What is an AIS - Automatic Identification Systems

By West Marine Staff, Last updated 1/6/2026

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AIS — the Automatic Identification System — is one of the most significant safety advances in recreational boating in the past two decades. It allows you to see every AIS-equipped vessel within VHF radio range on your chartplotter display, with real-time GPS position, speed, heading, vessel name, and collision avoidance data. For anyone who boats in busy shipping lanes, in restricted visibility, or at night, AIS is not optional equipment — it's essential.

Choosing the right AIS unit involves more than picking a price point. Class, networking compatibility, antenna configuration, brand integration, and MMSI registration all affect how well the system will work aboard your specific vessel. This guide walks through every decision point so you can buy with confidence.

What Is AIS and How Does It Work
Class A, Class B, or Receive-Only — Which Do You Need
MMSI Numbers and AIS Programming
Do You Need NMEA 2000 Networking
Should You Match Your AIS Brand to Your Other Electronics
Antenna Splitters and GPS Antennas
USCG AIS Carriage Requirements
AIS FAQ

What Is AIS and How Does It Work

The Automatic Identification System is a digital VHF radio-based transponder system that automatically broadcasts and receives vessel identification and navigation data. Every AIS-equipped vessel continuously transmits its GPS position, speed over ground, course over ground, vessel name, MMSI number, call sign, vessel type, and dimensions. Every other AIS-equipped vessel within VHF range receives that transmission and displays it — typically as an overlay on a chartplotter or dedicated AIS display.

The result is a real-time traffic picture of every significant vessel in your vicinity, accurate to GPS precision. By selecting a vessel's icon on your chartplotter, you can see its name, course, speed, classification, MMSI, call sign, and calculated collision avoidance data including Closest Point of Approach (CPA) and Time to Closest Point of Approach (TCPA). This is information that was previously available only to Vessel Traffic Service operators — now available on every AIS-equipped recreational vessel.

How AIS Differs from Radar

Radar detects physical objects by bouncing radio energy off them. AIS receives digital transmissions that vessels broadcast intentionally. The practical differences matter:

AIS provides vessel identity, name, and destination — radar shows only a return
AIS is not degraded by rain clutter, fog, or heavy weather — radar performance declines in precipitation
AIS propagation is slightly better than radar due to longer wavelength — signals can sometimes reach around bends in rivers or over low landmasses
AIS only shows vessels that are transmitting — a vessel with no AIS or a powered-down unit is invisible to AIS regardless of size
Radar detects any physical object — including vessels without AIS, logs, and rocks

AIS and radar are complementary systems, not alternatives. Offshore and commercial vessels typically run both. For recreational boaters, AIS provides collision avoidance coverage against the large commercial vessels most likely to cause fatal accidents, while radar provides awareness of everything else.

AIS Range

AIS range is essentially the same as VHF radio range — line-of-sight, typically 20 to 30 nautical miles depending on antenna height. A masthead-mounted VHF antenna on a sailing vessel provides significantly better AIS range than a deck-level antenna on a powerboat. The system works identically in all weather — rain, fog, and darkness have no effect on AIS performance.

Class A, Class B, or Receive-Only — Which Do You Need

Class A AIS

Class A AIS is the full-function standard required for commercial vessels over 300 gross tons on international voyages, passenger vessels of any size, and most commercial workboats operating in U.S. waters (see the USCG requirements section below). Class A units transmit at 20 watts with an update rate of every 2–10 seconds underway and every 3 minutes at anchor. They transmit the complete AIS dataset including vessel destination, ETA, draft, IMO number, and rate of turn — data that Class B units do not transmit.

Class A units are substantially more expensive than Class B and consume considerably more electrical power. They are not typically appropriate for recreational vessels unless the vessel is also used commercially.

Class B AIS

Class B AIS was approved for use throughout the U.S. in 2008 and is the appropriate choice for the vast majority of recreational boaters. It offers nearly identical functionality to Class A at a fraction of the cost and power draw. Key differences from Class A:

Transmits at 2 watts instead of 20 watts — sufficient for recreational vessel identification but shorter range than Class A
Update rate of every 30 seconds when traveling above 2 knots (versus 10 seconds for Class A) — adequate for collision avoidance in most recreational scenarios
Does not transmit vessel destination, ETA, draft, IMO number, or rate of turn
Significantly lower power consumption — suitable for vessels with limited electrical budgets

A Class B transceiver both transmits your vessel's data to other AIS-equipped vessels and receives all AIS traffic around you. This is the recommended choice for any recreational boater who regularly transits shipping lanes, operates at night, or navigates in restricted visibility. Being visible to commercial traffic on AIS is a meaningful safety advantage.

Receive-Only AIS

Receive-only AIS units display surrounding AIS traffic on your chartplotter without transmitting your own vessel's information. They are the lowest-cost entry point into AIS situational awareness and are appropriate for boaters who want to see commercial traffic without appearing on the AIS grid themselves.

The critical limitation: receive-only units do not make you visible to other vessels. A ship's officer monitoring their AIS display will not see you. For collision avoidance purposes, mutual visibility is significantly safer than one-way awareness. Think of a receive-only unit as radar without a radar reflector — you can see them, but they cannot see you. For most coastal and offshore boating, a Class B transceiver is worth the additional cost.

Which Class Should You Choose

Recreational vessel, any waters: Class B transceiver
Budget-constrained or inland lakes only: Receive-only receiver
Commercial vessel or vessel operating under SOLAS: Class A
Offshore passage-making: Class B transceiver at minimum; pairing with a Class B SOTDMA unit (the newer standard) provides better channel access in heavy traffic areas

Browse our full selection of AIS transceivers and receivers filtered by class and features.

MMSI Numbers and AIS Programming

What Is an MMSI Number

An MMSI (Maritime Mobile Service Identity) is a unique nine-digit number assigned to your vessel. It is the identifier that AIS broadcasts to other vessels — when another boater selects your vessel icon on their chartplotter, your MMSI is what links to your vessel name and registration data. MMSI numbers are also used by DSC (Digital Selective Calling) VHF radios for automated distress calls.

FCC Programming Requirements

FCC regulations require that all Class B AIS units have their static data — MMSI number, vessel name, call sign, vessel dimensions, and GPS antenna position — programmed by a qualified technician before operation. Self-programming is not permitted for FCC compliance. West Marine handles this programming for you: when you order an AIS transceiver, submit your vessel and MMSI information during checkout and your unit will arrive pre-programmed and ready to install.

How to Get an MMSI Number

For recreational vessels operating only in U.S. waters, a free MMSI number is available through BoatUS or Sea Tow. For vessels that operate in international waters or for any commercial vessel, an MMSI must be obtained through the FCC as part of a Ship Station License. Visit the FCC Ship Station Licensing page for current instructions and fees.

Information Required for West Marine Pre-Programming

When ordering an AIS transceiver from West Marine, have the following ready to submit at westmarine.com/ais:

Order number
Shipping and contact information (name, address, phone, email)
Manufacturer and model of AIS unit
MMSI registrant name
MMSI number (nine digits)
Serial number (required for em-trak brand units only)
Vessel name (up to 20 characters)
Vessel call sign (if available)
GPS antenna position in meters from bow, stern, port side, and starboard side

Some AIS units, including the em-trak B100, can operate in receive-only mode before MMSI programming is complete, allowing you to monitor traffic while waiting for your programmed unit or paperwork.

Do You Need NMEA 2000 Networking

NMEA 2000 is the current standard networking protocol for marine electronics. A vessel with an NMEA 2000 network can share data — GPS position, depth, wind, AIS targets — between all connected instruments over a single backbone cable. Approximately half of AIS units on the market support NMEA 2000; most are also backward-compatible with the older NMEA 0183 standard.

If your boat already has an NMEA 2000 network with a compatible chartplotter, choosing an NMEA 2000-capable AIS unit allows AIS targets to display directly on your chartplotter without additional wiring. If your boat uses NMEA 0183 or has no existing network, an NMEA 0183-only AIS unit connected by serial cable to your chartplotter achieves the same result — it just requires individual wiring rather than plug-and-play network connection.

For new electronics installations or upgrades, NMEA 2000 compatibility is worth prioritizing — it simplifies installation, reduces cable runs, and makes future expansion easier.

Should You Match Your AIS Brand to Your Other Electronics

Brand matching is worth considering when it unlocks cross-device features that provide genuine additional capability. Two examples worth knowing:

Garmin: Garmin's AIS units integrate with compatible Garmin chartplotters and VHF radios via NMEA 2000, enabling direct MMSI calling — you can initiate a DSC call to any AIS target directly from the chartplotter by selecting the vessel and choosing "call with radio," without manually entering the MMSI number.
Raymarine: Raymarine's AIS units support Buddy Tracking when used with compatible Raymarine multifunction displays — allowing you to save AIS targets with a custom icon and friendly name, making it easy to track specific vessels (a buddy's boat, a harbor patrol vessel) among a crowd of traffic.

If your chartplotter and VHF radio are already from the same manufacturer, check whether that brand's AIS unit unlocks integration features before defaulting to a standalone AIS receiver. If your electronics are mixed brands, any NMEA 2000 compatible AIS unit will display targets on your chartplotter without brand-specific features.

Antenna Splitters and GPS Antennas

VHF Antenna Splitter

AIS uses the same VHF frequency band as your VHF radio, which creates an antenna question: do you share your existing VHF antenna with your AIS unit, or install a dedicated AIS antenna? Sharing is possible with an antenna splitter — a passive device that allows both the AIS unit and VHF radio to connect to a single antenna.

Splitters introduce a small signal loss (typically 3 dB, equivalent to roughly half your antenna's effective power), which slightly reduces range. For most recreational applications this is acceptable; for maximum AIS performance, a dedicated antenna is better. A few AIS units include a built-in splitter (Garmin's AIS 800 is one example). Others require an add-on splitter sold separately — Simrad and em-trak both make compatible units. Verify whether a splitter is included before purchasing if you plan to share an antenna.

GPS Antenna

AIS transceivers require a GPS position fix to include accurate coordinates in their transmissions. Most AIS units include a dedicated external GPS antenna. Some units can share GPS data from an existing NMEA 2000 network if a GPS source is already connected — check the specifications for the unit you're considering to confirm what's included and what's required.

USCG AIS Carriage Requirements

AIS is not legally required for recreational vessels in the U.S. However, all commercial vessels operating in U.S. waters are required to carry a USCG-certified AIS transceiver under rules that took effect in 2015. This significantly expanded the previous requirement, which applied only to vessels over 300 gross tons on international voyages under SOLAS. The expanded USCG rule now covers:

Self-propelled commercial vessels of 65 feet or more
Vessels of 26 feet or more carrying more than 6 passengers for hire
Commercial towing vessels of 26 feet or more
Commercial dredges and floating plants on navigable waters

If you operate a vessel commercially in any of these categories, consult the USCG AIS requirements page for current regulations, exemptions, and certification requirements. Regulations can change — verify current requirements with the USCG or a maritime attorney if you are unsure whether your vessel is covered.

For recreational boaters, AIS carriage is voluntary but strongly recommended by maritime safety organizations for any vessel that transits shipping lanes, operates at night, or navigates in restricted visibility.

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AIS System — Frequently Asked Questions

What is AIS and how does it help boaters?

AIS is a VHF radio-based system that automatically broadcasts and receives vessel identification and navigation data — position, speed, heading, vessel name, and MMSI — between all AIS-equipped vessels within range. It displays surrounding traffic on your chartplotter with GPS-level accuracy and calculates collision avoidance data including Closest Point of Approach (CPA) and Time to Closest Point of Approach (TCPA). It is particularly valuable at night, in fog, and in busy shipping lanes where awareness of large commercial vessel traffic is critical.

What is the difference between Class A, Class B, and receive-only AIS?

Class A transmits at 20 watts with high update frequency and is required for large commercial vessels. Class B transmits at 2 watts with a 30-second update rate and is designed for recreational vessels — it both transmits your vessel's data and receives surrounding traffic. Receive-only units display surrounding AIS traffic but do not transmit, meaning you cannot be seen by other vessels on AIS. For collision avoidance, a Class B transceiver is significantly safer than a receive-only unit.

Can I program my AIS unit myself after purchase?

No. FCC regulations require Class B AIS units to be professionally programmed with your MMSI number and vessel data before operation. West Marine pre-programs AIS transceivers at no additional charge — submit your vessel information at westmarine.com/ais when placing your order and your unit arrives ready to install.

How do I get an MMSI number for my AIS unit?

For recreational vessels operating exclusively in U.S. waters, a free MMSI is available through BoatUS or Sea Tow. For vessels that travel internationally or operate commercially, you must obtain an MMSI through the FCC as part of a Ship Station License. Have your MMSI number ready before ordering your AIS unit — it is required for programming.

Do AIS units work with NMEA 2000 networks?

Many AIS units support NMEA 2000, which allows AIS target data to flow directly to compatible chartplotters over the boat's existing network backbone without individual wiring. Most NMEA 2000-capable AIS units are also backward-compatible with NMEA 0183. If your boat does not have an NMEA 2000 network, an NMEA 0183 connection to your chartplotter achieves the same display result via serial cable.

How far can AIS transmissions reach?

AIS range mirrors VHF radio range — line-of-sight, typically 20–30 nautical miles depending on antenna height. A masthead antenna provides significantly better range than a deck-level installation. Unlike radar, AIS is unaffected by rain, fog, or heavy weather, and signals can sometimes propagate around low landmasses or river bends where radar line-of-sight is blocked.

Should I match my AIS brand to my chartplotter?

If your electronics are already from a single manufacturer, check whether that brand's AIS unit offers integration features. Garmin AIS units enable direct MMSI calling from the chartplotter via compatible VHF radios. Raymarine AIS units support Buddy Tracking on compatible multifunction displays. If your electronics are mixed-brand, any NMEA 2000-compatible AIS unit will display targets on your chartplotter without those brand-specific extras.

Is an antenna splitter included with AIS units?

Not always. Some units — including the Garmin AIS 800 — include a built-in VHF antenna splitter. Others require a separately purchased splitter if you want to share your existing VHF antenna. Simrad and em-trak make compatible add-on splitters. Check product specifications before purchasing if you plan to share an antenna rather than installing a dedicated AIS antenna.

Is AIS required on recreational boats?

AIS is not legally required for recreational vessels in the U.S. It is required for most commercial vessels operating in U.S. waters. For recreational boaters, AIS carriage is voluntary but strongly recommended by maritime safety organizations for vessels that transit shipping lanes, operate at night, or navigate in restricted visibility or fog.

What information does a Class B AIS unit transmit?

A Class B AIS transceiver transmits your vessel's MMSI number, vessel name, call sign, vessel type, dimensions, GPS position, speed over ground, and course over ground. It does not transmit destination, ETA, draft, IMO number, or rate of turn — those fields are Class A only. The transmitted data is sufficient for other vessels to identify your boat, track your movement, and calculate collision avoidance information.

Marine AIS Buying Guide: How to Choose the Right AIS Unit for Your Boat