AIS is a useful tool to assist watchkeepers in carrying out their duties. It does improve the safety of navigation for everyone, but like everything, it does suffer from some limitations.

Limitations of AIS include:

  • AIS information may not be accurate;
  • The information could be misinterpreted;
  • AIS is not compulsory on every vessel;
  • The AIS system could become overwhelmed;
  • Vessels may deactivate their AIS;
  • You could become over-reliant on AIS.

Despite the limitations, the system is still important and helpful to navigators. All it requires is a proper understanding of the limitations to be able to use it safely.

AIS information may not be accurate

One of the biggest limitations of AIS is that the information transmitted may not be accurate. You may already have seen ships showing an incorrect destination. Maybe they left a port and forgot to input their next destination. They might even have forgotten to update their navigational status as well. I have seen countless ships showing “at anchor” when they are steaming along at 15kn.

It is too easy for this “voyage data” to be inaccurate because it relies on the active input from a human. You will find that most ships will include the correct setting of AIS data as part of a pre-departure check-list. Given that it is already acknowledged how easy it is to forget, it follows that you can never rely on the voyage data other vessels transmit. You have no way of knowing how comprehensive their checks are or whether they have been followed.

It could even be intentionally entered wrong. In some places, you will find entire fleets of ships that have all set their AIS status to be “not under command”. They are usually drifting, waiting to get into a port. They often change their status to try and keep other ships away, even though they don’t meet the criteria laid down in the International Regulations for Preventing Collisions at Sea (COLREGS) for a vessel not under command. 

It goes further than that though. Not only might the voyage data be inaccurate, but the static or dynamic data can be erroneous as well.

Static data

Static data, by its very definition, should never change. Once it is set in the AIS, it should always show the correct information. Static data includes things like the vessel’s name, MMSI, length & breadth. It is, however, still input by a human when the AIS is first set up so is still subject to human error. I have seen instances where the static data has been set incorrectly, leading to almost permanent transmission of the wrong data.

One particular example of incorrectly set static data is the offset in position of the GPS aerial. The effect of this is that when you zoom in close to an AIS target, the shape of the vessel that the system displays is centred on the wrong point. Normally the error is insignificant, but if you are feeding that data to a pilotage display and manoeuvring in a tight spot, that small error does become an issue.

Dynamic data

Dynamic data is the data that constantly changes as a vessel moves. It includes things like the vessel’s position; course; speed; rate or turn etc.

Normally you would think that the dynamic data couldn’t be incorrect because it isn’t manually entered by a human.

It is, however, still subject to errors. Take the GPS as an example. You can read about all the errors in GPS in my article: How Does Marine GPS Work?

Errors of marine GPS include:

  • Propagation
  • Multi-path
  • Ephemeris (Orbital)
  • Receiver Noise
  • Relativistic Errors

When the GPS itself suffers from errors, those errors pass straight into the AIS. The GPS likely feeds not only the position, but it will feed the course and speed as well. A single error on the GPS will lead to a lot of incorrect data being broadcast.

Other stations receiving AIS broadcasts have no way of verifying the accuracy of the data that they receive. They have no way of knowing whether a broadcasting vessel’s GPS is giving the correct data or not. You can confirm data with other means, like radar, but the underlying limitation still remains. The data broadcast by other vessel’s AIS may not be accurate.

AIS information could be interpreted incorrectly

If we make the assumption that the data being transmitted by another vessel is accurate, it now falls to the receiver to interpret it correctly.

To interpret the data, you usually have some kind of visual output. It could be an AIS feed into an ECDIS, or chart plotter. Maybe it is the visual “radar style” display on your AIS itself. It could even be a textual display just giving you the raw data itself. Either way, it is up to you to interpret what you are being shown.

On an ECDIS or a chart plotter, the easiest way to misinterpret AIS data is if you forget to acquire a target.

Examples of sleeping and active AIS targets
Three AIS targets. The left image is a sleeping target, middle is an active target, right is a dangerous target.

Sleeping AIS targets look surprisingly similar to active targets. As you can see from the diagram above, it is quite easy to glance at your screen and think that a target is not moving, when it is actually just a sleeping target.

If you think that a target is not moving, when in fact it is sleeping, you could miss a vessel that is on a collision course.

Most displays can be set to automatically acquire sleeping targets, or acquire sleeping targets that pose a threat. Either way, it is important to know how your display is set up and the settings that are in use.

Proper understanding and vigilance with your own display will reduce the chance of your misinterpreting AIS information that you receive.

AIS is not compulsory on every vessel

One of the bigger limitations of AIS is that it is not compulsory for it to be fitted to every vessel. According to SOLAS, AIS is only compulsory on the following types of vessel:

  • Cargo ships >300 GRT on international voyages;
  • Cargo ships >500 GRT on all voyages;
  • All passenger ships.

This means that actually, the majority of vessels on the water do not legally need to have AIS at all. Vessels not obliged to carry AIS include:

  • Non-passenger, commercial vessels <300 GRT;
  • Pleasure vessels not subject to SOLAS;
  • Small fishing vessels;
  • State-owned vessels like warships or law enforcement.

This is a serious limitation because there is always a chance that vessels in your vicinity will not be transmitting. If you are relying on your AIS too much, you will miss a lot of vessels entirely.

The International Regulations for Preventing Collisions at Sea (COLREGS) do not mention using AIS at all. It shouldn’t be used for collision avoidance, or for keeping a lookout. As it isn’t fitted on every vessel, you will miss some and there will always be an increased risk of collision.

Despite it not being compulsory, as the cost of AIS transmitters comes down, it is becoming more common for it to be fitted to smaller craft. Class B AIS is the sort fitted to these vessels where it isn’t compulsory. It has a reduced transmission power and will be the first sort to drop if the system gets overwhelmed.

The AIS system could become overwhelmed

The way AIS works is that each transmitter is assigned a timeslot. There are 2250 timeslots within each block, and 2 channels available. This gives a total of 4500 timeslots within a block.

Theoretically, if there are too many transmitters, there will not be enough timeslots and the system will become overwhelmed.

In reality, due to the limited transmission range, it is unlikely that the system will become overwhelmed. If it does happen, lower priority transmissions will be the first to be dropped. This creates the same limitation that we discussed above where we know that AIS is not fitted to every vessel.

When you are in a particularly busy area, the system may become overwhelmed and some targets will not show up.

You can deal with this limitation by assuming there will be targets around that are not transmitting an AIS signal. Just like you assume smaller vessels will not be carrying AIS, you can assume that the system could be overwhelmed and some targets may not be appearing.

As before, it is important to only use AIS as a tool to assist watchkeeping.

Some vessels may switch off their AIS

Although AIS may be compulsory on a lot of vessels, there is always the chance that the AIS has been switched off. For a detailed overview of when vessels can switch off their AIS, you can read this article: When Can Ships Turn Off AIS?

Generally, the only legitimate reason for a compulsory vessel to switch off its AIS is when the master deems it necessary for safety or security reasons.

In addition to legal reasons for switching off AIS, you will find other reasons for vessels turning it off. These include illegal fishing; smuggling; and unsanctioned trading. For example, it is common for vessels trading in areas like North Korea to switch off their AIS entirely.

Regardless of the reason for actually turning it off, the fact remains: vessels can, and do, turn off their AIS.

The same as we overcame the limitations of vessels not carrying AIS, we can deal with the limitations of vessels switching their AIS off in the same way. A watchkeeper needs to use AIS as a tool to assist watchkeeping rather than using AIS on its own.

Visual methods should always be the primary way navigating. It is still nearly impossible for vessels to hide visually. Electronically, however, it is all too easy to just switch off the AIS.

Over-reliance on AIS

Other limitations of AIS have all been solved by remembering that AIS is only a tool to assist watchkeeping. It should be used in conjunction with other methods to keep a safe watch.

Despite that, it is all too easy to become over-reliant on AIS. When used on an ECDIS, the information on the screen can look uncomfortably trustworthy.

Our own vessel appears to be placed in the right position, with the vectors all looking correct. The chart underneath corresponds with the landscape you see out of the window. There are other vessels showing up, their AIS plotting them in the same place as the radar, and it all looks correct visually out of the window.

The confidence with which the screen displays the information can quite easily lead to a false sense of security and over-reliance. If your cross-checks show it to be correct day after day, it becomes even harder to maintain the scepticism that you need to be safe.

As with all the other limitations of AIS, it is crucial to remember that it is just a tool to assist with watchkeeping. Despite the confidence with which it displays the information, it has to always be treated with scepticism.

Over-reliance on AIS is not so much an issue with the system itself, its more an issue with operators using the system. It isn’t something that can be solved through future upgrades to the system or through more widespread usage. It is an inherent problem, just like with every other piece of navigational equipment.

Training, practise and experience can all help to reduce over-reliance on AIS. Remembering the inherent limitations of the system will help you to reduce over-reliance.

Given the limitations, is AIS still useful?

AIS is a useful tool for watchkeepers to use to assist them in keeping a safe and proper watch. The information it provides can aid decision making and improve the safety of vessels overall.

In some situations, it can even provide more information than any other means. I have been on watch and seen small vessels on AIS miles before any sign on them visually or by radar. A small boat, in heavy seas and limited visibility, is very hard to see. If they use AIS, suddenly they stand out.

Another classic example is when you are approaching a tight bend in a narrow channel. If you can’t see around the corner, AIS can give an early indication of other vessels approaching. It doesn’t replace the need for a good visual lookout or appropriate speed. There could just as easily be a vessel without AIS approaching round the corner.

As long as AIS is used as another tool in the watchkeeper’s armoury, it is a very helpful and useful piece of equipment. In the same way that three lookouts on the bridge are better than two, AIS gives you another way of keeping your vessel safe. You wouldn’t dismiss the other two lookouts if a third turned up, but you’ll use all three if they are available.