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What is AIS and How Does it Work?

Garmin GPSMAP 8424 with AIS
The Garmin GPSMAP 8424 with AIS

A remarkable electronic navigation aid at an affordable price

AIS, which is short for Automatic Information System, is an easy-to-use navigation aid that transmits your vessel information to other AIS receivers in your area while receiving similar information from other vessels.

Using AIS technology, you can track multiple vessels in real time on a variety of applications, like a GPS or radar screen. Boats equipped with AIS-enabled displays can see the positions of other AIS-equipped boats up to 46 nautical miles away, even if they are behind obstructions like an island that would normally block radar. The information supplied for each vessel can vary, but the situational awareness that can include the targeted boat’s size, draft, position, speed, direction, cargo, destination, and more, is invaluable for navigation purposes.

Perhaps the best way to explain AIS is to give you an actual demonstration. Have you ever read an interactive boat article before? Lets give it a try right now.              

Please do this: On another device or in a separate tab go to You will see a world map with thousands of small icons representing ships and cruisers in real time. Zoom in towards a specific area. Then click on any icon to read about the boat you selected. Wow! If that didn’t surprise you, did you know your boat can be added to the ones you see? If you do this, you can search all of the information on other boats from your own boat as well.

The AIS electronic device was originally developed for pilots to identify air traffic around their plane. Realizing its potential for other forms of transit, it has since expanded to many forms of commercial and recreational transportation. It is becoming commonplace in smaller pleasure craft, as well as full sized ships.

During the mid-90s, the International Maritime Organization (IMO) and International Telecommunications Union (ITU) worked together to adopt a single worldwide identification system. They decided on the VHS-based AIS system that is used today.  The range for an AIS signal is about 46 miles (74 km), and it uses one of two VHF frequencies, 161.975 MHz or 162.025 MHz . Currently, the IMO requires AIS use on all passenger vessels, ships greater than 500GT, and any vessel weighing over 300GT that is traveling internationally. 

AIS ‘receivers’ will allow you to receive data from other vessels, while AIS ‘transponders’ allow you to receive data and continuously transmit your vessel's identity, position, speed, heading, and registry information to all other vessels.  AIS transmissions are almost unaffected by sea clutter or rain, which means they can operate effectively in conditions where targets might be invisible to radar. I should, however, emphasize that AIS does not replace radar because not all target vessels have AIS, and some that do may have them turned off.

Garmin AIS 800 Blackbox Transceiver
Garmin AIS 800 Blackbox Transceiver

Transponders that both send and receive data are referred to as Transceivers. As I noted, AIS for boats broadcasts data that consists of information about the vessel, as well as their navigation status. Transceivers use VHF radio and GPS technology to communicate with other nearby ships. Satellite tracking technology can be combined with other important navigation information and automatically communicated between AIS equipped vessels without any user interaction. 

“Satellite AIS” (S-AIS), greatly extends the range of traditional AIS, and because signals are sent and received from many kilometers above land and sea, the barrier of the horizon doesn't limit these signals. This satellite signal that identifies the positions of vessels is sent to a base station before being relayed to a boat’s smart navigational screen, or phone.

You may ask, “What if they attached an AIS transmitter to an existing fixed navigation aid, such as a channel marker?”  Well they already have, and they call them AIS AtoNs (Automatic Information System Aids to Navigation). This allows you to click the AIS icon showing up on your navigation system or radar and view full details regarding the marker.  You will be able to read the details that these AIS AtoNs provide on your screen before the nav-aid physically comes into viewing range. There are currently 3 types of AIS AtoNs: 

1) “Real” – This AIS AtoN is physically attached to a nav-aid

2) “Synthetic” – not attached, but broadcasting the nav-aid details from shore

3) “Virtual” – where there is no physical aid present, but the information for that location is still broadcast to an AIS receiver 

It is easy to understand how the navigation capabilities of these three AIS AtoNs can greatly help to keep you informed of your situational awareness. 

If you are thinking of ordering an AIS, you will be happy to learn that these transceivers are readily available from major electronic companies like Garmin and Lowrance, and for less money than you might think. For example, a Garmin, “Blackbox” 800 transceiver is less than $1,000 US and is easily connected to your existing navigation electronics. The unit comes with an internal VHF antenna splitter, which allows the VHF and AIS to share a single antenna. It has 5 watts of transmit power and provides fast position reporting. If you don’t already have a Mobile Marine Service Identity number (MMSI) with your VHF, you will need to apply for one. Several organizations such as U.S. Power Squadrons will supply you with a number at no charge if you belong to, or join, their organization.

I have not had much direct experience with AIS, however I can attest to the fact that it can be very comforting. One dark night while I was heading south in a 52’ Hatteras,150 miles off the west coast of southern Mexico, the radar identified a target at 12 o’clock and heading almost directly for me. The target icon showed up on my AIS screen so I quickly clicked on it. AIS immediately identified the vessel as a Holland America cruise ship and to my relief, its direction and speed posed no threat of collision.

AIS usage is becoming popular with boaters in North America, and AIS AtoNs are being attached to more and more navigation aids. Perhaps we should make our next boat accessory purchase an AIS system, instead of that portable bar we wanted.

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