Since the IMO made automatic tracking mandatory under SOLAS Chapter V in 2002, AIS has transformed collision avoidance and maritime navigation. But have you ever wondered why certain vessels disappear from public tracking maps while commercial container ships remain visible 24/7?
Regulatory Evolution – When AIS became mandatory
1998 - Performance Standards: IMO adopts technical performance standards for VHF automatic identification systems
2002 - SOLAS Convention: Mandatory rollout under SOLAS Chapter V for ships >300 GT and all passenger ships.
2008 - Full Commercial Fleet: Full implementation deadline for existing international commercial cargo ships.
2014 - EU Inland & Fishing: EU Directive expands mandatory AIS usage to inland shipping and fishing vessels >15m.
Class A vs. Class B Transponders
Key Comparison
Feature | Class A (Commercial/SOLAS) | Class B (Recreational/Small craft) |
Target Group & Duty | Mandatory for SOLAS ships, cargo >300 GT, passenger ships | Voluntary for yachts, pleasure craft, small workboats |
Power Output | High Power (12.5W) ~ 20–30 NM range | Low Power (2W std/5W B+) ~5–10 NM range |
Access Protocol | SOTDMA (Self-Organizing Time Division Multiple Access) | CSTDMA (Carrier Sense) or SOTDMA (Class B+) |
Network Priority | Top Priority (Guaranteed time slots) | Lower Priority (Transmits only in open slots) |
Update Interval | Every 2 to 10 seconds (dynamic based on speed) | Every 30 seconds (Standard B)/5–15 seconds (B+) |
Transmitted Data | Static + Dynamic + Voyage Data (ETA, Destination, Draft) | Static + Dynamic info only (No voyage/ETA data) |
Transmission Protocols: SOTDMA vs. CSTDMA
SOTDMA (Class A & Class B+)
Self-Organizing: Transponders pre-reserve time slots on VHF frequencies.
Guaranteed Slot: Commercial ships never lose broadcast access, even in crowded ports.
Long Range: Higher 12.5W power allows easy satellite reception.
CSTDMA (Class B Standard)
Listen before Talk: Scans for free slots; defers to Class A transmissions.
Slot Dropping: In extremely busy channels, Class B signals may get skipped.
Lower Wattage: 2W signal struggles to reach orbit for satellite tracking.
Commercial Fleet: 100% Transparency (Goal)
Strict SOLAS Mandate: Class A transponders must run continuously at sea and in port.
Global Satellite Reach: High power output (12.5W) enables seamless AIS tracking on oceans.
Rich Voyage Metadata: Provides real-time ETA, destination, current draft, and navigation status.
Yachts & Pleasure Craft: Satellite Gaps
Limited Power (2W): Terrestrial receivers detect signals easily, but satellites miss them offshore.
Manual Powering Off: Skippers often turn off AIS while anchored or docked in harbor.
Incomplete Static Data: Vessel dimensions, call signs, or ship types are frequently left unconfigured.
Fishing Fleet: Regulatory Gaps & "Dark Fleet"
Size Exclusions: Vessels under 15m in length are exempt from mandatory AIS in most regions.
Tactical Deactivation: Transponders are sometimes turned off to hide fishing grounds or evade regulations.
VMS vs. Public AIS: Fishing authorities rely on Vessel Monitoring Systems (VMS), which are not public.