The Automatic Identification System, more commonly known as AIS, is now one of the most valuable navigation tools for ensuring safety at sea. Whether you are an experienced boater or a professional sailor, understanding how to correctly interpret the data transmitted by this system can make the difference between a peaceful crossing and a potentially dangerous situation. AIS broadcasts a wealth of information about surrounding vessels in real time, but this data is only useful if you know how to decipher and use it intelligently. In this article, we'll explore the different facets of AIS together, from the basic information it displays to the advanced interpretation techniques that will allow you to anticipate the risks of collisions. You will also discover how to combine this data with other monitoring methods to navigate safely, regardless of the situation encountered.

Introduction to AIS

The Automatic Identification System is a maritime surveillance system operating using VHF radio waves. Initially developed for commercial vessels, it was gradually democratized among boaters thanks to the appearance of AIS Class B, which was more accessible and adapted to the needs of recreational boating. The basic principle is based on the automatic exchange of information between all vessels equipped with an AIS transponder within a radius of up to several tens of nautical miles depending on the conditions.

Each ship regularly broadcasts its position, identification, speed, heading and various other technical data that allow other navigators to visualize surrounding maritime traffic on a dedicated screen or a compatible GPS tracker. This transmission takes place automatically without crew intervention, thus ensuring that information is constantly updated. AIS operates on two dedicated maritime VHF frequencies and uses a technology called SOTDMA for class A vessels and CSTDMA for class B vessels, allowing thousands of ships to transmit simultaneously without interference.

The primary objective of this system remains the prevention of collisions at sea. By clearly displaying the vessels present in your navigation area, AIS allows you to quickly assess potentially dangerous situations and adapt your course accordingly. However, understanding the symbols, numbers, and abbreviations that appear on your screen requires some training, which we will gradually provide you with through this practical guide.

Understanding the information displayed by an AIS

When you look at your AIS screen, you are confronted with a multitude of data that may seem complex at first glance. Learning to decode this information is the first step in using this system effectively. Each detected vessel is generally displayed as a symbol or icon accompanied by alphanumeric data that provides information on its characteristics and current behavior.

Vessel identification: MMSI, name, type and dimensions

The MMSI number, or Maritime Mobile Service Identity, represents the unique identifier of each ship equipped with an AIS, comparable to an international maritime registration. This nine-digit number makes it possible to unambiguously identify a specific ship among all those present in the world. The first three digits generally correspond to the code of the country of registration, which gives you an immediate indication of the origin of the vessel you are viewing.

In addition to the MMSI, the AIS also transmits the name of the ship as programmed into the transponder. This information makes VHF communications much easier when needed, as you can directly call the vessel by name rather than having to describe it by its position or appearance. Another key piece of information that tells you whether it's a cargo ship, tanker, ferry, fishing boat, yacht, or passenger ship is the type of vessel. This classification helps to understand the probable behavior of the ship and its maneuverability constraints.

The dimensions of the vessel, including its length and width, allow you to visually assess its size even at a distance and better understand its maneuvering capabilities. A ship that is one hundred meters long will obviously not react as quickly as a small pleasure boat. This dimensional information is particularly useful when maneuvering in tight spaces such as channels or port entrances, where accurate knowledge of the size of other vessels helps you make the right decisions.

Navigation data: heading, speed, route followed (COG, SOG)

The COG, or Course Over Ground, represents the actual course that the ship follows in relation to the bottom. This value, expressed in degrees, indicates the effective direction in which the ship is moving, taking into account all external factors such as current and drift. This is crucial information because it sometimes differs significantly from the compass heading displayed by the ship, especially in the presence of strong currents or significant crosswinds.

The SOG, or Speed Over Ground, indicates the actual speed of the ship in relation to the seabed, which is also influenced by currents. This speed is generally expressed in knots and allows you to quickly estimate how long it will take for a ship to reach a given point. The combination of COG and SOG gives you a clear and accurate picture of the actual movement of the ship, which is the basis for any collision risk analysis.

Some AIS systems also show the ship's heading, called HDG for Heading, which is the direction in which the ship's bow is pointing. This information may differ from the COG when the ship experiences lateral drift due to wind or current. Observing the difference between the course and the course followed helps you understand the weather conditions in which the ship operates and to anticipate its future reactions. For a sailboat, this difference can be particularly marked when sailing upwind with a strong slope and a significant drift.

Ship position: GPS coordinates and signal accuracy

Each AIS transmission includes the exact GPS coordinates of the transmitting vessel, expressed in latitude and longitude with an accuracy generally of less than ten meters under normal conditions. These coordinates are automatically reported on your electronic map, allowing immediate visualization of the position of all detected vessels. The frequency of updating these positions varies according to the AIS class and the activity of the vessel, ranging from a few seconds for a fast-moving vessel to several minutes for a vessel at anchor.

GPS signal accuracy is an important parameter that some AIS systems explicitly display. This value tells you how reliable the coordinates received are, as a GPS can sometimes present errors due to poor satellite reception, interference, or technical problems. Degraded precision often results in a value greater than fifty meters and should attract your attention, especially when navigating in confined areas where every meter counts.

The age of the data is also something to consider, as the information displayed on your screen may not be instantaneous. A transmission delay of even a few seconds can create a discrepancy between the actual position of the ship and the one displayed on your system. Modern systems generally show how long it has been since the last update, so you can assess the freshness of the data. For an accurate analysis of collision risks, always use the most recent data possible and take this time into account in your mental calculations.

CPA and TCPA: key indicators for collision avoidance

CPA, an acronym for Closest Point of Approach, represents the minimum foreseeable distance between your ship and another vessel if both maintain their current course and speed. This critical value is generally expressed in nautical miles and is your main indicator of collision risk. A CPA of less than half a nautical mile should immediately catch your attention and encourage you to analyze the situation in detail, while a CPA close to zero indicates a major risk of collision.

The TCPA, or Time to Closest Point of Approach, indicates how much time is left before reaching this point of closest proximity. Generally expressed in minutes, this parameter allows you to determine the urgency of the situation and the time you have to react. A 20-minute TCPA with an acceptable CPA allows you to observe how the situation evolves, while a 3-minute TCPA with a low CPA requires immediate action on your part.

The combination of these two values offers a dynamic vision of risk. A CPA of two hundred meters may seem acceptable if the TCPA says thirty minutes, giving you time to communicate with the other ship or to change your course slightly. On the other hand, this same CPA with a TCPA of two minutes constitutes an emergency situation requiring a candid and immediate maneuver. Modern AIS systems automatically calculate these values and often trigger visual and audible alarms when predefined thresholds are crossed, alerting you to potentially dangerous situations before it is too late.

Status of the ship: anchoring, maneuvering, stopping, navigation

AIS also transmits the operational status of the ship, information that is often overlooked but is nevertheless very useful for understanding the intentions and constraints of the other navigator. A ship can indicate several different statuses depending on its current situation. The status “sailing with a motor” indicates a ship moving normally, while “at anchor” indicates a ship immobilized at anchor, which should theoretically not move unless there is a problem.

The status “under constrained maneuver” is of particular importance because it indicates a vessel whose ability to maneuver is limited for operational reasons. This typically concerns vessels being dredged, laid cables or carrying out towing operations. According to the international regulations to prevent collisions at sea, these ships are given priority and other ships must get out of the way. Recognizing this status on your AIS allows you to adopt appropriate behavior immediately.

Other statuses such as “restricted maneuvering capacity”, “fishing vessel” or “under sail” provide valuable information on the constraints of each vessel. A fishing trawler with its nets deployed cannot change its course easily, while a sailboat under sail alone has different maneuvering capabilities than a motor vessel. Some systems also make it possible to indicate “dangerous” or “ship without steering”, alerting all surrounding vessels to an emergency situation. Getting into the habit of systematically checking the status of nearby vessels helps you better understand maritime traffic and anticipate everyone's behavior.

How to interpret AIS data to anticipate risks

Beyond simply reading the raw data, the intelligent interpretation of AIS information requires an analytical approach that combines several elements. This analysis phase represents the core of the use of AIS for maritime safety, transforming numbers and symbols into concrete navigation decisions. Developing this skill requires practice and a thorough understanding of maritime principles.

Read the relative movement between your boat and the others

The concept of relative motion is the cornerstone of marine collision risk analysis. Instead of considering the absolute movements of each ship, you need to visualize how other ships move in relation to your own boat, as if you were stationary in the center of a reference system. This approach considerably simplifies the assessment of crossing or catching up situations.

To understand relative motion, imagine that your ship is fixed and all the others are moving around you. If a ship seems to be heading directly toward you with no change in bearing, that means you are on a certain collision course. On the other hand, if the bearing of the other ship changes progressively forward or backward, you pass safely, either in front of or behind it. This ancient technique used well before the invention of AIS remains perfectly valid and is now facilitated by the accurate data provided by the system.

Modern AIS systems often display motion vectors, represented by arrows extending from ship symbols. These vectors can be configured in “true” mode showing the real course of the ship, or in “relative” mode illustrating its movement in relation to your own ship. Relative mode is particularly useful for early detection of dangerous situations, as a vector pointing directly at your location indicates an imminent risk of collision. Learning to switch between these two modes depending on the circumstances will give you a complete mastery of marine traffic analysis.

Detecting a potential collision route (low CPA)

Early detection of a collision course is your first line of defense against accidents at sea. A low CPA represents the main alarm signal, but it is still necessary to know how to correctly interpret what is meant by “low” according to the different navigation situations. In offshore navigation with little traffic, a CPA of two nautical miles may be considered comfortable, while in a busy channel, even a CPA of half a mile can be a cause for concern.

The evolution of the CPA over time also provides valuable information. If you observe that an initially comfortable CPA gradually decreases over the minutes, it means that one of the two ships, or even both, have changed their course or speed slightly, thus making the situation worse. This trend should alert you immediately and encourage you to take preventive measures. Conversely, a gradually increasing CPA indicates that the situation is improving naturally, either through your own actions or those of the other ship.

AIS systems generally incorporate configurable alarm functions that are triggered when the CPA falls below a threshold that you have set. These alarms are an excellent safety net, especially during night shifts or solitary sailing where alertness may decrease. However, it is crucial not to rely solely on these automatic alarms and to maintain an active watch by regularly consulting your AIS. The Oria Marine solution with its IoT box also offers the possibility of receiving personalized alerts on your smartphone, allowing you to stay informed in real time of risky situations even when you are not directly in front of the navigation screen.

Differentiating commercial traffic, boating, fishing and ferries

Each category of ship has specific navigation characteristics that influence its behavior at sea. Freighters and container ships generally follow predictable routes with consistent high speeds, often in excess of fifteen knots. Their considerable inertia prevents them from making sudden changes in course or speed, which means that in the event of a critical situation, it is usually up to the smallest and most maneuverable vessel to react.

Ferries have particularly regular trajectories because they make the same crossings daily at fixed times. Their behavior therefore becomes very predictable once you know their usual routes in your navigation area. However, their high speed, often in excess of twenty knots for modern fast ferries, leaves little time to react in the event of a delicate situation. It is therefore important to pay particular attention to them and to always get out of their way early enough.

Fishing vessels are probably the most unpredictable category of marine traffic. Their behavior depends entirely on their fishing activity, which can cause them to make sudden course changes, unexpected stops, or maneuvers in a circle. An active fishing vessel can indicate the status “in fishing” on its AIS, a signal that must be respected by maintaining a sufficient distance. Boaters, on the other hand, show a great variability of behavior depending on their level of experience and the type of navigation practiced, which requires increased vigilance, particularly in coastal areas that are busy at weekends and during the summer season.

Identify atypical or dangerous behaviors

The experience at sea progressively develops your ability to recognize normal navigation patterns, making it easier to identify unusual behaviors. A cargo ship that zigzags instead of maintaining a constant course, a ferry that slows down significantly for no apparent reason, or a ship that stops suddenly in the middle of the sea are all warning signs that deserve your particular attention.

Some behaviors can betray an inexperienced or struggling crew. A sailboat that makes numerous close course changes in calm weather conditions may suggest a novice or lost crew. A ship that maintains an abnormally low speed for its type could experience mechanical problems. These observations, combined with AIS data, allow you to anticipate potentially dangerous situations and maintain an increased safe distance from these vessels.

The consistency between AIS data and what you observe visually is also an important element to check. If the AIS indicates that a ship is heading north at ten knots but visually you see that it seems to be sailing east instead, this may indicate a problem with its AIS system, a configuration error, or in rare cases even an attempt to hide illegal activities. In any case, such inconsistency should encourage you to be very careful and to prioritize your visual and radar observations over potentially erroneous AIS data.

Using AIS in a real situation

The theory makes perfect sense when it is applied to concrete navigation situations. Examining different typical scenarios allows you to better understand how to use AIS in your daily practice and to develop appropriate reflexes in the face of the common situations that you will inevitably encounter during your navigation.

Analysis of a typical case of approach at night

Nocturnal navigation completely transforms the perception of the maritime environment. Your usual visual cues are largely disappearing, replaced by navigation lights that can be confusing, especially when several ships are present simultaneously. In this context, AIS becomes your most valuable ally in maintaining clear and accurate situational awareness.

Let's say you're sailing at night and your AIS detects a ship seven nautical miles away showing a CPA of five hundred meters and a TCPA of twenty minutes. These values suggest a situation that requires your attention without still constituting an emergency. Your first action is to visually identify the lights on the ship in question. The AIS shows you its exact bearing, so you can point your eyes in the right direction. You may be seeing a white masthead light and a red light, indicating that you are seeing the ship from starboard.

By consulting the details of the ship on your AIS, you discover that it is a 150 meter cargo ship sailing at fourteen knots. Its route indicates that it is probably following a navigation channel, information confirmed by the superposition on your electronic map. Armed with these elements, you understand that this cargo ship will most likely follow its channel without deviating, allowing you to anticipate its behavior. You decide to slightly change your course by ten degrees to port to increase the CPA to a more comfortable value of one nautical mile, a maneuver you perform early enough to make it clear and obvious to the other ship.

Interpreting data in reduced visibility

Fog, heavy rain, or snow can drastically reduce visibility, turning even simple coastal navigation into a delicate exercise that requires your full attention. Under these conditions, AIS combined with radar becomes your extended sensory system, making up for the lack of direct visual cues. However, caution requires an even more rigorous interpretation of the data received.

When visibility falls below one nautical mile, international regulations require sailing at a reduced speed adapted to the circumstances. Your AIS may show you multiple targets within a five-mile radius, but you can't visually confirm any of them. This situation requires maximum vigilance because other vessels not equipped with AIS can also navigate in the area without you being able to detect them other than visually or by radar.

The methodical approach consists in identifying on the AIS all vessels with a CPA of less than two nautical miles and in particular monitoring those whose TCPA is less than thirty minutes. For each, you assess whether your current route is safe or requires adjustment. There is a temptation to rely only on AIS data, but maritime caution also requires careful listening to the sound signals emitted by other vessels in accordance with international regulations, as well as a speed sufficiently reduced to be able to stop within half of the available visibility distance.

Confirm AIS data with radar and visual intelligence

The fundamental principle of safe navigation is based on the intersection of several sources of information. AIS, as efficient as it may be, is only one element of your overall situation picture. Crossing it systematically with your radar observations and visual intelligence creates redundancy that minimizes the risks of error and compensates for the possible failures of each system taken individually.

When your AIS shows a ship three nautical miles away, your reflex should be to check its presence on the radar. This confirmation brings several advantages. First, it validates the consistency of AIS data by ensuring that a radar echo corresponds to the indicated position. Second, it allows you to detect possible additional ships not equipped with AIS that would only appear on radar. Third, radar can sometimes provide a better estimate of the exact distance, especially for ships that are very close.

Visual monitoring remains mandatory in all circumstances, even with the best embedded electronic systems. Your eyes can detect things that neither AIS nor radar pick up, such as small boats, floating objects, fishing buoys, or marine animals. In addition, visual observation tells you about local weather conditions, the state of the sea, and can reveal information about the intentions of other ships through their navigation lights and movements. This multi-sensory approach represents the essence of careful and professional navigation.

React correctly in the event of a risk of collision

When your analysis of AIS data reveals a real risk of collision, your reaction must be both rapid and in accordance with international regulations for the prevention of collisions at sea. There is sometimes a temptation to make small, gradual adjustments in course, but this approach poses a major danger because it makes your intentions unclear for the other ship. International regulations state that any evasive maneuver must be candid, clear, and done early enough to be obvious.

In a crossing situation where you see a ship from starboard, the regulations require you to manoeuvre to pass behind it. Your action therefore consists in reducing your speed or in coming frankly to starboard to significantly increase the CPA. A change of course of thirty degrees is generally the minimum for a maneuver to be considered clear and obvious. Then follow the evolution of the CPA on your AIS to confirm that your action is producing the expected effect, the CPA should increase gradually if your maneuver is appropriate.

In some complex cases involving several vessels simultaneously, direct VHF communication may be necessary to coordinate maneuvers. The AIS provides you with the exact name of the ship and sometimes even its call sign, greatly facilitating this communication. Never hesitate to call a ship if the situation seems confusing or if you are unsure of the appropriate maneuver. Clear communication is always better than a collision that was narrowly avoided by chance. After any evasive maneuvers, continue to monitor the situation until the ship has clearly passed and all dangers have been removed before returning to your original course.

Best practices to avoid common mistakes

The effective use of AIS comes with some pitfalls that less experienced mariners frequently fall into. Knowing these common mistakes and the methods to avoid them will allow you to get the most out of your system while maintaining the highest security standards.

Do not rely solely on the map: cross-check is essential

The most frequent and potentially the most dangerous mistake is to develop an excessive trust in AIS to the point of neglecting other means of monitoring. Seeing all of the surrounding ships clearly displayed on a screen creates a sense of control and safety that can be deceptive. This misplaced trust can cause you to relax your visual alertness or to navigate faster than conditions allow.

The maritime reality still includes a significant proportion of vessels not equipped with AIS, especially among small pleasure boats, traditional sailing boats, tenders, kayaks and other small boats. These ships remain completely invisible on your AIS screen but nevertheless constitute a real danger of collision. In busy coastal areas, these small boats can represent the majority of real traffic, creating a false sense of security if you rely solely on AIS.

The professional method therefore requires constantly maintaining an active visual alert, regularly consulting the radar, and systematically corroborating AIS information with these other sources. If your AIS shows an empty space in front of you but visually you see a small boat, visual information is obviously the most important thing. This discipline of constant checking may seem tedious but it is your best guarantee against unpleasant surprises at sea. Modern connected systems such as the Oria Marine IoT box facilitate this global approach by centralizing various data sources and alerting you intelligently to situations requiring your attention.

Verify the reliability of the AIS data received

Not all AIS signals are the same in terms of reliability. Several factors can affect the quality of the data you receive, from transmitter transponder configuration errors to transmission problems due to radio propagation conditions. Developing a critical eye on the data received is an integral part of mature use of AIS.

There are warning signs that should immediately draw your attention to the possible data failure. A ship whose position jumps abruptly several hundred meters between two successive updates probably indicates a GPS problem. A cargo ship with a speed of forty knots is clearly a data error because no commercial cargo ship can reach such a speed. Incoherent information, such as a ship that is categorized as a 100-meter cargo ship but whose behavior is more like a small fishing boat, should also arouse your suspicion.

Another common pitfall is the presence of old data. If a ship was last updated more than ten minutes ago, the information displayed probably no longer reflects its current position. Some systems visually signal the age of data by changing the color or transparency of the symbols, making it easier to identify targets whose information is out of date. In any case of doubt about the reliability of AIS data, go back to the basics of traditional navigation and prioritize your visual and radar observations over suspicious electronic information.

Know how to interpret a ship without AIS or with a weak signal

The absence of an AIS signal for a ship that you are observing visually or on radar is a common situation that requires an adapted approach. This absence can have multiple causes, from the simple fact that the ship is not equipped to a technical failure of its transponder, through insufficient radio range or geographical obstacles blocking transmission.

Small pleasure boats under fifteen meters are generally not required to have an AIS, which explains their frequent absence on your screens. Traditional sailing boats, artisanal fishing boats and rental boats also represent categories that are often not equipped. Faced with these electronically invisible ships, you must rely solely on your visual watch and radar, applying the classical rules of maritime navigation without the comfort of AIS data.

A weak or intermittent AIS signal, characterized by frequent link losses or partial data, may result from the excessive distance between the two vessels, geographic obstacles such as islands or cliffs, or from a poorly positioned or faulty transmitting antenna. Under these conditions, never assume that you have a complete picture of the situation. Maintaining increased safety margins and increased vigilance compensates for the uncertainty created by this incomplete data. The principle of maritime caution always requires acting on the basis of the worst-case scenario that is reasonably foreseeable rather than hoping for the best.

Set up your own AIS correctly to be visible

Your responsibility as a navigator equipped with AIS is not limited to receiving data from other vessels. You must also ensure that your own transponder is transmitting correctly and that the information broadcast is accurate, allowing other browsers to identify you and avoid you effectively. A poorly configured AIS can cause confusion for other browsers and unintentionally create dangerous situations.

Key parameters to check include the name of the vessel, its MMSI number, exact dimensions, type, and call sign. This information is generally programmed during the initial installation of the system but deserves periodic verification, especially if the vessel has changed owners or if significant changes have been made on board. Wrong dimensions can mislead other mariners about your actual congestion, while the wrong type of ship can mislead their expectations about your sailing behavior.

The AIS antenna also requires special attention because its position and condition directly affect the range and quality of your broadcasts. A poorly positioned or damaged antenna significantly reduces your visibility for other vessels, especially at long distances. Conducting regular range tests, either by comparing with other equipped vessels or by using coastal AIS receiving stations, ensures that your signal is well received within an appropriate radius. Think of your AIS as a two-way safety device whose effectiveness depends as much on its good reception as on its good transmission.

Additional tools for safe browsing

AIS is part of a larger ecosystem of electronic navigation tools that, when used together, offer maximum situational awareness. Understanding how these different systems complement and interact allows you to build a modern, safe, and efficient navigation approach.

Combining AIS and radar for comprehensive traffic analysis

Radar and AIS have complementary characteristics that, when used together, create a powerful synergy for the detection and monitoring of maritime traffic. Radar detects all objects that reflect radio waves indiscriminately, regardless of whether they are equipped with AIS or not, and operates independently of any cooperative system. On the other hand, AIS only detects ships equipped with a transponder but provides much richer information on their identity and intentions.

Overlaying AIS data on the radar screen, a feature available on most modern systems, is the optimal approach. This fusion allows you to instantly correlate each AIS target with its corresponding radar echo, thus validating the coherence of the two information sources. When a radar echo appears without an associated AIS target, you immediately identify an unequipped ship in need of increased surveillance. Conversely, an AIS target without a corresponding radar echo indicates either an AIS data problem, or more likely a ship that is too far away to be detected by your radar.

Radar also excels at detecting fixed objects such as buoys, emerging reefs, or offshore structures, elements that AIS obviously cannot detect. In addition, in difficult weather conditions, radar can sometimes detect intense precipitation or squalls before they reach you, valuable information for your safety. This complementarity makes the Radar-AIS pair the standard combination for serious sailing, whether commercial or high-end recreational.

AIS-compatible mobile applications and navigation software

The digital age has democratized access to AIS data well beyond traditional dedicated equipment. Many mobile applications and navigation software now incorporate AIS functionality, often by operating networks of terrestrial receiving stations that collect and share AIS data via the Internet. These solutions have considerable advantages in terms of cost and flexibility, while at the same time having some limitations that should be understood.

Applications such as MarineTraffic, VesselFinder or Navionics allow you to visualize global maritime traffic in near real time from your smartphone or tablet. This global visibility is particularly useful for route planning, analyzing traffic patterns in an unknown area, or simply to satisfy your curiosity about the identity of a ship observed in the distance. These terrestrial applications generally receive AIS signals up to forty or fifty miles from the coast, an area covered by the network of terrestrial receiving stations.

However, these Internet-based solutions present significant limitations for real-time browsing. Data latency, typically a few minutes, makes it inappropriate for critical collision avoidance decisions. In addition, their dependence on an Internet connection makes them unusable on the high seas or in areas with limited network coverage. These applications are therefore excellent complementary tools for planning and general supervision, but cannot in any way replace a dedicated embedded AIS receiver for safe navigation. Maritime IoT systems like those proposed by Oria Marine offer precisely an interesting compromise by combining local AIS reception and Internet connectivity to offer the best of both approaches.

Use of AIS Class B for boaters

AIS Class B was specifically developed to meet the needs of recreational vessels and small commercial vessels, offering a more affordable alternative to the mandatory AIS Class A for large commercial vessels. Understanding the differences between these two classes and the specific capabilities of Class B helps you make informed choices for your equipment and better interpret data from other vessels.

AIS Class B transponders transmit the same essential information as Class A, including position, speed, heading and vessel identification. The main difference lies in the update frequency, which is lower for class B. A ship equipped with AIS Class B updates its position every thirty seconds when sailing at more than two knots, compared to two to ten seconds for a class A ship. This difference remains generally acceptable for recreational boating where the speeds and weights involved are lower.

The transmission power is the other notable difference, with class B emitting at two watts against twelve watts for class A. This lower power results in a slightly reduced theoretical range, generally around ten to fifteen nautical miles under ideal conditions compared to twenty to thirty miles for class A. In practice, this range remains largely sufficient for the needs of coastal and semi-offshore yachting. For offshore sailors making long crossings, AIS Class B nevertheless offers valuable visibility in the face of large commercial vessels, which are all equipped with Class A and will be able to detect you at reasonable distances.

Conclusion

Today, AIS represents an indispensable tool for modern navigation, profoundly transforming our ability to perceive and understand the maritime environment that surrounds us. Mastering the reading and interpretation of AIS data cannot be improvised and requires a thorough understanding of the various parameters displayed, their practical meaning and their limitations. The concepts of CPA and TCPA form the fundamentals of collision risk analysis, while reading relative motion allows you to anticipate dangerous situations with remarkable precision.

However, the effectiveness of AIS is based on intelligent use that never replaces traditional monitoring but complements it. The systematic cross-referencing of AIS data with radar and visual observations creates this redundancy that is essential for maritime safety. Each source of information has strengths and weaknesses, and it is precisely their combination that provides the best possible situational awareness. Never forget that a significant proportion of maritime traffic remains invisible to AIS, justifying the constant maintenance of active vigilance.

Regular practice remains the key to transforming this theoretical knowledge into operational reflexes. Each sea trip offers an opportunity to analyze real situations, to observe the navigation patterns of different types of vessels, and to refine your ability to quickly interpret AIS data. Over time, reading the AIS becomes intuitive, allowing you to assess the complexity of a traffic situation in seconds and immediately identify vessels requiring your priority attention. This control contributes not only to your personal safety but also to that of all mariners sharing the same waters, creating a safer maritime environment for the entire boating community.

FAQ — Frequently asked questions

What are the most important AIS parameters to monitor to avoid a collision?

The essential parameters are the CPA and the TCPA, which respectively indicate the minimum approach distance and the time before reaching this point. Also, monitor the COG and SOG to understand the actual direction and speed of the ship. The constant bearing between your ship and another is the main warning sign of an impending collision. Finally, always consult the type and status of the ship to anticipate its behavior and maneuvering constraints.

What should I do if a ship approaches and does not show AIS data?

Use only your visual and radar observations to assess the situation. Rigorously apply the rules of the bar and route of the international regulations. Increase your safety margins because you don't have the detailed information that AIS would normally provide. If necessary, do a candid and obvious maneuver early enough to make your intentions clear. In case of doubt, do not hesitate to communicate by VHF on channel 16 to establish contact with the ship.

What is the difference between AIS Class A and Class B?

AIS Class A equips commercial vessels and transmits data every two to ten seconds with a power of twelve watts, offering a range of twenty to thirty miles. The AIS Class B, intended for yachting, updates the data every thirty seconds with a power of two watts and a range of ten to fifteen miles. Both classes transmit essential identification and navigation information, but Class A offers better responsiveness and a greater range suitable for large, fast vessels.

Does AIS work in the event of a GPS failure?

No, AIS depends entirely on GPS to determine the position of the ship it is transmitting. In the event of a GPS failure, the AIS transponder can no longer transmit reliable position data and most systems report this failure. However, the ship is still visible on the radars of other ships. This dependence on GPS highlights the importance of maintaining traditional navigation methods such as compasses, paper maps and estimated navigation techniques to mitigate any electronic failure.

Why are CPA and TCPA essential for risk analysis?

The CPA indicates the minimum distance the other ship will pass if both maintain their current course, thus directly quantifying the risk of collision. The TCPA specifies when this point will be reached, informing you of the urgency of the situation and the time available to react. Together, these two values transform a complex situation into objective data that can be easily interpreted, allowing quick and appropriate decision-making even under stress or fatigue.

Can AIS completely replace radar?

Absolutely not. AIS and radar are complementary and not substitutable. AIS only detects vessels equipped with a transponder and depends on their correct functioning and configuration. Radar detects all objects indiscriminately, including ships without AIS, buoys, reefs, and even weather conditions. Furthermore, the radar operates independently of any cooperative system and cannot be tampered with or misconfigured. Modern safe navigation requires the joint use of both systems.

Is it mandatory to have an AIS on a pleasure boat?

Regulations vary by country and vessel size. In Europe, AIS is generally not mandatory for pleasure boats under fifteen meters, but it is highly recommended to improve your visibility and safety. Some particularly busy maritime areas or certain types of navigation may require AIS even for small vessels. Always consult the regulations specific to your flag and the areas where you sail to know your exact obligations.

How do I know if the AIS data received is reliable?

Check the consistency between the AIS data and your visual or radar observations. Watch out for outliers such as speeds that are impossible for the type of ship shown or positions that jump abruptly. Check the age of the last update, as data older than ten minutes is likely to be out of date. Observe the continuity of the signal, with frequent interruptions that suggest technical problems. When in doubt, always focus on direct observations on suspicious electronic data and maintain increased safety margins.