AIS Class A vs Class B: Differences and Installation Guide
AIS shows the identity and course of nearby ships during night and fog passages. We explain the difference between Class A and Class B and how to install it correctly.
AIS (Automatic Identification System) broadcasts a nearby vessel's identity, position, speed and course over VHF frequencies in real time. In fog, at night or in busy traffic, knowing exactly which ship a radar echo belongs to, where it's headed and how fast it's moving substantially reduces collision risk. This guide covers how AIS works, the differences between Class A and Class B transponders, and the correct installation steps.
How does AIS work?
AIS units broadcast GPS position and vessel data (name, MMSI, length, type, speed, course) on VHF frequencies at regular intervals; other AIS units listening on the same frequencies pick up this data and display it as a target on a chartplotter or radar screen. To avoid collisions between transmissions, the system uses SOTDMA (Self-Organizing Time Division Multiple Access), a time-sharing protocol in which each unit transmits within its own allotted time slot.
Broadcast frequency depends on a vessel's speed and manoeuvring state: a fast vessel that's turning broadcasts position several times a second, while an anchored boat might broadcast only every few minutes. This dynamic reporting rate lets the system use its limited frequency capacity efficiently.
AIS also runs on two separate VHF channels (AIS 1 and AIS 2) simultaneously, so a temporary interference or congestion issue on one channel doesn't interrupt the data stream. Think of the system less as a plain receiver-transmitter and more as a "digital lookout": it announces the identity of a distant target the human eye can't yet resolve on the horizon, even while you're inside the helm station.
Class A vs Class B
AIS transponders fall into two main classes:
| Feature | Class A | Class B |
|---|---|---|
| Requirement | Mandatory for commercial vessels under SOLAS | Generally optional, for private craft |
| Transmit power | 12.5 W | Typically 2 W (some SO variants up to 5 W) |
| Reporting rate | Every few seconds, scaling with speed | Slower than Class A (typically 30 seconds to a few minutes) |
| Data broadcast | Comprehensive static and voyage data, destination, draft | Basic identity and position data; some models offer extended data |
| Display/integration requirement | Standalone display or MFD integration mandatory | Usually viewed via MFD or chartplotter |
| Typical cost | High | Noticeably lower than Class A |
For private yachts and boats, Class B is usually enough: both unit cost and installation complexity are lower than Class A. In areas with heavy commercial traffic, the higher-power "Class B/SO" (SOTDMA) variant broadcasts more often and appears closer to a Class A target's priority on large ships' Class A receivers.
Installation: antenna, splitter and MMSI
Steps to get an AIS installation right:
- Antenna choice: A dedicated VHF antenna gives the best performance. Those wanting to share an existing VHF antenna can use an AIS/VHF splitter, which briefly prioritises AIS reception while the VHF radio is transmitting.
- GPS source: Most AIS units come with a built-in GPS antenna; on larger boats an external GPS antenna, or sharing the existing GPS over NMEA 2000, may be preferred.
- NMEA connection: The AIS unit passes target data to the chartplotter over NMEA 2000 or NMEA 0183. If you need to bridge the two protocols, use a suitable gateway; we cover this in detail in our NMEA 2000 network installation guide.
- MMSI registration: The vessel's MMSI number is entered into the unit; if the VHF radio has DSC, the same number must be defined there too. See our VHF radio and antenna installation guide for more.
- Entering static data: Vessel name, length and type are entered once and don't need re-entering unless they change.
- Testing: After installation, confirm your own vessel's data appears on another AIS receiver — a marina office, another boat, or an online AIS tracking site.
Using AIS alongside radar and the chartplotter
AIS alone isn't a complete collision-avoidance solution — small craft, fishing boats and some buoys don't transmit AIS. That's why it should be considered alongside the radar system covered in our radar installation guide: radar shows every object that returns an echo, while AIS supplies that object's identity and intent (course, speed). Most modern chartplotters and MFDs overlay AIS targets on the radar picture, giving integrated situational awareness on a single screen.
On long passages with the autopilot engaged, AIS alarms are especially valuable for spotting approaching traffic in time; we cover this integration in our autopilot faults and calibration guide too. The IMO's AIS page summarises the system's international carriage requirements and technical background, while the USCG Navigation Center's AIS requirements page is a practical reference for which vessel types need which class.
Common issues
- My own vessel's data isn't visible to others: Check the antenna connection, a splitter fault, or an MMSI that was never entered.
- Targets update slowly on screen: Network load or an incorrect baud rate (on NMEA 0183 connections) can slow the data stream.
- Some vessels don't appear: Small craft not carrying AIS simply won't show; a large ship not appearing may instead point to antenna shadowing or VHF reception conditions.
- DSC and AIS MMSI mismatch: If different numbers were entered for the same vessel, distress calls can become confused; confirm the same MMSI is defined in both units.
Maintenance and winterizing after installation
AIS units are generally low-maintenance electronics, but the antenna and connectors are exposed to the marine environment. Corrosion or moisture at antenna cable connections can gradually reduce receive/transmit power, so an annual visual check and corrosion protection on connectors is worthwhile. On installations using a splitter, occasionally test that it still correctly prioritises AIS reception during VHF transmission — an old or faulty splitter can degrade both VHF and AIS performance.
It's worth including AIS in your season-end review of the boat's overall electrical and electronic systems; see our winterizing guide for the detailed steps. At season opening, confirming MMSI and static data (vessel name, length, type) are still correct, the software is current, and vessel data is actually being broadcast is a natural part of the general electronic checks we recommend in our season opening checklist.
Work with Aslan Yacht on AIS installation
Our team in Turgutreis handles Class B AIS installation, VHF antenna/splitter integration and MMSI programming end to end across marinas around Bodrum. See our electronics and navigation services or get in touch for details.
Frequently asked questions
Does a yacht need Class A or Class B?
Class A is mandatory for commercial vessels under SOLAS. Private yachts and boats are generally well served by Class B, which also costs less; owners wanting greater visibility in dense commercial traffic can consider higher-power Class B/SO variants.
Does AIS need its own antenna, or can it share the existing VHF antenna?
A VHF splitter lets you share the existing VHF antenna with AIS, though this causes a small signal loss. A dedicated AIS antenna is recommended for boats that use VHF heavily or want the best possible performance.
How do I get an MMSI number?
In Türkiye, MMSI is issued through the maritime authority as part of vessel registration. Once assigned, enter the same number into both the AIS unit and any DSC-equipped VHF radio; mismatched numbers across the two devices cause confusion between the systems.
