Article by Capt. Steve Katz
This week, we are following up on our previous column about the marine MMSI number to dive deeper into the Automatic Identification System (AIS). While we briefly touched upon AIS before, we will now provide a detailed review of its technical aspects and functionality.
Interestingly, AIS has been a prominent topic in marine news recently due to a new law in the Bahamas. This law, set to become effective this week, will require AIS on all visiting vessels over 50 feet operating in Bahamian waters, among other new regulations for foreign vessels. While there has been considerable discussion surrounding the Bahamian AIS requirements, it is worth noting that the USA also mandates AIS on many vessels, though generally not for traditional pleasure boats.
What is Marine AIS and How Does It Work?
AIS stands for Automatic Identification System, and it is an essential tool for modern marine navigation and safety. Utilized by commercial vessels, many recreational boats, and shore-based stations, AIS significantly improves situational awareness and helps prevent collisions at sea.
AIS is an automatic tracking system that employs transceivers on ships and is also monitored by vessel traffic services (VTS). The system continuously transmits and receives vessel information over dedicated VHF radio frequencies. For recreational boaters, installing AIS is a voluntary option that offers enhanced safety.
AIS operates using marine VHF radio channels, specifically 161.975 MHz and 162.025 MHz, to digitally broadcast and receive information between ships and shore stations. There can be some confusion regarding AIS due to the types or classes available:
Class A: This class is required on large commercial ships. It broadcasts at a high power of 12.5 watts, updates every 2–10 seconds, and operates with a higher transmission priority in congested waters. The hardware for Class A is typically the most expensive.
Class B+: Often referred to as “Class B SOTDMA” or “Class B 5W,” this class bridges the gap between Class A and Class B. It operates at a medium power of 5 watts and has a higher transmission priority than standard Class B. The hardware costs are in the medium range.
Class B: Designed for recreational and smaller vessels, Class B operates at a lower power of 2 watts, updates every 30 seconds, and has a lower priority in congested waters. This class offers the lowest cost hardware.
Additionally, many marine VHF radios offer a “Receive only” AIS capability. This allows them to receive an AIS transmission and display the received vessel information on the VHF screen, as well as on a connected chartplotter or similar device. Due to the complexity of FCC and international standards, an AIS transmitter is not currently offered as an internal capability of a VHF radio; a separate AIS transmitter is required.
AIS Transmits the Following Crucial Data:
Dynamic Data (automatically updated):
GPS position (latitude & longitude)
Speed overground (SOG)
Course overground (COG)
Heading
Rate of turn
Navigational status (e.g., underway, at anchor)
Static & Voyage Data (entered manually in Class A):
MMSI number (Marine ID)
Vessel name and call sign
Type of ship and cargo
Length and beam
Destination
Estimated time of arrival (ETA)
Where Does AIS Data End Up?
The information transmitted by a ship’s AIS is utilized in several ways:
Nearby vessels: To monitor traffic and help avoid collisions.
Coast stations and VTS: To manage harbor or waterway traffic efficiently.
Satellites: For tracking vessels beyond the horizon or offshore.
AIS data is typically displayed on chartplotters, radar overlays, ECDIS (Electronic Chart Display and Information Systems), and various marine navigation applications.
Internet AIS
Many websites and apps, such as Marine Traffic or AIS Live, now display ship AIS data on computers or smartphones. This can be an enjoyable way to monitor ship traffic around the world or to track family members on a voyage. These systems often rely on land-based volunteers who share their local AIS transmissions with these websites. This is accomplished using an AIS base station, which typically consists of an outdoor AIS antenna and an indoor receiver connected to the internet.
There is often confusion when someone’s boat (or a friend’s boat) does not appear on an AIS website. This is frequently due to the absence of a local base station to receive their signal and send it to the internet, rather than a problem with the vessel’s AIS unit itself.
Why AIS Matters
Collision avoidance:
AIS provides crucial information, allowing mariners to know who is around them and where they are going, significantly reducing the risk of collisions.
Situational awareness: It enables mariners to “see” beyond their radar range or through fog and other obscured conditions, enhancing overall awareness.
Search and Rescue (SAR):
AIS greatly assists in locating vessels in distress, making search and rescue operations more efficient.
Regulatory compliance: Commercial vessels are required to carry AIS under IMO and U.S. Coast Guard regulations.
