Digital Selective Calling is the part of a modern marine VHF that most crews never use and every crew should understand. It is not a mode you talk on. It is a short digital message the radio sends on a dedicated channel to get someone's attention, and in a distress situation it does in under a second what a spoken mayday call takes a frightened person a minute to get out, assuming they get it out at all.
Channel 70 is not a voice channel
Every DSC-equipped VHF set watches 156.525 MHz, marine channel 70, continuously whenever it is powered. That channel carries no speech at all. It carries short bursts of data, sent as 16K0G2B emission rather than the 16K0G3E frequency modulation used for voice, and the receiver decodes them and acts on them automatically. This is why a DSC set has to keep a watch on two things at once, channel 70 for data and channel 16 for voice, and why fixed-mount sets are specified with that dual watch built in.
What actually happens when the distress button is pressed
The sequence is short and worth knowing by heart. The operator lifts the guarded distress cover and holds the button, typically for three to five seconds, so that a knock or a sleeve cannot trigger it. The radio then transmits a distress alert on channel 70 containing three things: the vessel's nine-digit Maritime Mobile Service Identity, the nature of the distress if one has been selected, and the vessel's position with the time it was valid.
That last item is the reason DSC installation practice matters so much. A DSC alert without a position tells a rescue coordination centre that someone within VHF range is in trouble and nothing more. The position comes either from a GNSS feed wired into the radio over NMEA, which is why fixed sets list input sentences such as RMC, GGA, GNS and GLL, or from an internal receiver in sets built with one, such as the Icom IC-M424G or the handheld IC-M94D. If neither is present the position has to be keyed in by hand, and it will be as stale as the last time somebody remembered to do it.
Having transmitted, the radio switches itself to channel 16 and waits. Coast stations and other vessels in range receive the alert, their radios sound an alarm and display the calling vessel's identity and position, and the exchange then continues by voice on channel 16 in the normal way. The digital alert does not replace the mayday call. It precedes it, and it means that even if nobody manages to speak, the identity and position are already out.
The MMSI is the whole system
A DSC radio is useless until its MMSI is programmed, and the number is issued to the vessel by the national administration, not by the equipment supplier. Two practical consequences follow. First, most sets allow the MMSI to be entered only a limited number of times, sometimes once, precisely so that the identity stays tied to the vessel. Second, if a radio is sold on or the vessel is renamed or re-registered, the registration record has to be updated, or a rescue centre will be handed the wrong owner, the wrong emergency contact and possibly the wrong vessel description at the worst possible moment.
Beyond distress
The same mechanism carries routine traffic. A DSC set can place an individual call to another vessel's MMSI, which alerts that specific radio and proposes a working channel, and it can send an all-ships urgency or safety call. Class A DSC, as fitted to GMDSS installations such as the Icom GM600 and the Samyung STR-7000A, supports the full set of call types required of a SOLAS ship. Class D DSC, found in commercial and leisure fixed sets such as the Icom IC-M330 and IC-M220, covers distress and routine calling for vessels outside the SOLAS carriage regime.
Three things to check on your own set
- Is the MMSI programmed, and is it the vessel's own current number?
- Does the radio have a live position, from an internal GNSS receiver or an NMEA feed, and does the display confirm it?
- Does the crew know where the distress cover is and how long to hold the button, without having to look for it?
None of that costs anything to verify, and all three are the difference between an alert that starts a search and an alert that starts a question.