Sailing Safety Guides

DSC And AIS Check

DSC And AIS Emergency Communication Check For Sailboats

DSC and AIS are only useful in an emergency if they are configured before the emergency.

This guide is a plain check of the two systems many sailors mention but do not always verify: the red DSC distress button on the VHF, and AIS for identification, traffic awareness and nearby MOB/location help.

First Principle

The button is not the safety system

A red distress button, an AIS icon and a chartplotter alarm can give a strong feeling of safety. But the real safety is in the configuration behind them: correct MMSI, valid GPS position, good antenna, working power, correct vessel data and a skipper who knows what the display is telling him.

In an emergency I want the boat to identify itself, send its position, make noise where help can hear it, and still leave me enough brain space to speak clearly on the radio.

What DSC does

DSC means Digital Selective Calling. On a leisure boat it is most often part of the fixed VHF radio. In distress, the DSC alert sends a digital call with the vessel identity and, if the radio has GPS position, the position.

The identity is the MMSI number programmed into the radio.
The position comes from internal GPS or from a connected GPS/plotter.
The digital DSC call uses VHF channel 70; the spoken Mayday still belongs on channel 16.
The alert can be received by coast stations and other DSC-equipped vessels in radio range.
After sending the DSC distress alert, you normally make the voice Mayday on VHF channel 16.

DSC should reduce the chance that a tired skipper forgets identity or position, but it does not replace the voice call.

What AIS does

AIS is a VHF data system used to exchange vessel identity, position, course and speed with other AIS stations. It is mainly a navigation and traffic-awareness tool, not a rescue service by itself.

An AIS receiver can see other transmitting vessels.
An AIS transponder also transmits your own vessel data.
AIS MOB devices can help nearby AIS-equipped vessels find a person in the water.
AIS alarms can warn about CPA and TCPA, but only for targets that transmit and are received correctly.

For many ordinary private leisure sailboats, AIS transmitting is voluntary, not mandatory. A private yacht below about 20 m / 65 ft is often outside mandatory AIS carriage rules, but do not use that number as law. Rules depend on flag state, waters, vessel type, commercial/passenger use and sometimes gross tonnage.

The practical point is simple: with receive-only AIS you can see AIS-transmitting vessels around you, but they cannot see you on AIS. If your transponder is set to silent mode, the result is similar. You still receive targets, but your boat stops transmitting, so other vessels cannot rely on AIS to notice you.

AIS is excellent help, but it is not a lookout replacement. Small boats, fishing gear, floating objects and some vessels may not appear on AIS.

Check that DSC is really ready

Do not only ask whether the radio has DSC. Check whether this boat's DSC is useful today.

Check	What you want to see
Radio type	Fixed VHF with DSC, usually with a protected red distress button.
MMSI	The correct nine-digit MMSI programmed, matching the vessel and registration data.
GPS position	Radio shows a current position, not blank, old, manually forgotten or "no GPS".
Antenna and power	Main VHF has reliable power and antenna. If the mast antenna can be lost, know the emergency antenna plan.
Crew knowledge	At least one other person can find the distress button, read position and make a basic voice call.
Radio region	Correct channel plan and licensing/certificate rules for where the boat is used.

A handheld DSC radio may have different licensing, MMSI and area rules from a fixed ship radio. Check the rules for your flag state and the waters where you sail.

Testing DSC without creating a false distress

Do not press the distress button to "see what happens". That is not a test. Use the radio's DSC test-call function or the local coast radio/radio-check practice.

Find the DSC test-call menu before leaving harbor.
In areas where an official automated DSC test service exists, use that published test MMSI and procedure.
Check that the radio shows the test call was sent and acknowledged.
Confirm the displayed MMSI and position before trusting the result.
Also do a normal VHF voice radio check using the locally accepted method.

If your country or coast station does not provide an automated DSC test service, use the radio manual and local guidance. The important rule stays the same: never send a distress alert as a test.

If a DSC distress alert is sent by mistake

A false alert must be cancelled properly. Do not just switch the radio off and hope it disappears. Some radios may continue or repeat when powered again.

Use the radio's distress-cancel function if available.
Then call on VHF channel 16 and say the alert was accidental.
Give boat name, MMSI and position.
Follow instructions from the coast station or other authority.
Write down what happened and fix the cause: button cover, crew briefing, radio handling or menu confusion.

Using DSC in real distress

If there is grave and imminent danger and immediate assistance is needed, DSC is normally used first if it can be done quickly and the radio is ready.

Open the distress-button cover.
If there is time and the radio allows it, choose the nature of distress.
Press and hold the distress button as required by the radio manual.
Watch that the radio shows the alert is being transmitted.
Switch or stay on VHF channel 16 as the radio instructs.
Make the voice Mayday: who you are, where you are, what is wrong, what help is needed, and how many people are on board.
Listen for acknowledgement or instructions, but keep solving the emergency.

If there is no DSC acknowledgement and nobody answers the voice call, continue with all suitable means: VHF voice, EPIRB/PLB, satellite device, mobile phone near shore, visual signals and contact with nearby vessels.

How to know whether the DSC alert probably worked

In stress, the skipper needs simple confirmation signs. Different radios behave differently, but useful checks are:

The radio screen says the distress alert was transmitted or is transmitting.
The radio gives the distress alarm state and may repeat the alert until acknowledged or stopped.
A coast station or vessel acknowledges digitally or answers on channel 16.
The answering station knows your MMSI or position without you reading everything first.
The radio remains powered and attended after the alert.

Do not sit quietly waiting only because the button was pressed. DSC starts the alerting. The voice call and the ongoing communication still matter.

Check AIS configuration

If the boat transmits AIS, the transmitted data should describe the real boat. Bad AIS data can make you harder to identify, or make another vessel judge the situation wrongly.

MMSI matches the vessel and the VHF/DSC registration.
Boat name, call sign, type and dimensions are correct.
GPS position updates normally.
AIS antenna and splitter, if used, are understood and checked.
Only one AIS transmitter is active for the boat unless a specific device is meant to be personal AIS MOB.
The skipper knows whether the unit is transmitting, receive-only or in silent mode. If you expect other vessels to see you, silent mode must be off and the transponder must actually transmit.
The display shows target age, CPA/TCPA, vectors and alarm settings in a way the skipper understands.

Verify that AIS is actually seen

Seeing your own AIS on your own plotter is not always the same as knowing others receive it. Use several practical checks when possible.

Ask another boat, marina or club with AIS reception whether they see you correctly.
Check an internet AIS site only as a secondary hint; shore coverage is not guaranteed and delay is normal.
Look for correct name, MMSI, position, course, speed and vessel type.
Check both harbor-low-speed and underway situations.
If the displayed name or static data is missing, give AIS time to send static messages before concluding.

AIS classes and emergency marking

It is easy to mix up AIS traffic visibility with distress alerting. They are related only because both can help another vessel identify or find you.

Equipment	What it normally means on a leisure sailboat
AIS receiver only	Shows AIS-transmitting vessels around you, but does not transmit your own boat. Other vessels cannot see you on AIS because of this unit.
Class B transponder	Common on recreational boats. It transmits your position and static vessel data, but at lower power and usually slower reporting than Class A. It should not be expected to send a proper emergency mark.
Class B+ / Class B SOTDMA	Often a better recreational or offshore choice. B+ is a common name for Class B SOTDMA, normally with higher transmit power and better time-slot handling than older Class B CS equipment. It is still a normal yacht transponder, not an emergency beacon.
Class A	Normal on commercial, SOLAS and larger vessels. It has higher transmit power, more frequent reporting, voyage data, navigational status and safety-related AIS message capability. It is uncommon on ordinary private sailboats.
AIS MOB, AIS SART, AIS EPIRB	Special emergency locating devices. These are meant to mark a person, liferaft or emergency position for nearby AIS-equipped vessels.

AIS can help nearby vessels see your position, but AIS is not the primary distress system. For distress, use DSC Mayday and EPIRB/PLB first. AIS MOB, AIS SART and AIS EPIRB devices are useful local-position aids, but a normal Class B or Class B+ yacht transponder should not be assumed to mark the boat as being in distress.

Using AIS for collision awareness

AIS gives useful numbers, but the numbers must be understood. The target that matters is not always the closest one. It is the one that will be close at the wrong time.

CPA: closest point of approach. How close the target is predicted to pass.
TCPA: time to closest point of approach. How soon that closest point will happen.
COG/SOG: course and speed over ground. This may differ from heading and water speed.
Target age: old data can make a target look safer than it is.
Vectors: a quick visual way to see where the target and your boat are going.

If CPA is small and TCPA is short, act early. Confirm visually and by radar if fitted. If needed, call the vessel by name or MMSI on VHF. Do not assume the other vessel has seen you just because you see its AIS.

AIS MOB and personal beacons

AIS MOB is very useful near the boat and near other AIS receivers. It can place a person in the water on the plotter and can make recovery faster. But it is short-range VHF equipment, not a global rescue beacon.

Test only according to the device manual.
Know whether the device activates automatically, manually or both.
Make sure the plotter/radio actually alarms when a test or training mode is used.
Do not treat AIS MOB as a replacement for staying clipped on.
For offshore distress, understand the separate role of EPIRB or PLB.

Alarm settings before night or rest

AIS alarms should wake you early enough to think. If they are too tight, they wake you too late. If they are too noisy, you may stop trusting them.

Set CPA and TCPA for boat speed, visibility, traffic density and your reaction time.
Use more conservative alarms at night, in poor visibility and when singlehanded.
Check alarm volume from the place where you may rest.
Remember that AIS does not see non-transmitting targets.
Use radar, visual lookout and conservative course planning where traffic risk is high.

Before passage: one practical DSC/AIS pass

Item	Pass condition
Fixed VHF	Powers up, receives, transmits, channel 16 works, volume and squelch sensible.
DSC	MMSI correct, GPS current, distress button known, test-call method known.
Handheld VHF	Charged, waterproofed, reachable and not buried.
AIS transmit	Correct MMSI/name/data and verified if possible from another receiver.
AIS receive	Targets display correctly with CPA/TCPA, vectors and target age understood.
Emergency card	Boat name, call sign, MMSI, position format and safety gear locations written down.
Crew briefing	Crew knows the first radio call, the distress button, and where the backup devices are.

How this connects to SailingKSF

This guide sits beside the emergency-call article and the collision-avoidance article. The common idea is simple: do not wait for stress before checking the system.

In the SailingKSF suite, AIS/RADAR alarms, voice alarm, Passage Companion boat details and printable emergency cards all point in the same direction. The skipper should not have to remember every number while wet, tired and frightened. The important data should already be correct, visible and easy to read.

Calling For Help Collision Avoidance Printable Emergency Cards

USCG DSC distress alerting - Official guidance on sending, receiving and cancelling DSC distress alerts, including why the voice Mayday still follows the digital alert.
USCG DSC test calls - Official DSC test-call procedure. Useful for checking that a radio can make a DSC call without sending a false distress alert.
USCG MMSI information - Official MMSI background. The MMSI links DSC, AIS and registered rescue information, so it must match the boat and owner data.
IMO AIS carriage overview - Official IMO background on AIS and SOLAS carriage requirements, including the gross-tonnage and passenger-ship thresholds.
USCG AIS requirements - Official U.S. AIS carriage and operation rules. Useful because some thresholds are commercial, passenger, cargo or gross-tonnage based, not simply private-yacht length.
USCG NAVCEN AIS FAQ - Official AIS background and limitations. AIS is very useful, but the data must still be checked against lookout and radar.

Feedback For DSC And AIS Emergency Communication

If you have marine radio, DSC, AIS, coast radio, rescue or offshore experience, I would be glad to read what should be corrected or added.

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Check the system before the call is emotional

DSC, AIS and VHF voice are not magic. They are tools. The useful work is to make sure identity, position, alarms and crew understanding are correct while the boat is still calm.