AIS certification study rewards one habit: treat every displayed field as a claim with a category, an age, and a source. Static data rarely changes but can be wrong at the source; dynamic data is fresh but can drift for nearly stopped vessels; voyage data lags by minutes. Map each message type to the decision it supports, expect Class A and Class B targets to behave differently on the VHF data link, and corroborate AIS with radar and visual observation before acting on any single field.
Why the Three AIS Data Categories Change How You Read a Target
AIS reports combine three data categories: static (identity, entered once), dynamic (motion, refreshed every report), and voyage (changes per voyage). Knowing which category a field belongs to tells you how quickly it can go stale.
Static data — the MMSI, call sign, name, ship type, and the A/B/C/D dimensions locating the antenna relative to bow, stern and both sides — is entered once and changes rarely. Dynamic data — position with an accuracy flag, speed over ground, course over ground, heading, rate of turn, and navigational status — is refreshed with every position report. Voyage data — draught, hazardous cargo category, destination and ETA — changes only per voyage, which is why it can silently lag reality.
Apply the categories when you judge a display. A heading that lags course over ground during a turn is normal kinematics within the dynamic set, not an error. A navigational status of 'at anchor' combined with six knots of speed over ground is a voyage-status conflict worth questioning rather than accepting. In scenario work, first label every quoted value with its category, then ask how stale that category can plausibly be at the moment of the decision.
Short exercise: take any target list and write S, D or V beside each field. Fields you hesitate over are exactly the ones to review.
Class A and Class B Transponders Behave Differently — Read Them Differently
Class A units on larger commercial vessels use SOTDMA, reserving slots and carrying the full message set. Class B units on smaller craft use CSTDMA: lower power, fewer fields, and reports that can be dropped when the link is congested.
Class A units transmit at higher power and use self-organizing time division multiple access, reserving slots on the data link so their reports keep a steady rhythm. Class B units use carrier-sense TDMA: they listen for free slots and give way to Class A traffic. Their position report (message 18) omits fields such as rate of turn, navigational status and voyage data; the static details arrive separately in a two-part message 24.
The interpretation consequences are concrete. A Class B target can skip reports when the link is busy, so an intermittent track is an expected property of the equipment, not evidence that a vessel is concealing itself. Because Class B carries no navigational status field, a missing status on such a target is a design fact, not a defect. When a scenario asks whether a small craft appears on every display with full details, the answer turns on which class it carries.
Match Each Message Type to the Decision It Supports
Each AIS message type answers a different operational question — where is the target, what is it, or is it a fixed aid or a hazard. Map every type to the bridge decision it supports before attempting scenario items.
Read the table below as a mapping from information to action, not as a list to recite. In scenario-based study, treat every display excerpt as an instruction to name the message type behind each field before deciding anything. If you cannot say which message produced a value, you also cannot say how often it refreshes or when it might be outdated.
Timing is part of the mapping. The static and voyage message (type 5) repeats on the order of every six minutes, so a Class A target can be tracked for several minutes before its name appears, and a vessel that recently changed destination may still show the old one. Separately, a position-report target whose MMSI begins with a 970-series pattern identifies a distress beacon such as an AIS-SART: treat it as a distress source, not as ordinary traffic.
| Message | Typical source | Carries | Decision it supports |
|---|---|---|---|
| 1 / 2 / 3 | Class A transponder | Position, SOG, COG, heading, ROT, navigational status | Track targets and judge motion |
| 5 | Class A transponder | Static and voyage data | Identify the hull; plan for draught and cargo |
| 18 | Class B (CS) transponder | Position, SOG, COG, heading | Track smaller craft not carrying Class A |
| 24 | Class B transponder | Static data in two parts | Attach a name and type to Class B tracks |
| 21 | AIS aid to navigation | AtoN position, type and status | Compare the displayed aid against the chart |
| 4 | Base station | Position and time reference | Recognize shore infrastructure on the link |
| 14 | Any AIS station | Safety-related broadcast text | Heed a hazard reported nearby |
The VHF Data Link: Slots, Channels and Why Targets Go Missing
AIS shares two VHF channels (AIS 1 and AIS 2) using a one-minute frame divided into thousands of time slots that stations allocate themselves. Congestion means skipped reports, so an absent target is not necessarily an absent vessel.
Each channel organizes transmissions into a frame of 2,250 time slots per minute. SOTDMA stations reserve slots in advance; ITDMA supports shorter, transitional transmissions such as an initial burst after activation; CSTDMA stations (Class B) simply transmit when they detect a free slot. This allocation machinery is why AIS can carry hundreds of units in a busy area — and why it degrades gracefully by dropping lower-priority reports rather than failing outright.
Reporting intervals vary with the situation: as typical values, a Class A unit under way updates every few seconds, while anchored or moored units may update only around every three minutes. Use this when judging report age: a five-minute gap for a moored container ship is unremarkable, but the same gap for a fast-moving target is a genuine data hole. In scenarios, state the expected interval before you call a track stale — the exercise below trains exactly this habit.
Worked Scenario 1: A Nearly Stopped Target on Your Track
When a target shows SOG near zero with a COG across your course, the common mistake is treating that COG as the vessel crossing your bow. The better decision is to check report age, plot motion across several reports, and corroborate with radar and visual observation.
Scenario: you are overtaking a target in a traffic lane. Its display shows SOG 0.3 knots, heading 090, COG 175, sitting ahead of a slower convoy. A plausible mistake is to read COG 175 as the vessel crossing your bow and maneuver around a 'moving' obstacle. The error is mixing data categories: for a nearly stopped vessel, SOG and COG come from the position sensor and can drift with current and set without reflecting hull motion at all.
The better decision is procedural. First, check the age of the last position report against the interval expected at that speed — a stopped Class A unit may report only about every three minutes, so a five-minute-old fix is plausible. Then judge motion from the change in position across two or three reports, not one COG readout, and confirm extent and movement on radar and by eye. This matters because collision-avoidance decisions rest on all available means; a single stale or drifting AIS field is not a basis for altering course.
Worked Scenario 2: Static Data That Contradicts What You See
When static data contradicts observation — a name or type that does not match the hull — the mistake is quietly accepting it. The better decision is to verify identity through the MMSI and call sign, log the discrepancy, and report it through established channels.
Scenario: during a night watch the display shows a target named 'COASTAL TUG' typed as a pilot vessel, yet the radar picture suggests a vessel several hundred meters long. A tempting choice is to assume the name field is merely miskeyed and track by position alone. The trap is that one wrong field usually means the whole static record is unreliable — the dimensions, ship type and destination may all belong to a different unit entirely.
The better decision is verification. The MMSI is nine digits whose first three digits are the Maritime Identification Digits (MID) identifying the country; note that a trailing check digit appears only in certain series — such as distress transmitters and aids to navigation — not in standard ship-station MMSIs, so verify identity primarily by cross-checking the call sign on the VHF watch and comparing the reported A/B/C/D dimensions with the radar extent. Log the mismatch and report it through your company's or administration's established procedure instead of quietly editing your own notes to fit. This matters because if two nearby units carry corrupted static records, you can attach the wrong identity — and the wrong expectations — to the wrong hull.
AIS Aids, Distress Beacons, and a Logbook Exercise With a Prep Sequence
Recognize the special AIS units — aids to navigation (message 21, 99-series MMSI pattern) and distress beacons (970-series) — then build a logbook of ten logged targets. A four-week sequence moves you from vocabulary to audited scenario decisions.
Exercise: take thirty minutes of AIS traffic from a simulator, a coastal feed, or a ferry passage and log ten targets. For each, record the data categories present, the class if determinable, the message types you observed, the age of the newest position, and one decision the data supports. Expected observations: moored units updating only every few minutes; Class B targets whose names arrive after their first positions; occasional skipped reports. If your log shows none of these, you were reading a summarized display rather than raw AIS behavior. A virtual aid to navigation deserves special attention — it exists only as an AIS message with no physical mark on the water, so it can warn of a hazard no buoy marks yet.
A workable preparation sequence: weeks one and two, rebuild the vocabulary — data categories, Class A versus Class B, the message table, reporting intervals — until you can reproduce the table and typical intervals from memory. Week three, run written scenarios and log decisions as in the exercise. Week four, audit: re-answer every scenario cold and repeat any item where you cannot justify the decision in one sentence. Readiness checks: (1) place any displayed field in its data category instantly; (2) given a speed, name the expected Class A interval and flag older reports as stale; (3) sort a mixed message list into position, static, aid and broadcast groups unaided; (4) every logged decision cites AIS plus at least one corroborating source. Treat any self-check score as a learning milestone, not a prediction of a result — and consult the awarding body directly for the administrative details of the specific certification. This guide covers subject knowledge; the IMO and ITU-R sites linked below are the reference points for the underlying international framework.
- Rubric line 1: you can state the three data categories and place any displayed field in one of them from memory.
- Rubric line 2: given a speed, you can name the typical Class A reporting interval and flag a report older than that as stale.
- Rubric line 3: you can sort a list of message types into position, static, aid and broadcast groups without notes.
- Rubric line 4: in your logbook, every decision entry cites at least two corroborating sources — AIS plus radar, visual observation, or the chart.
References and further reading
Use these references to explore the concepts and check the latest information from the relevant organizations.