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Get the best from your AIS

AIS is a significant help to navigation and safety. Once you’ve programmed in your boat’s details you’re ready to go – but there’s more to AIS than meets the eye. Here are some useful pointers to enable you to get the best from your AIS setup.

AIS vessels on a B&G chartplotter

Some key data:

Vessel name: The vessel’s name is transmitted – some AIS transmitters may take a while to send this, but you will always have the MMSI (see below) should you wish to call a vessel.

MMSI: This is a unique identifier that is programmed into the AIS transmitter and the DSC radio, issued by a national authority (Ofcom in the UK). 

Call Sign: The vessel’s unique callsign is assigned by the national authority and allows you to identify the ship or craft and call them if necessary.

SOG: Speed over ground.

COG: Course over ground.

CPA: Closest Point of Approach. This is the closest that you will come to the vessel in question should you both maintain your current course and speed.

TCPA: Time to Closest Point of Approach. This states when the closest pass will occur.

Dangerous vessels

You can set your chartplotter to display ‘dangerous’ vessels that you need to keep an eye on. These can be defined in the settings, where you can set Closest Point of Approach (CPA) and Time to Closest Point of Approach (TCPA) parameters. Here, we’ve set the system to classify a target as dangerous when it comes within 3048m, and/or when its TCPA is under 5 minutes.

Dangerous vessels are depicted on Zeus and Vulcan chartplotters and on Triton2 instruments with a heavy outline.

Vessel Alarms

In some situations, it’s useful to be able to set alarms to warn you when an AIS target becomes dangerous – offshore, and when crossing the channel, for instance, when there aren’t many boats around. You can do this in the settings menu – just enable Alarms >Vessels > Dangerous vessels. Conversely, the alarms will be going off constantly if alarms are enabled in a busy waterway or marina – so it’s worth getting used to switching them on and off.

Extension Lines

It’s really helpful to enable extension lines. These give a vector of a vessel’s course and heading (you can set the length to a set time). If you also enable your own heading lines, you will see where both you and any vessels around you are and, more importantly, where you’ll all be in 10 minutes, which can be a big help if you’re trying to work out if a situation is dangerous or not. Here, the other vessel will pass safely ahead. These extension lines also help to show relative speed, so you can easily identify a high-speed craft.

Last position report

Vessel icons will continue to be displayed even when the signal is lost for a preset period of time. If a target hasn’t moved for a while and you’re worried, you can check the ‘age’, or the time the signal was last received. A different icon (strikethrough) appears when the signal has been lost.


Recently drift nets, buoys, offshore oil platforms and other hazards have been fitted with AIS transmitters. These Aids to Navigation or AtoNs give a useful real-time location of marks, which can be especially useful for location in strong tidal flows where they may be dragged down-tide. They have a different icon to vessels -  a diamond with an 'x' inside. 


Personal AIS-enabled rescue beacons are becoming more popular to help crews locate a Man Overboard. These transmit an MMSI number with the prefix of 972 to identify them. Up-to-date chartplotters will display these correctly, but older ones may need a software update to correctly identify the signal and give you the option of initiating a Man Overboard based on its position.

Check you have the latest software here


As Class B AIS transmitters become more popular, so the number of icons increases in busy waterways. If you’re struggling to make sense of your charts, you can filter the targets, for instance only displaying dangerous vessels. You can also hide all, if you don’t need AIS at present.  This must be used with care, but can be a useful way to increase the visibility of the screen. This was the Round the Island race in the UK...

Linked VHF DSC radio

If you have a compatible VHF DSC radio on your network, such as the B&G V60 and V90S, you can initiate DSC calls to vessels with a single tap on the screen.

Explore B&G VHF radios

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