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Understanding AIS

The Automatic Identification System uses marine VHF radio channels to exchange vessel identity, position, course, speed and other reported information between equipped vessels and shore stations.

An AIS installation may include a transceiver, GNSS input, VHF antenna or splitter, navigation-data connections and one or more displays. Its usefulness depends on correct programmed information, valid sensor data, a dependable radio path and an operator who understands the system’s limitations.

This free technical guide gives yacht owners and prospective buyers a practical introduction to common Class A and Class B AIS installations. It explains how AIS information is created, transmitted and displayed, and why it should always be cross-checked against radar, visual lookout and other navigation evidence.

Download the Free Guide

Why AIS matters

AIS can improve traffic awareness by showing information reported by other equipped vessels, including identity, position, course and speed.

However, AIS is a reporting system rather than a complete detection system. A vessel may be absent because it is not fitted with AIS, is not transmitting, has faulty equipment, is hidden by display filters or is reporting incorrect information.

Information that appears precise may also be stale or based on incorrect GNSS, heading or programmed data. Target vectors and closest-point calculations should therefore support—not replace—radar observation, visual lookout and collision-avoidance judgement.

Shared equipment can create additional weaknesses. A failed VHF antenna splitter, navigation-network gateway, power supply or GNSS source may affect AIS alongside voice communications or other navigation systems.

What this guide covers

The guide explains:

  • How AIS exchanges vessel and navigation information

  • Static data including MMSI, name, call sign, dimensions and vessel type

  • Dynamic data including position, course, speed and heading

  • AIS transceivers and their transmission status

  • Class A, Class B and receive-only equipment

  • GNSS antennas and navigation-data inputs

  • Dedicated VHF antennas and active antenna splitters

  • Displays, chart integration and navigation networks

  • Target age, filters and CPA/TCPA alarms

  • Voyage data and antenna-reference dimensions

  • Silent or transmit-off modes

  • Reception range and antenna diagnostics

  • Shared power, antenna, GNSS and network dependencies

  • Stale targets, incorrect data and common warning signs

  • Verification using another receiver, radar and visual lookout

  • Offshore readiness and operation after AIS failure

Who is this guide for?

This guide may be particularly useful if you are:

  • Considering the purchase of a motor yacht

  • Preparing a yacht for extended or offshore cruising

  • Trying to understand an unfamiliar AIS installation

  • Comparing Class A and Class B equipment

  • Checking programmed vessel identity and dimensions

  • Investigating short reception range or missing targets

  • Reviewing shared VHF antenna or splitter arrangements

  • Experiencing stale data, GNSS alarms or unreliable target information

  • Planning navigation-equipment maintenance or replacement

  • Looking to improve crew understanding of AIS limitations

Questions the guide will help you answer

After reading the guide, you should be better placed to answer:

  • What class of AIS equipment is installed?

  • Is the unit transmitting, receiving or receive-only?

  • Are the yacht’s MMSI, name, call sign, type and dimensions correct?

  • Does the antenna-reference position match the actual installation?

  • What GNSS and navigation-data sources supply the transceiver?

  • Is the AIS using a dedicated VHF antenna or a shared active splitter?

  • Could a splitter fault affect both AIS and voice VHF?

  • Does another receiver see the yacht’s transmitted information correctly?

  • Is reception range reasonable for the antenna height and installation?

  • Are target age, filters and alarm settings understood?

  • Could targets be hidden, slept or removed by the current display settings?

  • Is silent or transmit-off mode active?

  • Could incorrect or stale data be affecting CPA and TCPA calculations?

  • Do multiple displays depend on one network gateway or AIS source?

  • What radar, visual and communication options remain if AIS becomes unavailable?

Download the guide

Gain a clearer understanding of how AIS transmits and displays vessel information, how to verify its performance and why its limitations must remain part of safe navigation.

Download the Free Understanding AIS Guide
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