What is an AIS?
The acronym AIS stands for Automatic Identification System. It is used by ships, pleasure boats and traffic control stations. The system allows information on the position of nearby ships and shore stations to be exchanged electronically. The tool supplements the information provided by the RADAR which still remains the main anti-collision tool for commercial ships today. The radar watch with automatic MARPA mapping remains mandatory for large vessels.
However, for years AIS has become mandatory for ships over 300 tons. It is also rapidly spreading in pleasure craft, particularly among fishing boats and racing boats. Initially, receivers became popular, relatively cheap compared to instruments capable of receiving and transmitting. However, more and more offshore regattas have made it mandatory to have a transceiver. This is starting from Category 2 regattas according to the World Sailing classification.
The AIS transmits and receives on dedicated VHF digital bands, on channels not selectable by the on-board radio. Each vessel or shore station equipped with a transmitter periodically sends accurate information about its position, speed and course. It also sends other useful data such as the type, size and name of the vessel, the number of MMSIs. For large ships, the destination, estimated time of arrival, type of cargo, etc. are also indicated.
This allows any boat equipped with a receiver to have all the information necessary to avoid collisions and much more. With the radar it is possible to identify another boat in situations of bad visibility. With AIS it is also possible to instantly determine its speed and course and, if there is a risk of collision. Consequently, who has the right of way according to the type of boat. Moreover, knowing its name, it will be possible to call via VHF or DSC digital channel using the MMSI number.
AIS class A, B and C
There are instruments of various classes based on functionality and scope. All commercial ships over 300 tonnes and all passenger ships are equipped with AIS of class A. These instruments transmit with very high frequency through a dedicated antenna and receive data from all types of AIS. These units have a system of prioritization of the transmission of navigation data. This ensures that if there are many ships in the same area, none of the signals overlap with others.
The system architecture manages up to 4500 stations in the same area. Class A must also be equipped with a dedicated display and a calculator that analyses the risk of collision with any other signal received.
On the smaller ships, on many fishing boats or on pleasure boats there are Class B instruments. These also transmit and receive but are less powerful than Class A. They are also not equipped with a prioritisation system for the transmission of navigation data. They can have a dedicated screen or they can provide information to be displayed on the chart plotter or laptop. The installation of class B units is very often voluntary but, as already specified, it has become mandatory for Category 2, 1 or zero regattas. For example the Rolex Fastnet, a Route du Rhum or the Vendee Globe respectively.
Finally, there are simple receivers, called Class C, which, although not transmitting any information, are able to receive the data transmitted by others. This allows you to see all those around us equipped with an AIS class A or B. They are therefore very useful to avoid collisions with commercial traffic even if we will have to be vigilant. By not conveying our position to others, we see but are not seen. These units too can be equipped with a dedicated screen or receive information to be integrated on the chart plotter or laptop.
In some races with the illusion of obtaining a great strategic advantage, many keep the unit off. This should be sanctioned with disqualification because it affects the safety of the other competitors. In many French regattas, the operation of each unit is checked before the start. Some irresponsible sailors sometimes choose to put the tool into receive-only mode. This would allow them to have information about others without disclosing their position. An unsportsmanlike, dangerous and absolutely unseemly behaviour that should find a mechanism of punishment and disqualification.
Installation on a sailboat
On board a sailing boat we can therefore find a Class C receiver or a Class B transceiver installed. The instrument can have a dedicated screen or be integrated with the on-board chart plotter or laptop. In both installations, it can have a dedicated antenna or share the same antenna as the VHF. In this case, a special splitter must also be installed which protects the AIS when transmitting with the VHF radio. If you are replacing a Class C AIS receiver with a Class B transceiver, remember that you will also need a new splitter. With this additional device you will be able to use the masthead antenna shared with the VHF. Otherwise the risk is to damage the new instrument or the VHF due to the intensity of the transmitted signal compared to those received.
The alternative is to install a dedicated antenna, often installed on the transom. The idea has its own solid logic because it makes the installed instrument independent of the mast. In case of dismasting we will continue to be able to use the instrument. However, the antenna installed on the pushpit is much lower, reducing its range. In races, the use of the masthead antenna has become the strongly recommended option. However, you can still install an antenna on the pushpit or other suitable place. This will count as your emergency antenna, and will be especially useful in the event of dismasting. Just swap the input cable of the splitter to be immediately operational with the antenna on the pushpit.
The AISs operate on VHF 87B and 88B digital channels and therefore their range is roughly dictated by the visual range. Land traffic control stations can act as repeaters for signals transmitted by individual vessels. Therefore, at times you will be able to receive AIS signals from ships hidden by a promontory or much farther than the maximum range of our receiver. The architecture is very complex but very intelligent, and represented a real breakthrough in safety at sea to prevent collisions.
A valuable tool for avoiding collisions
Anyone who installs and uses even just a receiver for the first time will immediately realise how valuable this tool is. Situations that were previously stressful such as the risk of collision with a container carrier that has just appeared on the horizon will instead become easily manageable. The instrument uses data relating to our speed and course and to make a continuous calculation of the collision risk. The calculation performed is a vector calculation of the trajectories providing two key information. The CPA, Closest Point of Approach, is trivially the closest point to which we will pass from another ship. The TCPA, Time to Closest Point of Approach is the time it will take us to find ourselves in that point.
It is possible to set sound alarms on the basis of these parameters, in order to be alerted in case of potentially dangerous situations. When we are offshore we can be warned of any ship that passes within a mile of us in the next 30 minutes. This is just one example, we need to set up the instrument to give ourselves ample time to react. Each parameter is easily customisable and it is advisable to learn how to use the instrument to adapt to the navigation area.
A warning to those who use AIS connected to a computer instead of having a dedicated display. Navigation programs do not always handle audio alarms or they may not work. For example if the computer automatically goes into standby after a period of inactivity. In all cases you will have to check the volume of the alarm which may not be sufficient to be heard in the cockpit especially when you are sailing in strong winds or your engine is turned on.
The advantage of seeing and being seen
If our boat is equipped with a Class B transceiver unit we will not only be able to see commercial traffic but we will also be seen with confidence. You will notice to your enormous surprise that in some cases some commercial vessels, even large ones, will change their course. A change of course of a few degrees may be enough to avoid you or pass with a larger safety margin. That said, if you are sailing, even when you have right of way, stay away from ships on trade routes.
Lots of information at your fingertips
On the instrument display or on your integrated on-board computer you can see all the boats around you. Above all, you will be able to identify those with which there is a risk of collision. For each you can easily view a lot of useful information to make the safest choices. In addition to the route and speed also the name, the destination, the type of vessel, the size, the type of cargo and the MMSI number. With this data available, if needed, you can call a vessel directly by name via VHF radio.
This way you can communicate safely without being forced to make generic calls. When this information was not available, we found ourselves using the VHF hoping to be heard by the right vessel. The VHF can be useful for example to communicate our intentions when our manoeuvres may be limited by the wind direction. If you have a VHF radio enabled for DSC digital calls you can also call the other vessel by MMSI number. The receiving radio will be alerted by an audible alarm and will hardly be able to ignore your call.
PLB personal beacons
Among the safety equipment, the evolution of the PLB (personal locator beacon) is certainly very interesting. Until a few years ago there were only beacons capable of calling for help via satellite, or personal EPIRBs. The advantage of this system was to send an emergency signal directly to the search and rescue (SAR) services.
The downside is that from activation to when you could expect to receive help depended on many factors. One of these was the response time of the SARs, which always verify that it is not a false alarm before mobilising. If we fell overboard in cold waters this can really be a limit.
Furthermore, only if we sail within 200 miles of the coast can a helicopter reach us quickly. In the case of offshore navigation, only a plane will be able to reach us and attempt to launch a liferaft. However, this could only happen as early as a few hours after the initial call. In cold waters we probably wouldn’t be able to survive that long.
The SARs station will also attempt to identify comm