The Universal Shipborne Automatic Identification System (AIS) is a vessel tracking system capable of communicating navigation information automatically between AIS equipped vessels and coastal authorities. Vessels equipped with a receiver can also benefit by knowing the whereabouts and intentions of these ships. AIS provides a tool for improved safety and collision avoidance. Since 2004, the International Maritime Organization (IMO) has instituted carriage requirements for vessels affecting both worldwide and local shipping traffic. These carriage requirements apply to commercial vessels subject to the Safety of Life at Sea (SOLAS) convention.

In a very general sense, the AIS system is similar to the air traffic control system only applied to marine traffic. This mandatory regulation requires all vessels over 300 tonne on an international voyage and all domestic vessels over 500 tonne to have an AIS transponder installed. Passenger ships irrespective of size are also required to carry an AIS transponder. Local authorities may have additional requirements subject to the area's traffic. Ships are aware of each other's position and harbour control can use AIS to increase transportation efficiency and safety by identifying, tracking and supervising the movement of these large vessels as they head into harbour, or navigate along in-land waterways or dangerous coastlines.

AIS data is sent every few seconds over two dedicated digital marine VHF channels. The transmission interval varies based on speed and for those changing course; faster and turning vessels are updated more frequently. The AIS transponder works in continuous mode regardless of whether the vessel is off-shore, within coastal or inland waters, or at anchor. Moored and anchored vessels broadcast their position less frequently.

Ships send data related to vessel movement including current position, speed, course, rate of turn (ROT) and others. Information about the ship and its voyage is sent less frequently. This data includes the IMO number (unique to the ship), call sign, name, length and beam of vessel and the location of the position fixing antenna for the AIS signal. Voyage information including the type of vessel (ex. cargo, tanker, hazardous cargo), destination and ETA are sent out as well.

Signal coverage for AIS is similar to other VHF applications and depends on the height of the antenna. Typical ranges are 20 nautical miles. We have have seen this range with our antenna on the radar pole which is just above the bimini. Others have successfully placed a spare antenna on the rail. Even a coat hanger lying on the chart table serving as a temporary antenna was receiving ships 8-12 nautical miles away!

Because VHF frequencies have a longer wavelength and better propagation, AIS signals have an ability to "see" behind islands or around bends in a river, where a radar cannot. This aspect of the AIS signal can add to safer navigation by detecting the whereabouts of a ship, even when it is out of sight behind a headland or island.

Aids to navigation can be transmitted over AIS. These can be physical aids like buoys or "virtual" ones to mark a new or transient danger such as a wreck. AIS can also identify navigational aids that are not in their charted position.

Additionally, safety messages can be issued from either a ship or shore-based stations. A ship that is adrift may issue a broadcast warning "adrift with no engine". Safety messages also may include meteorological broadcasts or search and rescue information.

How does this system benefit coastal mariners?

The AIS system has many features that can bring enormous benefits and collision avoidance data to safety-minded mariners especially when maneuvering in crowded commercial harbours, along busy coastal waterways and shipping channels and for short-handed crews. AIS does not replace standing a proper watch, but it can add improved situational awareness for the watch-keeper and since the system constantly updates, real-time changes of another ship's movements are immediately recognized.

Targets (ships) are easily identified because the name is broadcast to the receiving station. Making contact by actual ship name, instead of calling "ship off my port bow", or "tanker at position latitude X, longitude Y" increases the likelihood of a positive response to the call. If your vessel has digital selective calling, you can punch in the MMSI number that AIS provides to ring the bridge of the ship directly.

The system also greatly increases safety at night and AIS signals are received in poor conditions such as heavy rain and squally conditions where a radar would show only noise.

What options are available & useful for interpreting AIS data?

The transmissions from AIS equipped vessels are received and decoded by an AIS receiver. The AIS receiver produces a NMEA data stream which when input to one of a variety of electronic options for further decoding provides usable navigation data. The data can be displayed on modern chart plotters, with a dedicated AIS display or with computer software.

Products vary widely in their features for display and alarm capabilities. Many calculate and sound alarms based on the closest point of approach (CPA). The CPA is the minimum distance your vessel will come with the target if both maintain their speed and course. AIS enhances this feature because signals are received miles in advance of the CPA actually occurring. Based on your current course and speed you know that an intercept of your vessel and the on-coming ship will occur in a given time (TPCA); you also know just how close that distance will be (CPA).

Systems with CPA alarms sound a warning when the AIS target is first received and typically sound only when alarm criteria are met. Customizable alarm parameters allow the user to define the distance a ship will be predicted to come before an alarm is sounded. This may be especially important depending on whether you are maneuvering in a crowded harbour or off-shore. For harbour sailing one would typically want a smaller CPA because of the amount of traffic operating closely and specifically; coastal or off-shore scenarios generally have greater CPA values.

CPA alarms are superior over range alarms because of their advance warning and they are target specific. Similar to radar guard zones, when alarms are based on range the target must already be within the predefined circle or range before an alarm sounds. Some ships may be within range, hence sounding the alarm, but pose no collision risk because they are headed away or running parallel for example. The alarm will sound constantly with this scenario and most mariners turn the alarm off until the vessel passes out of the prescribed range but this means another target may go undetected. Often times the watch-keeper forgets to return the guard alarm to its active state, providing no alarm benefit at all.

Another benefit of AIS includes the ability to filter and prioritise targets. In a crowded harbour many vessels are moored in port or anchored awaiting to off-load their cargo. False alarms are greatly reduced by filtering out vessels or suppressing alarms for targets that are not moving, hence posing no collision risk, as one moves through the harbour.

AIS data can be displayed providing a live, real-time graphical display of marine traffic. The display provides an interesting visual representation for the location and movement of the ships and the target's AIS navigation data, but without some kind of prioritisation system, the navigator won't necessarily know which of the vessels pose a collision risk. The closest ship may not be the number one safety concern. Crowded harbours and busy shipping lanes typically have numerous targets. How do you know which of the vessels pose the highest risk, especially when there are 80 to 100, or even 30 ships on your chart?

Cruising with AIS

As we all know, large ships move fast, have limited maneuverability, can't always see smaller vessels or sadly, aren't always looking for them. Given their mass and speed, they can't stop without planning for this event well in advance. Sailors hold these behemoth vessels with great respect and most identify with an underlying fear of being run down ranking high on the safety checklist.

As interested cruisers, we bought an AIS receiver a couple years ago intrigued by the system and its potential benefits for marine safety. We soon found out that just a receiver was not going to provide much information - you also needed some way to interpret and display the data. We started evaluating what products were available and in particular how alarm features worked for given systems.

Through this discovery process we developed a stand alone collision warning display and alarm. It is one of several options that can help you maintain a safe watch at sea. AIS is an invaluable tool and source of navigation information which can be used to your benefit: it should not be solely relied upon in making navigation and collision-avoidance decisions.

The Options:

The available options for interpreting, displaying and utilizing AIS depends in part on your boat systems and personal choice. If you already have a chart plotter that supports AIS, it may be the fastest and simplest road to using the benefits of AIS. Many modern chart plotters support receiving AIS data. The targets are represented as triangles which overlay an electronic chart, each typically showing a vector to indicate the direction of movement. Additional data about a specific target can be gained by moving the cursor over each target.

Smaller and less expensive chart plotters tend to have very crowded screens displaying lots of chart information condensed to a small area. All this data, along with the icons for each AIS target are present. Many manufacturers have CPA alarms to warn if a ship will pass dangerously close, but it is often necessary to cursor to the ship in question to determine more information about the alarm. If it doesn't have filtering the false alarms may be a big nuisance. If it doesn't prioritise targets and alarms, then determining which targets to pay attention to first may be tricky, particularly in crowded shipping areas.

If power consumption is not a concern, another means of using AIS data is with a receiver and charting software for the computer. There are a variety of packages that support AIS incorporating an external receiver. They all work basically like the chart plotting scheme mentioned previously, but because laptops usually have a larger screen there is more area for all the data.

The challenge comes with leaving the laptop running: power consumption is high for computers. Most sailing vessels do not have the luxury to run their computers all the time. If you shut off the computer you have no alarms and you must wait to boot up the computer before you can learn anything about a ship you might see coming at you.

One of the many beauties of the AIS system is that it has the potential to notify you well in advance of an oncoming ship, along with collision avoidance alarms for ships you might not see. Laptops are also known to mysteriously shut down, break down, or go flying, all of which will hinder AIS data interpretation.

There are also dedicated stand-alone AIS display units. These devices are designed to be left running at all times and are solely dedicated to AIS interpretation.

Regardless of the type of display, features to consider for AIS interpretation are their alarm capabilities, especially for CPA alarms and filtering options. Does it prioritize targets and alarms? Is it waterproof? Can it be at or near the helm to provide benefit when and where its needed? Does it receive AIS transmitted aids to navigation and safety messages ? Is the unit 'upgradeable'? Does it support the ability to switch to a different type of AIS receiver or possibly to a class B transponder? How many targets can it track? Does the display support both class A and class B AIS data? How much power does the unit use? Can it be left on all the time? These are some of the questions you might keep in mind when evaluating your navigation needs and the applications of AIS technology.

Receivers and Transponders

A variety of receive-only devices is currently available. All receiving units accept AIS data from either of two dedicated VHF channels. The data is sent every few seconds, but alternately between each channel. The two channels are in place for redundancy.

Dual channel receivers are available as scanning (multiplex) or full-time dual channel units. The multiplex unit receives data over one channel at a time, but will automatically switch to the alternate channel if interference is detected. The dual channel receiving unit receives both channels simultaneously and consolidates the information into a single data stream. All AIS data will be received, but the dual channel unit updates the data more frequently. Some AIS receivers have a built-in GPS unit. Single channel receiving units are also available, but are not recommended.

Transponders not only send data but are also two channel receivers. However, like a receive only unit, transponders also require an interpretive or display head in order to gain the navigation benefits of AIS. There are two types of transponders used for AIS. Class A is intended for use to meet the IMO mandated carriage requirements for commercial ships. Class B was developed after the introduction of class A and is designed to be compatible with the the safety operation of class A. It is targeted for use on smaller vessels but has a simplified installation and a lower cost as compared to class A. Class B does not send data as frequently, nor does it use all of the same fields of data as class A, but can still provide safety and navigation benefits to smaller vessels.

The availability of class B transponders is increasing amongst smaller commercial boats, fishing and work boats and for recreational boaters as manufacturers bring more products to market. Being "seen" by using a Class B transponder does not guarantee that the bridge of the receiving ship reads your signal. The ship may not have AIS data displayed or may not be looking at it. Carriage requirements specify that they only send their AIS data and does not require them to have more than a minimal display device aboard. Some ships may not receive class B signals at all or may choose to filter them out. How the class A traffic deals with increasing and potentially high numbers of class B fitted boats remains to be seen; this situation may become a problem as class B traffic increases.

As teenagers we learned defensive driving when we first received our license. As prudent mariners and sailors it is to our benefit to take an active role in defensive collision avoidance. Your vessel may have right of way, but it is much easier for you to maneuver out of a big ship's way to avoid a dangerous situation - especially when you know more about his movement. AIS data is there for the taking. It's use has the potential to save lives.

More Resources

For further reading on AIS and product information:

www.navcen.uscg.gov/marcomms/ais.htm -- US coast guard site on AIS
www.shipadm.org/upload/1486/a171_2.pdf - Swedish Maritime Administration

www.panbo.com - blog site with valuable and interesting AIS Section
www.gcaptain.com - blog by ship captains about ships
www.vespermarine.com - AISWatchMate™ Collision warning alarm