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 (AIS) is a digital, VHF-based transponder system that enables vessels to automatically share identification and navigation data. AIS transceivers broadcast information at dynamically adjusted intervals based on vessel speed and maneuvering state. Depending on the unit's class, transmitted data includes dynamic information—such as position, speed over ground, and course over ground—as well as static details like vessel name, MMSI number, call sign, ship type, and dimensions. Receive-only AIS units process incoming signals without transmitting any data, while nearby vessels within VHF range receive these broadcasts and display them, typically as an overlay on a chartplotter or dedicated screen.
By integrating these transmissions with onboard electronics, AIS displays a real-time traffic picture of transmitting vessels in your vicinity. Selecting a target icon on your chartplotter reveals the vessel's name, course, speed, MMSI, and call sign, along with calculated collision-avoidance metrics such as Closest Point of Approach (CPA) and Time to Closest Point of Approach (TCPA), providing mariners with detailed information about AIS-equipped traffic that radar alone cannot provide.
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 can provide vessel identity and, when transmitted, voyage-related information such as destination and ETA; radar primarily shows physical targets and does not inherently identify them
- AIS is not affected by rain clutter or fog in the way radar can be; radar performance can decline in precipitation
- AIS is primarily a line-of-sight system, so land, structures and other obstructions can block transmissions just as they can affect other VHF communications
- 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 can provide valuable collision-avoidance information about AIS-equipped commercial vessels, while radar provides awareness of physical targets whether or not they carry AIS.
AIS Range
AIS is primarily a line-of-sight system, and its range is influenced by the height and performance of both the transmitting and receiving antennas, transmitter power, and physical obstructions. A properly installed masthead antenna generally provides greater range than a deck-level antenna. Under typical conditions, AIS and VHF radio have comparable ranges, although actual AIS range can vary considerably.
Class A, Class B, or Receive-Only — Which Do You Need?
Class A AIS
Class A AIS is the full-function standard required for certain commercial vessels under SOLAS and U.S. regulations. Under SOLAS, AIS is required on ships of 300 gross tons and above engaged on international voyages, cargo ships of 500 gross tons and above not engaged on international voyages, and passenger ships regardless of size. U.S. domestic AIS requirements also apply to specified commercial vessels under 33 CFR 164.46.
Class A units transmit at 12.5 watts nominal RF power, with position reports typically transmitted every 2–10 seconds while underway and every 3 minutes when at anchor or moored. Class A units support a more comprehensive AIS data set than Class B units, including voyage-related information such as destination, ETA, and draft, as well as the vessel's IMO number and, where available, rate-of-turn information.
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 is the appropriate choice for the vast majority of recreational boaters. It provides the core transmit-and-receive functions recreational boaters typically need at a fraction of the cost and power consumption of Class A. Key differences from Class A:
- Class B/CSTDMA units transmit at 2 watts, while Class B/SOTDMA units transmit at 5 watts nominally. Both are lower-power alternatives to Class A's 12.5-watt nominal transmit power.
- Class B/CSTDMA units generally transmit position reports at longer intervals than Class A, typically every 30 seconds while underway. Class B/SOTDMA units can transmit position reports more frequently, with the interval varying according to vessel speed and operating conditions.
- Class B units do not transmit the full range of voyage-related and vessel data supported by Class A, including destination, ETA, draft, and IMO number.
- Class B units have significantly lower power consumption, making them suitable for vessels with limited electrical budgets.
A Class B transceiver both transmits your vessel's data to other AIS-equipped vessels and receives AIS traffic within reception range. 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 operating in areas with significant AIS traffic: Class B transceiver
- Budget-constrained or primarily interested in monitoring commercial traffic: Receive-only receiver
- Commercial vessel subject to applicable Class A requirements: Class A
- Offshore passage-making: Class B transceiver at minimum; a Class B/SOTDMA unit can provide more frequent position reports and more efficient access to transmission slots in areas with heavy AIS traffic.
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 used to identify your vessel in AIS transmissions. It is also used by DSC (Digital Selective Calling) VHF radios for automated distress and calling functions.
FCC Programming Requirements
Class B AIS units must be programmed with accurate static vessel information, including the MMSI number. For FCC-certified Class B AIS devices, static vessel information must be entered by the vendor or an appropriately qualified marine-electronics installer. Follow the manufacturer's instructions and applicable FCC requirements.
West Marine offers AIS pre-programming for eligible units. When you order an AIS transceiver, submit your vessel and MMSI information during checkout and, where the service applies, your unit will arrive pre-programmed and ready to install.
How to Get an MMSI Number
For recreational vessels operating exclusively in U.S. waters, an MMSI may be obtained through authorized recreational MMSI providers such as BoatUS or Sea Tow. Vessels that require an FCC Ship Station License obtain their MMSI through the FCC as part of the licensing process. Check current FCC requirements to determine which applies to your vessel. 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)
- GNSS 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. Many AIS units support NMEA 2000, while others use NMEA 0183 or both standards.
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.
Antenna splitters introduce some insertion loss, although the amount varies by design. A quality AIS-rated splitter can have relatively little loss, but a dedicated AIS antenna generally provides the best performance and also gives you a separate backup antenna for the VHF system.
GPS Antenna
AIS transceivers need a reliable GNSS position source to transmit accurate position data. Many units have a built-in GPS/GNSS antenna, while others require or optionally support an external antenna. Some can also receive position data from another compatible navigation source through the boat's network. Check the manufacturer's specifications to determine what's required.
USCG AIS Carriage Requirements
AIS is not legally required for recreational vessels in the U.S. Federal AIS carriage requirements apply to specific categories of commercial and other vessels, including:
- 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 can provide a significant safety advantage for vessels that transit shipping lanes, operate at night, or navigate in restricted visibility.
Browse All Marine Electronics and Navigation Gear
- Chartplotters, GPS units, radar systems, and integrated navigation displays
- Sonar, CHIRP, downscan, and fish-finding electronics
- Marine stereos, speakers, amplifiers, and audio accessories
- VHF radios, AIS, satellite communication equipment
- Autopilot systems and steering control components
- Wind, depth, speed, and multifunction instruments
- Onboard monitoring and security systems
- Paper and electronic navigation charts
- Traditional navigation plotting tools
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 — including position, speed, heading, vessel name, and MMSI — from AIS-equipped vessels within reception 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 12.5 watts nominally, with position reports typically transmitted every 2–10 seconds while underway, and is required for certain commercial vessels. Class B/CSTDMA typically transmits at 2 watts with position reports generally every 30 seconds while underway, while Class B/SOTDMA units transmit at 5 watts nominally and can provide more frequent position reports. Both Class B types transmit your vessel's data and receive AIS traffic within reception range. Receive-only units display surrounding AIS traffic but do not transmit, meaning your vessel cannot be seen by other vessels on AIS. For collision avoidance, a Class B transceiver provides a significant safety advantage over a receive-only unit.
Can I program my AIS unit myself after purchase?
Programming procedures vary by manufacturer and model. Some AIS units are programmed by the seller or installer, while others provide controlled owner programming. Follow the manufacturer's instructions and applicable FCC requirements. If you purchase an eligible AIS transceiver from West Marine, pre-programming is available when you provide the required vessel and MMSI information.
How do I get an MMSI number for my AIS unit?
For recreational vessels operating exclusively in U.S. waters, an MMSI may be obtained through authorized recreational MMSI providers such as BoatUS or Sea Tow. Vessels that require an FCC Ship Station License obtain their MMSI through the FCC as part of the licensing process. 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, while others use NMEA 0183 or support both standards. With a compatible NMEA 2000 network and chartplotter, AIS target data can be shared over the network without a dedicated serial connection. If your boat does not have an NMEA 2000 network, an NMEA 0183 connection to your chartplotter can provide AIS target data through a serial connection.
How far can AIS transmissions reach?
AIS is primarily a line-of-sight system, so range depends largely on antenna height, the performance of the transmitting and receiving antennas, transmitter power, and physical obstructions that can block or reduce reception.
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. Federal AIS carriage requirements apply to specified categories of commercial and other vessels, including certain self-propelled commercial vessels, towing vessels, passenger vessels, dredges, and vessels carrying certain dangerous or bulk flammable or combustible cargoes.
What information does a Class B AIS unit transmit?
Class B AIS transceivers transmit your vessel's MMSI number, vessel name, call sign, vessel type, dimensions, position, speed over ground, and course over ground. They do not transmit the full range of voyage-related and vessel data supported by Class A, such as destination, ETA, draft, and IMO number. The transmitted data is sufficient for other vessels to identify your boat, track your movement, and calculate collision-avoidance information.