How Does AIS Work?
When you check websites like marinetraffic.com or vesselfinder.com, you can see the location of any ship in the world. They draw data from AIS, then display it on a user-friendly web interface that anyone can access.
AIS is the “Automatic Identification System” which broadcasts navigational data from seagoing vessels through a VHF transmitter integrated within a transceiver. It transmits information such as position, course and speed to other vessels or shore stations equipped with an AIS receiver.
AIS was developed as a way of reducing VHF radio reporting by automating the process of transmitting navigational data. Over time, it has evolved into the system we know today, which is used far more broadly than was originally intended.
It works by dynamically drawing information from a vessel’s navigational system and supplementing it with its own data. It adds some fixed data, like the vessel’s name and identity, along with information that can be changed periodically, like navigational status and destination.
All this data is broadcast as a single package using a VHF antenna.
The transmission can be received by any other vessel equipped with an AIS receiver.
Shore stations such as VTS services, the coastguard, or port authorities also have AIS receivers so that they can receive the same information.
The information is then interpreted by the AIS equipment and displayed to the user. It can be displayed directly on an AIS unit, or it can be integrated into another navigational system such as an electronic chart or a radar.
Regardless of how it is displayed, the end-user then has access to the data that was broadcast by the original vessel.
Using the name, position, course and speed, the other vessel can be accurately plotted by the receiving station even when they are outside of visual or radar range.
What are the components of the AIS system?
The original AIS system has expanded over time and is now comprised of many more components than are strictly necessary for its operation.
With two vessels in the middle of an ocean, AIS works using only the equipment that is fitted to each vessel.
When vessels come closer to land, the system expands to include shore-based receivers as well. These are similar to ship-based systems, but they don’t need to transmit their own navigational data.
Vessels do not actually need to be close to these shore-based receivers to be detected because satellite-based receivers also detect AIS signals from the most remote parts of the oceans.
Finally, there is the public interface of the AIS system. This covers those websites that we mentioned at the beginning, such as marinetraffic.com and vesselfinder.com. They take the data received by shore-based and satellite-based receivers and present it in a user-friendly format online.
AIS equipment fitted to vessels
The backbone of the AIS system is the equipment that is fitted to vessels themselves.
In the case of ships, they carry an AIS transceiver which handles both transmission and reception of data. Smaller vessels might only carry a receiver, which means they cannot transmit their own data, but they will be able to see other vessels close to them.
The transceiver collects together all the information that is needed for an AIS transmission. It is connected to the ship’s navigational equipment. For example, the GPS for position, course and speed.
It listens for incoming AIS signals, determining a good time slot for its own transmission.
At the right time, it uses the VHF antenna to transmit its data package to other AIS receivers in the area.
When it isn’t transmitting, it listens out on VHF channels 87B and 88B for transmissions from other vessels. This is the same as receiver-only units, that will only listen out for messages and will not transmit themselves.
When a signal is received, the transceiver (or receiver) passes that data back to the vessel’s navigational system so that it can be displayed to the navigators on board.
Satellite-based AIS receivers
AIS receivers are fitted to satellites to receive signals from vessels when they are beyond the range of shore-based antennas.
They enable the tracking of commercial shipping from anywhere in the world.
It costs more to access satellite data, but companies are often happy to pay as it gives them a way of tracking their own vessels throughout their voyage.
Satellite-based receivers are not required for the AIS system to function, but they increase the usefulness of the overall system by providing a service beyond the original core functionality.
Shore-based AIS receivers
In its original form, AIS was established to streamline vessel reporting to shore stations for things like VTS monitoring.
To accomplish this, shore stations use AIS receivers to detect signals from ships.
They work in the same way as ship-based receivers, using a VHF antenna to detect AIS transmissions on channels 87B and 88B.
In addition to receiving data, shore-based stations operated by competent authorities also play a role in assigning the mode of operation of AIS in the area.
AIS modes of operation
There are three different modes of operation within the AIS system.
- Autonomous and continuous;
- Assigned or Controlled;
- Polled.
Autonomous and continuous
In “autonomous and continuous” mode, AIS is operating in its usual state.
Each AIS unit adjusts its transmission frequency based upon its vessel’s speed, rate of turn and navigational mode. The idea is that the more frequently the data is changing, the more often it will be updated.
| State | Transmission Interval |
| At anchor or moored | 3 min |
| At anchor or moored & speed >3kn | 10 sec |
| Speed <14kn | 10 sec |
| Speed <14kn & altering course | 3.3 sec |
| Speed 14-23kn | 6 sec |
| Speed 14-23kn & altering course | 2 sec |
| Speed >23kn | 2 sec |
| Speed >23kn & altering course | 2 sec |
While in autonomous and continuous mode, a competent authority ashore can automatically switch the AIS to another mode.
Assigned or controlled
In “assigned or controlled” mode, the AIS reporting interval is set by the competent authority.
This may be the case in an area with high traffic density where the system may be at risk of overload otherwise.
In this mode, vessels will report at the designated intervals, irrespective of their course and speed alterations.
Polled
In “polled” mode, an AIS is responding to an interrogation by a competent authority.
They can interrogate the AIS and receive an immediate response.
While it is being interrogated, the AIS is still in either automatic or assigned mode and the interrogation does not interfere with that. It will still continue to broadcast at its designated intervals.
AIS Classes
AIS works by assigning different classes to different vessels.
Class A includes vessels carrying mandatory AIS in accordance with the SOLAS convention.
Class B includes all vessels, other than Class A vessels.
The different classes of AIS only have an effect on their own transmissions. Both Class A, and Class B AIS, will receive the same data from other vessels that are transmitting.
Class A AIS
Class A AIS is the compulsory version of AIS that you will find on most commercial vessels. It is the most powerful version of AIS, having a typical transmission power of 12.5W.
The International Convention for the Safety of Life at Sea (SOLAS) defines which vessels must carry AIS.
“All ships of 300 gross tonnage and upwards on international voyages and cargo ships of 500 gross tonnage and upwards not engaged in international voyages and passenger ships irrespective of size…”
SOLAS
Class A AIS must meet strict performance standards, including the requirement for an integrated display.
They transmit more frequently than Class B AIS and take a higher priority if the level of traffic starts to increase towards the limits of the system capacity.
Class B AIS
Class B AIS is designed for everyone who does not need to carry a Class A AIS. It is lower powered, only needing around 5W of transmission power.
These AIS units are used by pleasure sailors, small boats, and commercial vessels that fall outside of the SOLAS requirements we discussed above.
They transmit less frequently than Class A, on the assumption that it is fitted to smaller craft which does not pose as much of a hazard to other traffic as Class A vessels.
Class B AIS still transmits the same message types as Class A AIS. It’s only the frequency of transmission that changes.
AIS messages
AIS works by sending a block of data within a pre-defined transmission slot.
These slots are assigned using a technology called “Time Division Multiple Access”, which allows multiple users to access a single transmission channel.
TDMA (Time Division Multiple Access)
The principle of TDMA is that a single transmission channel is split up into multiple timeslots.
In the case of AIS, the slots are 26.6ms long, giving 2250 possible slots per minute.
As there are two AIS channels, 87B and 88B, this gives a total of 4500 possible transmission slots every minute.
Each 26.6ms slot is treated the same, and the data transmitted within the slot follows an identical pattern.
The main block of AIS data fills the middle portion of the message, buffered either side by signals that the system as a whole needs.
We won’t go into the technical details of the buffering messages in this article, but you can read more about it at the International Telecommunication Union: Technical Characteristics For An Automatic Identification System Using Time-Division Multiple Access In The VHF Maritime Mobile Band.
The part we are interested in is the data contained in the centre of the AIS message.
This data is split up into different message types that can be sent.
AIS message types
There are 27 possible message types within the AIS system. The table below has been created using the information within the ITU “Technical characteristics for an AIS system using TDMA in the VHF maritime mobile frequency band” document.
| Message ID | Purpose |
| Message 1 | Class A position data report |
| Message 2 | Class A position data report |
| Message 3 | Class A position data report |
| Message 4 | Base station data report |
| Message 5 | Class A static and voyage data report |
| Message 6 | Binary data for addressed communication |
| Message 7 | Receipt acknowledgement for binary addressed data |
| Message 8 | Binary data for broadcast communication |
| Message 9 | Airborne SAR position report |
| Message 10 | Request for UTC and date |
| Message 11 | Current UTC and date |
| Message 12 | Safety-related data for addressed communication |
| Message 13 | Receipt acknowledgement for addressed safety-related data |
| Message 14 | Safety-related data for broadcast communication |
| Message 15 | Request for a specific data type (from an interrogating station) |
| Message 16 | Assignment of a specific report behaviour |
| Message 17 | DGNSS corrections |
| Message 18 | Class B position report |
| Message 19 | No longer required; Class B voyage and static data |
| Message 20 | Reserve slots for base stations |
| Message 21 | Position and status reports for ATONs |
| Message 22 | Channel management by a base station |
| Message 23 | Assignment of a specific report behaviour |
| Message 24 | Additional data assigned to an MMSI |
| Message 25 | Short unscheduled binary transmission |
| Message 26 | Scheduled binary data transmission |
| Message 27 | Position report for long-range applications |
Within these 27 message types, there are three categories that we, as navigators, are concerned with.
- Static data
- Voyage data
- Dynamic data
These are the messages that we actually read from an AIS, and are the ones that we want to be able to plot onto a chart.
Static data
Static data includes everything that does not change. It is programmed into the AIS when it is installed and should not change throughout the lifetime of the AIS on one particular vessel.
Static data includes:
- MMSI number
- Name
- Type of vessel
- Call sign
- Dimensions, and reference for position
Voyage data
Voyage data includes everything that can be changed on a regular basis by a vessel’s navigator. It is usually set once per voyage, or when a significant navigational change occurs.
Voyage data includes:
- Navigational status
- Cargo
- Destination
- ETA
- Draft
Dynamic data
Dynamic data includes everything that is likely to continuously change. It is fed by a vessel’s navigational system and should update automatically.
Dynamic data includes:
- Position
- Time
- COG
- SOG
- Rate of turn
- True heading
How often are AIS messages transmitted?
The frequency of transmission of AIS messages depends on the class of AIS, and how the vessel is moving.
A Class A AIS will broadcast at the following intervals:
- Static and voyage data: 6 min
- Dynamic data: according to the vessel’s speed and rate of turn (see table below)
| State | Transmission Interval |
| At anchor or moored | 3 min |
| At anchor or moored & speed >3kn | 10 sec |
| Speed <14kn | 10 sec |
| Speed <14kn & altering course | 3.3 sec |
| Speed 14-23kn | 6 sec |
| Speed 14-23kn & altering course | 2 sec |
| Speed >23kn | 2 sec |
| Speed >23kn & altering course | 2 sec |
A Class B AIS will broadcast at the following intervals:
- Static and voyage data: 6 min
- Dynamic data: according to the vessel’s speed and rate of turn (see table below)
| State | Transmission Interval |
| Speed <2kn | 3 min |
| Speed 2-14kn | 30 sec |
| Speed 14-23kn | 15 sec |
| Speed >23kn | 5 sec |
| SAR aircraft | 10 sec |
| ATON | 3 min |
| AIS base station | 10 sec |
AIS channels and frequencies
AIS operates across two main VHF channels, 87B and 88B. Longer range transmission also takes place on VHF channels 75 and 76, but that is mainly for communications with satellites for deep-sea tracking purposes.
For the purposes of understanding how regular AIS works, we only need to consider the two main channels.
| VHF Channel | Frequency | |
| AIS 1 | 87B | 161.975 MHz |
| AIS 2 | 88B | 162.025 MHz |
As AIS operates over VHF, its performance can be assessed in the same way as normal VHF performance.
This means that the range that you can expect from your AIS is the same as the range you can expect from your VHF. In general, the higher your antenna, the further you can expect the transmission to go.
There are slight refractive and atmospheric effects, but for the most part, you can expect your AIS to work over a line-of-sight distance.
An antenna on the water’s surface would only send a signal a few miles. That same antenna at the top of a large ship’s mast would send the signal over 30 miles.
Limitations of AIS
Due to the way AIS works, there are some inherent limitations in its operation.
The most important limitation is that a vessel receiving data transmissions from other vessels does not know how accurate that information is.
We can start by considering the static data. The accuracy of static data depends entirely upon the inputs from the person that installed the AIS. If that information was incorrect, it is always going to be incorrect.
I have piloted ships that have incorrect static data. When I take a feed off their system, I can immediately see when things like the offsets for the vessel’s length and beam are incorrect.
Similar inaccuracies can occur with voyage data. This is input by the user, so is subject to errors as it is entered. Using the example of navigational status, there are often ships that set their status to “not under command” when they are drifting. Often this is an inaccurate representation of the true situation.
Finally, dynamic data is also subject to data inaccuracies. This time it is due to the accuracy of the data that gets fed into the AIS. If there are inaccuracies in a vessel’s GPS these will be broadcast to other vessels with no indication about the accuracy of the data.
Moving away from inaccuracies in the data itself, there are limitations due to the 4500 slots assigned to AIS transmissions every minute. If you are in an area with high traffic, there is the possibility that the system could become overwhelmed.
It’s not so much of an issue with commercial traffic, but as more and more leisure vessels install AIS as well, it may become an issue in the future.
There are numerous other limitations within the AIS system itself, but we won’t go into detail about them in this article. For example:
- AIS is not compulsory on every vessel;
- Vessels may deactivate their AIS;
- You could become over-reliant on AIS.
If you would like to read more about the limitations of AIS as an entire system, you can read this article: What Are The Limitations Of AIS?
Who uses AIS?
Although AIS was originally designed to be used by vessels to reduce the need for VHF reporting, the modern system far exceeds those original design parameters.
Nowadays, the system is used for a range of different purposes by different users.
Vessels
Ships and boats were the original intended users of AIS, and they still form the backbone of the system today.
Rather than simply using it to reduce radio reporting, however, it is useful for so much more.
They use it to identify other vessels, to be able to positively identify ships on VHF. They can identify a particular vessel, and call them by name on the radio.
In addition to communication, AIS is used as an aid for collision avoidance. Data feeds directly onto electronic charts and radars, so you can use it to help determine when a risk of collision exists.
AIS itself is not a legitimate way of determining the risk of collision in accordance with the International Regulations for Preventing Collisions at Sea, but it does fall into the category of “all available means”.
More and more vessels are using it as a means to generally improve the safety of navigation at sea.
ATONs
The AIS architecture is built with “virtual aids to navigation” in mind.
The idea is that a navigational marker can be broadcast to vessels over AIS, and appear virtually on their navigation systems.
As the system is virtual, the broadcast can be made by a competent authority ashore, and then all vessels within reception range will be able to “see” the marker on their screens.
It cannot replace traditional visual markers, but it can be useful in some situations.
For example: if there is a new danger to navigation and there hasn’t been time to set buoyage, a virtual maker can be used instead.
The AIS message contains the position that needs to be marked, placing a new buoyage symbol in the correct location.
In addition to virtual markers, AIS can also be fitted to traditional buoys and navigational marks. In the same way that it makes other vessels easier to see, it can help with buoyage as well.
VTS
Along with vessels, VTS were the original intended users of AIS.
They use it for positively identifying vessels, and having immediate access to some navigational data.
It dramatically improves safety in busy areas because VTS operators can effectively monitor the traffic and assist any vessels that require.
EPIRBs & SARTs
A new use of AIS is within electronic distress equipment like EPIRBs and SARTs.
These both send electronic distress signals to different recipients. In the case of EPIRBs, they signal satellites which relay it on to shore stations. In the case of SARTs, they signal X-Band radars that are operating in the vicinity.
While both do raise attention, they are invisible to anyone close by that doesn’t have the correct equipment.
Nowadays, AIS is being fitted to EPIRBs and SARTs as well.
Including AIS makes them visible to any vessel that is equipped with an AIS receiver. Not only that, but any shore station that monitors AIS will also be able to detect them.
Although it is not strictly necessary for their core functionality, including them within the AIS system increases the chances of a successful rescue.
