What Is AIS On A Boat? Transponder Systems Explained
Understanding Automatic Identification Systems (AIS) and their role in ship-to-ship and ship-to-shore communication, tracking and safety.
April 23, 2024
Once only found on commercial vessels such as cargo ships and passenger ferries, AIS is now commonly carried by recreational boaters. Touted as a “must have,” it can be difficult to navigate the lingo and sift through the options to determine what exactly AIS does, and which AIS set-up is right for your vessel.
Above: An illustration of communication systems and VHF frequencies broadcasting a vessel's location to other ships and back to shore. Image created using OpenAI's DALL-E.
What is AIS?
AIS stands for Automatic Identification System. It is a ship-to-ship, and ship-to-shore, transponder system that uses VHF to exchange static information about a vessel - name, call sign/MMSI, length, draft – and dynamic information about a vessel- GPS position, speed over ground, course over ground, and sometimes, intended destination. The primary purpose of AIS is to aid in collision avoidance by providing concise and automatic information exchange between vessels.
Since 2008 it has been mandatory under SOLAS (Safety Of Life At Sea) regulations for commercial vessels 200 GRT and over to carry a Class A AIS, regardless of where they sail. Certain large vessels such as war ships and government vessels are exempt from carrying AIS. There are only a few countries, including Singapore, Indonesia, Thailand, that currently require recreational vessels to carry a Class B, AIS transponder when navigating in their waters.
How does AIS work?
AIS is based on technology abbreviated as SOTDMA – Self Organized Time Division Multiple Access – that is recognized and used world wide. Static information about the vessel must be input by the user when installing the AIS unit. Dynamic information about the vessel is gathered via a dedicated GPS antenna. Whether that GPS that is integrated into the AIS transponder or has to be installed separately varies from model to model.
Broadcasting On VHF Frequencies
The AIS transponder unit does all the heavy lifting for the user, parcelling the unique vessel information and automatically broadcasting it at timed intervals on a specific VHF frequency. There is no need to acquire targets or schedule transmission, if your AIS is on, it is sending and receiving information to and from every AIS equipped vessel within range.
AIS utilizes a small portion of the VHF bandwidth, so it is possible to piggyback on an existing VHF antenna if a proper VHF splitter is installed. This type of splitter acts as both a buffer and amplifier, so signal strength is not compromised. However, VHF antennas can be affected by EMI (electro-magnetic interference), so proper installation is a must.
When using a splitter only one device will broadcast at a time, so if you are having a lengthy conversation on the VHF radio, your AIS will not be able to update. As well, since AIS uses only a small portion of the VHF range, a purpose installed AIS antenna will not double as an antenna for a traditional VHF radio. As with any VHF antenna, the higher the antenna is mounted, the greater the potential range of transmission.
Displays
Information received by an AIS transponder can be displayed on a standalone display, a computer, or overlayed on a chart plotter with an NMEA port, which modern chart plotters have. As well, some VHF radios are able to display AIS information, and AIS transponders that are WIFI compatible are able to display on devices such as phone or tablet. Either in a dedicated app or as an overlay on a chart plotter program being used on that device.
What’s the difference between Class A and Class B AIS?
There are two different classes of AIS transponders available. Class A AIS is a more powerful 12.5 watt transponder that larger vessels are legally required to carry. Class B AIS transponders can either be 2 watts or 5 watts, and are what most small fishing boats and recreational vessels install.
The 5-watt Class B transponder uses the same SOTDMA system as the Class A transponder. A 2-watt Class B transponder uses a slightly different CSTDMA- Carrier Sense Time Division Multiple Access - system to parcel the information. The difference between the two systems is that the AIS using SOTDMA are given transmission priority, which means the lower watt system may have to wait in line to transmit. As well, SOTDMA AIS are able to transmit AIS message 27, which is short packet information that can be picked up by AIS satellite in extreme remote places, allowing shore parties with internet access to track a vessel.
Class A AIS have a range of 25-30 NM. 5 watt Class B AIS have a range of 10-15 NM, and 2 watt Class B have a range of 5-7 NM. So, if you are operating a Class B AIS, it may be possible that you can see a vessel transmitting via a Class A AIS, but they cannot see you because they are beyond your broadcast range. However, it is important to note that all AIS transponders will communicate with one another, regardless of Class.
Cost And Comparison Of AIS Systems
Class A AIS transponders start around $2000, which put them out of reach for the average recreational boaters considering that it is an optional piece of kit. Class B AIS transponders vary in price from approximately $500 to well over $1000.
The price difference is influenced by manufacturer brand – the larger and more well-known marine brands such as Raymarine, Simrad, Furuno, Garmin, em-trak, Si-Tex, and Vesper typically asking premium prices. However, with the hefty price tag of an established brand you are guaranteed to get a AIS that has undergone rigorous testing and meets international standards.
There are cheaper AIS units available from online discount stores, but it is buyer beware when purchasing unestablished brands as not all overseas manufacturers adhere to such high standards. Some users have reported that their budget AIS functioned properly in one geographical area but not in another. As well, AIS that are not compliant with international standards may not be legally used all international waters.
The second factor in price difference is what is included with the basic transponder. For instance, will you need to buy a VHF splitter or install a separate antenna? Is the GPS integrated into the transponder or will an external GPS antenna need to be fitted Does the device have it’s own display? Does the unit support WIFI?
A base priced model may look like the more economical way to go, but once the necessary add-ons are purchased and installed the end price is often similar to the higher priced option that is ‘plug-and-play.’
If AIS Worth The Investment?
There are many advantages to carrying an AIS, the main one is that it makes it easy for your vessel to be seen by others. Too many boaters have reported attempted VHF calls to cargo chips and freighters, only to be greeted with dead air and an eerie feeling that possibly no one is paying attention to what’s going on beyond the bright lights of the bridge displays. Carrying an AIS means your vessel will automatically be seen by other vessels carrying AIS including the largest ships, regardless of how fast your are travelling, or how small you are. This is especially important if you regularly navigate in a busy shipping port or through shipping lanes.
Additionally, some AIS units are compatible with online tracking apps, which means friends and family can track your progress on longer passages or when sailing in remote locations. This feature can all be used as a sort of boat monitoring feature, allowing you to know the exact GPS location of your vessel when not on board.
Conclusions: Safety First
Every boater knows that when it comes to enhancing safety at sea beyond the legally required basics, carrying gear that makes you more visible is always money well spent. It might be near impossible to put a price on your safety, but it’s hard to ignore the benefits of investing in an AIS.
