatically.
If a distress call is sent from a vessel via DSC, nearby shore stations and other vessels with DSC will receive this call, and it will alarm.
This call alarm is repeated until acknowledged, to ensure it's not missed or forgotten about.
DSC has a greater range than voice calling, around 15-25%, meaning it's much more likely to reach someone else.
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Technical Details
The DSC system uses synchronous technology to transmit characters encoded with a ten-bit error detecting code. This code is used to ensure the accuracy of the message.
Each character is transmitted twice with a time delay, allowing for redundancy and error correction. The time delay is a crucial aspect of the system's reliability.
For High Frequency and Medium Frequency, two tones 170 Hz apart are used, with a symbol rate of 100 Baud. This setup is specifically designed for these frequency ranges.
In contrast, VHF uses tones 1300 and 2100 Hz, with a symbol rate of 1200 Baud. The choice of tones and symbol rate is optimized for the unique characteristics of VHF.
The detailed specification of the DSC system is published in the International Telecommunication Union recommendation ITU-R M.493, revision 16, published in 2023. This document provides a comprehensive overview of the system's technical requirements.
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MMSI (Maritime ID)
An MMSI, or Maritime Mobile Service Identity, is a unique 9-digit number that identifies vessels, coast stations, and SAR aircraft.
This number is assigned to a DSC Radio or an AIS, making it an individual calling number for the vessel.
To get an MMSI number, one needs to have a ship radio license, which is required for safety and marine traffic management purposes.
All radio equipment must operate under this MMSI once the license is obtained.
There are six categories for MMSI, including ship station, group ship station, coast station, group coast station, SAR aircraft, and navigational aids and crafts.
The MMSI is used for identification purposes, similar to a vehicle registration number.
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DSC Alerts and Calls
Digital selective calling (DSC) is a system that sends a digital call or alert to one or more radio stations, making it a lifesaver in distress situations.
The main advantages of DSC alerts include quick and efficient transmission of necessary vessel information, rapid and repeated transmission, and less room for error due to digital transmission on a dedicated frequency.
In a distress situation, a DSC distress alert allows for accurate and quick reporting of the situation to another radio station, which can provide assistance or help coordinate it.
A DSC distress alert is transmitted repeatedly every four minutes until a DSC acknowledgement is received, and it should always be followed by a radiotelephone distress transmission using normal distress procedure.
You can send an undesignated DSC distress alert by pressing one or two buttons, which will automatically transmit the MMSI number, position, and time and date on Channel 70.
Designated alerts are similar, but they also include the type of distress and provide a few more seconds to input the information.
Here are some key benefits of DSC alerts:
- In cases of emergency, a distress alert will be sent quickly and efficiently.
- The alert will be transmitted rapidly and repeatedly.
- Since the information is transmitted digitally, there is less room for error.
- DSC virtually guarantees that someone will get the alert.
The International Telecommunications Union Sector for Radiocommunications recommends limiting live testing on DSC distress and safety frequencies with coast stations to once a week to minimize possible interference.
The US Coast Guard urges mariners to interconnect their GPS and DSC-equipped radio to save their lives in a distress situation, and to consult the owner's manuals before doing so.
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DSC Benefits and Purpose
Digital Selective Calling (DSC) is a game-changer for maritime safety. It was developed primarily to send a digital call, or an alert, to one or more radio stations, similar to a pager.
In cases of emergency, a distress alert will be sent quickly and efficiently since it includes all the necessary vessel information. This is thanks to the digital transmission of the alert, which is less prone to error.
The primary use of DSC is to send distress calls to onshore systems, as well as to other sea vessels nearby. This is crucial for improving safety and response times in emergency situations.
A DSC alert will be transmitted rapidly and repeatedly, ensuring that someone will get the alert. This is especially important in situations where every second counts.
Digital Selective Calling (DSC) allows you to directly 'call' or receive a call directly, rather than having to listen to a speaker. This makes it a more efficient and effective way of communicating at sea.
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Here are the main benefits of DSC alerts:
- In cases of emergency, a distress alert will be sent quickly and efficiently since it includes all the necessary vessel information.
- The alert will be transmitted rapidly and repeatedly.
- Since the information is transmitted digitally using a dedicated frequency known as Channel 70, there is less room for error.
- DSC virtually guarantees that someone will get the alert.
By using DSC, you can send a distress call that includes the precise position of the vessel, which can be sent in seconds. This can massively improve response and rescue times in emergency situations.
Alert Types and Designation
There are two types of DSC alerts: undesignated and designated.
Undesignated DSC distress alerts can be sent quickly by pressing one or two buttons simultaneously, automatically transmitting the MMSI number, position, and time and date on Channel 70.
A designated alert is very similar, but it takes a few more seconds to send, and includes the type of distress, such as a vessel on fire or sinking.
Both types of alerts will continue to be transmitted repeatedly, every four minutes, until a DSC acknowledgement is received.
Here's a quick summary of the key differences between undesignated and designated alerts:
Designated alerts provide more information, but both types of alerts will get the message out quickly and efficiently.
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