AIS technology: a simple definition

AIS is an automatic identification system used at sea to exchange standardized information between vessels (and with some coastal stations). The goal is simple: better understand the traffic around you and anticipate close-quarters situations.

Important: AIS is an aid. It does not replace visual watchkeeping, attention to the water, or the rules of the road. It is part of an overall safety approach: observation, anticipation, communication, and redundancy (because electronics can fail). And in busy traffic, a maneuverability issue can quickly complicate the situation: if your engine tends to stall, it’s best to fix the problem before heading out again (see our full diagnosis on an engine that stalls).

How AIS works (GPS, VHF, transponder)

An AIS system is built on three components: a GPS (position), a VHF radio (transmit/receive), and an AIS device (receiver or transponder). Information is sent digitally on dedicated VHF channels, then received by compatible vessels.

AIS receiver vs AIS transponder

The difference is simple and important: an AIS receiver lets you see others (you receive their messages); an AIS transponder lets you see and be seen (you receive and you transmit). For leisure boating, being visible to a cargo ship or a ferry—especially at night or in busy areas—can be a real advantage.

AIS classes: A, B, and B+ (with practical examples)

You will often come across these terms: Class A (mostly professional vessels subject to requirements), Class B (common in leisure boating), and Class B+ (an evolution designed to be more “present” than Class B in certain situations).

To better visualize the differences, here are practical examples (exact values depend on standards and models, but the idea remains the same):

• Transmit power: Class A generally transmits stronger than Class B. Result: it is often received farther away and more consistently.
• Update rate: while underway, Class A updates very frequently. Class B updates less often. At 25 knots, a boat travels about 400 meters in 30 seconds: if updates are too slow, the “snapshot” of traffic can become less precise at higher speed.
• Behavior in very busy areas: in dense traffic, a Class A unit generally has better “priority” access to the channel. A Class B+ is intended to improve the presence of leisure boats (faster updates than Class B, depending on the case).

Key takeaway: for typical coastal boating, Class B is often sufficient. However, if you travel fast, at night, or in a very busy area, the value of a more capable device (or better display/alert integration) can be justified.

Which AIS data matters for navigation (COG/SOG, CPA/TCPA)

AIS is not only about plotting dots on a chart: it helps you understand trajectories. Among the truly useful information:

• MMSI: the vessel’s radio identifier. It is used for ASN/DSC: a targeted digital call via VHF (see dedicated section).
• Position: latitude/longitude.
• COG (Course Over Ground): track made good over the ground.
• SOG (Speed Over Ground): speed over the ground.

Most importantly, many displays calculate two very clear indicators: CPA (Closest Point of Approach) and TCPA (Time to Closest Point of Approach). In plain terms: how close you will pass, and in how much time. This is exactly the kind of information that helps you decide early, rather than maneuver at the last moment.

What AIS is used for in leisure boating: real-life examples

1) Night navigation or reduced visibility

When visual recognition becomes difficult (night, fog, rain), AIS helps identify vessels and track their movement. It does not replace observation, but it provides a structured reference: who is coming, from where, at what speed, and what the close-quarters risk is.

2) Harbors, channels, busy areas

In busy areas, AIS helps keep an overall picture of traffic: ferries, cargo ships, fishing vessels, leisure craft… On a good display, higher-risk targets can be highlighted (priority, emphasis, alarm). The goal is to save mental bandwidth: spot what matters without drowning in information.

3) Alarms: collision, anchor drag, man overboard (depending on equipment)

Depending on the equipment and onboard integration, AIS can trigger collision-risk alerts, help detect anchor drag, or signal a man-overboard situation (via dedicated beacons). It all depends on your installation and associated devices.

Personal AIS MOB beacons on a lifejacket: man overboard

On a boat, the most critical event is not always a breakdown: it is often man overboard. In that context, AIS MOB beacons are a very interesting solution: small personal beacons that attach to the lifejacket (or harness).

When activated (manually, and sometimes automatically depending on the model), they transmit an “MOB” signal. This signal can appear as a specific target on nearby AIS displays (chartplotter, AIS VHF, multifunction display…) and can trigger an alarm. This helps the crew locate the person quickly and return to the spot.

• Range varies: AIS works on VHF, so range depends heavily on antenna height and sea state.
• Complementary: an AIS MOB beacon does not replace the lifejacket, harness, tether, or crew reflexes.
• Routine: know where it is mounted, how it activates, and how to recognize the alert on the screen.

For your onboard safety equipment: see the Safety category.

AIS, VHF and ASN/DSC: contacting a vessel quickly

AIS uses VHF to transmit/receive. But there is an additional practical point: AIS can make communications easier when a situation becomes tense.

ASN (Appel Sélectif Numérique) is the same as DSC (Digital Selective Calling): it is a feature on some VHF radios that allows you to send a targeted digital call (or a distress alert) using the MMSI identifier. Concretely, instead of only voice calls, you can send a digital call to a specific vessel.

In a close-quarters situation, this can help you make contact faster: you identify the target, then call it using its MMSI (if your equipment allows it, without having to retype the number). It does not replace vigilance, but it can save time when stress rises.

Displaying AIS on a chartplotter (NMEA 0183 / NMEA 2000) or a tablet

AIS becomes truly comfortable when you display targets on a chartplotter / multifunction display or on an onboard charting solution. To connect devices, marine communication standards are used:

• NMEA 0183: a “legacy” connection, still very common.
• NMEA 2000: a network where devices share a main line (often called a “backbone”). Each instrument connects to it via a connector, making it easier to share data between multiple devices.

On a properly configured display, you can see multiple targets, their course, their speed, and trigger alerts. On some setups, AIS can be correlated with radar: radar detects a target, AIS provides its identity and declared trajectory.

Relevant DAM Marine products (when you are on NMEA 2000)

If you build or expand an NMEA 2000 network, some accessories are genuinely useful (power, terminators, cables). For example:

• NMEA 2000 cables & connectors
• NMEA 2000 power cable with fuse
• NMEA 2000 terminator (120 ohms)

AIS vs GPS vs radar: differences and complementarity

GPS vs AIS: what’s the difference?

GPS gives you your position. AIS is used to exchange information between vessels: position, course, speed, identity. In short: GPS = “where am I?”; AIS = “who is around me and how is it moving?”.

AIS vs radar

Radar sees echoes (even without AIS). AIS provides identity and a declared trajectory, but only if the target is equipped and configured. Together, they complement each other.

Which AIS for your boat? Receiver, AIS VHF, transponder

The right choice depends on your boating program, your area (dense traffic or not), and your existing equipment (chartplotter, VHF with ASN/DSC, display, network). Here is a simple way to think about it:

Option 1: AIS receiver

You see other AIS targets around you. This is already very useful to understand an area, anticipate a cargo ship and navigate with more confidence. Limitation: you do not improve your visibility to other vessels via AIS.

Option 2: AIS transponder (Class B / B+ depending on needs)

You see others and you are visible. In busy areas, this is often the most reassuring option. A more capable solution can be justified if you travel fast or in very dense areas.

Installation: VHF antenna, splitter, wiring, best practices

A reliable AIS installation rests on three pillars: a good VHF antenna, clean wiring, and consistent integration with your display (chartplotter / multifunction display / tablet).

Step 1: VHF antenna

Because AIS uses VHF, performance depends heavily on the antenna: the better it is installed (height, clear view), the more range and reliability you get. Relevant examples:

• VHF antenna RACING 156–162 MHz
• VHF antenna RA1225 6 dB (2.4 m)

Step 2: dedicated antenna or shared antenna (splitter)

Two approaches exist: dedicated AIS antenna (robust solution if you have space), or a shared antenna between VHF and AIS via a splitter. A splitter avoids multiple antennas, but you need the right model and a clean installation.

Step 3: connect AIS to your chartplotter (NMEA 0183 / NMEA 2000)

If you want comfortable display, connecting AIS to your screen is key. On NMEA 2000, take care of the structure: protected power, terminators at both ends, quality cables.

Step 4: settings (alerts and “alert overload”)

Take the time to set alerts (CPA/TCPA). Too many useless alerts and you stop paying attention. Better fewer alerts that are meaningful, adapted to your speed, your area and your boating style.

Don’t rely only on electronics: paper charts and a backup plan

Electronics can fail (low battery, poor connection, fuse, humidity, dead screen). Even though AIS and GPS are very useful, you should keep a minimum navigation capability as a backup.

• Up-to-date paper nautical charts for your area.
• Compass and the ability to follow a simple heading.
• Basic route plan: departure, destination, reference points, alternatives.
• Something to write with: pencil, notebook, light.

To keep a boat reliable all year round, the best time to check your connections, antennas, power supply and safety equipment is often the off-season: winterization is an excellent opportunity to do these checks calmly.

Power consumption: what to anticipate

An AIS unit typically draws a few watts in continuous reception, and more during transmissions (for a transponder). It’s not huge, but on a small battery bank (especially at anchor), everything adds up: lights, instruments, autopilot, fridge… The right approach is clean power supply and a healthy electrical installation.

MarineTraffic: how to use it and why it matters

MarineTraffic is a platform that shows vessels on a map using AIS data received by coastal stations (and depending on services and areas, sometimes via satellite). It is particularly useful to:

• Understand traffic in an area (harbor, bay, channel, ferry route).
• Identify a vessel (name, type, course, speed) when you see it pass.
• Prepare a trip by visualizing active areas and traffic “habits”.

Key point: MarineTraffic is excellent for observing and learning, but it does not replace onboard AIS. Coverage and latency depend on available reception.