AIS Class A vs Class B: Differences, carriage requirements, and inspection points. A practical guide for marine radio inspectors

#General 11 min 146 views

AIS transponder

AIS transponders: why class matters

The Automatic Identification System (AIS) is a VHF-based system that automatically exchanges vessel data (identity, position, course, speed, etc.) between ships and shore stations.

For inspectors, the key point is simple: AIS is not just equipment installed onboard — it determines whether a vessel can be reliably detected and tracked.

This functionality is provided by AIS transponders. Their class (A or B) defines:

how consistently the vessel appears in traffic
what data is transmitted
whether regulatory requirements are met
how detailed the inspection needs to be

AIS Class A vs B — core differences

Technical comparison relevant for marine radio inspectors

AIS Class A vs B — core differences. Table

Key insight:

Class A uses SOTDMA for predictable, prioritized transmission.

Class B relies on opportunistic channel access (CSTDMA or SOTDMA in Class B “CO”), which directly affects visibility under load.

Operational behavior — what inspectors should understand

Class A

Periodic transmission using reserved time slots (SOTDMA)
High, dynamic reporting rate (speed‑ and manoeuvre‑dependent), enabling reliable tracking
Full AIS dataset, supporting traffic monitoring and management systems
Stable performance in congested VHF environments, due to transmission priority

Class B

Transmission depends on channel availability
Lower transmit power, typically resulting in reduced effective range
Lower and variable reporting rate
Reduced data set compared to Class A

Important:

A Class B target not visible on AIS does not necessarily indicate a fault — it may result from channel congestion, transmission delays, or RF limitations.

Inspection context:

Operating conditions matter. In congested ports or high-traffic coastal areas, Class B performance may degrade due to channel load. In quieter RF environments, intermittent visibility is less likely and may point to installation or transmission issues.

Carriage requirements (SOLAS & beyond)

Mandatory carriage (Class A)

AIS Class A is required for:

All ships ≥300 GT on international voyages
Cargo ships ≥500 GT (non-international)
All passenger ships (regardless of size)

Non-SOLAS vessels (Class B)

Recreational vessels
Small fishing vessels
Coastal workboats

Generally not mandatory, but widely used for visibility.

Note:

Some flag states may extend AIS carriage requirements. In certain cases, they may allow Class B transponders in limited mandatory scenarios under specific conditions.

Inspection points — practical checklist

✅ General (applies to both Class A and Class B)

MMSI correctly programmed
Static data (name, call sign, dimensions) verified
Position data valid (GNSS input confirmed)
Transmission confirmed externally
Reception verified
Antenna condition and VSWR within acceptable limits
No active alarms or fault indications

How to verify transmission in practice:

Use an independent AIS receiver or approved tester
Confirm that the target is visible with the correct MMSI and position
Check that updates are received consistently for the vessel’s operating condition
Do not rely only on local TX indication on the unit

✅ Class A specific checks (compliance-focused)

Reporting intervals compliant with IMO-defined dynamic reporting rates
Continuous automatic position reporting confirmed
AIS safety-related (ASM) messaging capability verified
Detection and correct interpretation of AIS-SART targets verified
RF output power within specification
Interfaces with external sensors verified:
- GNSS
- heading
- rate of turn (ROT), where applicable

Why it matters:

Class A is part of mandatory safety compliance — deficiencies may lead to detention.

What is not acceptable:

Missing or invalid position data
Incorrect static or voyage-related data
Reporting behavior inconsistent with vessel status
Transmission present, but data incomplete or non-compliant

✅ Class B specific checks (functionality-focused)

Reporting intervals (including high-speed mode, where applicable) correct
Correct operation in CSTDMA or SOTDMA mode (depending on device type)
Transmission confirmed (target visible on independent AIS or tester)
AIS safety messaging capability (if supported by the device)
Antenna installation quality
Output power sufficient for reliable detection
Internal or external power backup (if applicable) operational

Key point:

For Class B, the focus is not formal compliance — but actual visibility and correct configuration under real operating conditions.

What to look for in practice:

Delayed or inconsistent visibility
Reduced detection range
Incorrect static data
Unstable performance caused by poor antenna installation, RF losses, or power supply issues

Typical inspection pitfalls

“AIS is transmitting” ≠ AIS is compliant
Class A operating without valid voyage data => non-compliant
Class B installed where Class A is required
Misidentified transponder class on small commercial vessels
Poor antenna or RF path issues:

reduced effective transmit power
intermittent or lost visibility

Incorrect MMSI or static data => false vessel identity in AIS network

Diagnostic logic:

If a target is intermittently visible, check antenna condition, cabling, connectors, transmit power, and channel load
If transmission is present but data is incomplete, verify GNSS input and connected sensor data
If Class B performance appears weak in a congested area, consider channel access limitations before concluding that the unit is faulty

Why the distinction matters

Class A and Class B AIS transponders serve different operational roles.

Class A is designed for:

safety-critical environments
high traffic density
predictable and prioritized transmission

Class B provides:

basic situational awareness
lower-cost visibility
non-mandatory applications

Understanding this distinction allows inspectors to focus on parameters that directly affect navigational safety and regulatory compliance.

Field summary & inspection focus

In practice, inspecting AIS transponders means looking beyond checklists.

Class characteristics

Class A — mandatory, predictable, high reporting rate, full dataset
Class B — optional, variable transmission, lower priority, reduced dataset

Inspection priorities

Verify Class A for regulatory compliance
Verify Class B for actual detectability in real conditions

Quick assessment

Class A not transmitting correctly => non-compliant
Class A transmitting with invalid or incomplete required data => non-compliant
Class B intermittently visible => verify conditions before concluding fault
Incorrect identity or vessel data => always a defect