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What is AIS

MariTrace primarily tracks vessels using a system called AIS, but what is AIS and how does it let us track ships?



AIS is short for "Automatic Identification System" and it's hardware that sits on a vessel. The primary purpose of this hardware is to ping out messages at regular intervals with a ship's current coordinates, speed, course, and dimensions (along with a few other fields). AIS is designed to be used as an anti-collision system. Vessels send out these messages in order to say "here I am, please don't hit me." Other vessels in the area pick up these signals and they use that data to avoid accidents.


One happy side-effect of vessels transmitting all of this data is that it can be hoovered up by anyone with the correct hardware (not just other vessels). This means that somebody sitting in a port with an AIS receiver can watch the traffic in that port.


So, all I need to do is buy an AIS receiver and I can pick up on vessel movements worldwide?

AIS data is sent by VHF radio and has a limited horizontal range of about 40 nautical miles (74 km), so these signals can't reach all around the world. Not even close. Also, big objects like buildings stop these signals so to get half-decent data you would need to put your AIS receiver high-up where it has an uninterrupted view of the sea. But still, the best you're likely to do is get signals from vessels a maximum of 40 nautical miles away. To get good AIS coverage you need a network of lots of receivers all around the coast. We would call these receivers Terrestrial-AIS (or T-AIS) as they sit on land and they send us data via the internet.


But that's not the end of the story... The moment these ships drop over the horizon, they cannot be seen by T-AIS. Many AIS providers only supply T-AIS data which means when a vessel is far from land it is effectively lost until it gets near land / another T-AIS receiver.


How can you view vessels which are far from land?

We tap into a network of satellites overhead which hoover up all of the AIS signals they can and transmit that data back to us. We call these receivers Satellite-AIS (or S-AIS) for obvious reasons. You might wonder why we need T-AIS if we have S-AIS?


Well, the amount of data that satellites can pick up (the range) is so vast that the AIS signals can overlap making them very difficult to decipher. This means that heavily congested areas (such as the English Channel or South China Sea) are sending too many signals to properly understand each one. Here, the naturally limited range of T-AIS means that the number of signals are reduced to the point that they can be effectively read. So, to get the best coverage, you need a combination of S-AIS and T-AIS.


Do all vessels carry AIS?

Not quite. Way back in 2002, the IMO (International Maritime Organization, a specialised agency of the United Nations responsible for regulating shipping) mandated that the vessels which need to carry AIS equipment were those over 300 gross tonnes on international voyages. The rules these days are a little bit more complex, but effectively most commercial vessels will carry AIS equipment.


Are there different types of AIS?

Yes. Vessels will carry Class A or Class B AIS equipment. The differences between the two are a little technical, but the shorthand is that larger commercial vessels will carry Class A equipment, and smaller vessels (e.g. yachts, small fishing vessels...etc.) will tend to carry Class B equipment. Class B signals are much harder to detect using S-AIS which means yachts in the middle of the ocean can be hard to track.


What are the challenges with AIS data?

Using AIS to track vessels worldwide is not as straightforward as it may sound. These are some of the challenges that MariTrace (and all other AIS tracking providers) face:

  • AIS can be switched off. Although vessels aren't supposed to switch AIS off in normal circumstances they will sometimes do so anyway which makes them impossible to track using AIS

  • AIS can malfunction. Should a vessel's AIS equipment be faulty, there is little that we can do to track it.

  • AIS can be deliberately misleading. It is possible for a vessel to falsify AIS data if they choose to do so. This could range from changing their identity, to amending their reported coordinates.

  • MMSI numbers may not be right. A vessel's MMSI number is how we uniquely identify a vessel. It's a nine-digit number that is broadcast as part of the AIS message and tells us which vessel is transmitting that data. It is quite common for vessels to transmit the wrong MMSI number because they weren't paying attention when setting up their AIS equipment, because their MMSI number has changed and they haven't updated their AIS equipment, or because they've put in "any-old" number such as 123456789, or 111111111. This means that we detect signals from multiple vessels around the world claiming to be the same vessel and we need to discern which vessel is which, and which is legitimately transmitting on that MMSI number.

  • AIS in congested areas can be difficult to read. As above, particularly when using S-AIS, it can be difficult to detect AIS where there is heavy traffic.

  • AIS data can get garbled. Since this data is being sent by radio the signals sometimes make no sense and we need to try and weed out these nonsensical messages.

There are a myriad of other reasons why AIS is an imperfect system, particularly for the use to which we put it, but it is our job to overcome these challenges and present information that is as accurate as possible. Nobody providing AIS data to clients as tracking data should claim that AIS is a perfect system. However, considering that we use the data in a manner for which it was never intended, it is astonishing how concise a picture of world maritime traffic we can build.


This all sounds fascinating! How can I look at this AIS data on your map?

Simply click here to set yourself up with a free MariTrace trial. We look forward to welcoming you on board.