All fixed VHF radios produced today have DSC radio (Digital Selective Calling) functionality built in as standard. Prior to the advent of DSC equipped VHF radios, all VHF communication was limited to open-channel voice traffic.
Everyone could eavesdrop on your conversations, and distress calls could go unanswered due to the widespread misuse of channel 16.
All this changed with the arrival of the Global Maritime Distress and Safety System (GMDSS) and the DSC VHF radio.
DSC uses VHF channel 70 to transmit and receive digital messages.
No voice communication is permitted on that channel; it's reserved solely for DSC which enables you to selectively contact other stations and in digital format (an MMSI number), or nominate another channel for voice communication.
In the UK, Maritime Mobile Service Identity (MMSI) numbers are assigned as a part of the ship's radio licensing procedure. These nine digits number are used to uniquely identify a radio station.
The MMSI number has a standard format and identifies the type of station, country of registration and vessel identity, and can be programmed into a DSC equipped VHF radio just like entering a number into a mobile phone.
Alternatively, groups of MMSI's can be programmed in, effectively creating a net to which communal broadcasts can be made.
Here's how to
But by far the most important feature of DSC is that it provides a safer way of placing a distress call to the coastguard.
A DSC distress message includes all the vital information required by the authorities - MMSI, current position and time of call - to initiate a rescue mission.
Sending a distress message is as simple as selecting the nature of the distress from a scroll-down list on screen and pressing the distress button, dramatically improving the chance of a successful message than a MAYDAY call on channel 16.
If the VHF set is interfaced with the GPS, then the position information is fully automated; otherwise it must be entered manually.
It had to come of course; handheld VHF radios with both DSC and GPS built in.
If you're unlucky enough to have to take to your liferaft, then one of these will greatly increase your chances of early rescue.
Predictably perhaps, they're not cheap...