AIS Basics for Recreational Boats
What AIS is, how it works, and why every offshore boater should understand
What Is AIS?
AIS stands for Automatic Identification System. It was developed to improve maritime traffic safety and is now mandatory for most commercial vessels worldwide. The system uses VHF radio frequencies (channels 87B and 88B, around 162 MHz) to broadcast a vessel's identity, position, speed, course, and other navigational data.
AIS-equipped vessels continuously transmit this information, and any AIS receiver in range — aboard another vessel or at a shore station — can decode and display it. The result is a real-time map of all nearby AIS traffic on your chartplotter or phone app.
Class A vs Class B: What's the Difference?
Class A (Commercial / Mandatory)
- Required on SOLAS vessels (commercial ships 300+ GT, passenger vessels)
- Transmits every 2–10 seconds underway
- 12.5W transmit power
- Sends full voyage data: destination, ETA, draft, cargo type
Class B (Recreational / Optional)
- Designed for voluntary use on recreational and small commercial vessels
- Transmits every 30 seconds at 2W (SO-TDMA) or every 5 seconds at 5W (CS-TDMA)
- Sends position, speed, heading, MMSI, and vessel name
- Receives Class A and Class B transmissions equally well
- Far less expensive than Class A — most units $150–$600
AIS Receivers vs AIS Transponders
There are two categories of AIS devices for recreational boaters:
- AIS Receiver (receive-only): Shows you other vessels but does NOT broadcast your position. Good for situational awareness at low cost. Other ships cannot see you on their AIS displays.
- Class B AIS Transponder: Both transmits and receives. Commercial ships and ferry operators will see you on their display. Significantly improves your visibility — especially critical in shipping lanes, harbors, and restricted-visibility situations.
For offshore or coastal cruising, a transponder is strongly recommended over a receive-only unit. Being seen by commercial traffic is the primary safety benefit.
What AIS Displays on Your Chart Plotter
- Vessel position — shown as a triangle icon on the chart
- COG/SOG — course and speed over ground, displayed as a vector line showing predicted track
- MMSI and vessel name — tap any target to see ID
- CPA / TCPA — closest point of approach and time to CPA — critical for collision avoidance
- Vessel type and length — helps you understand traffic context (tanker vs cargo vs passenger)
- Heading and rate of turn (Class A only)
MMSI: Your Vessel's AIS Identity
Every AIS transponder requires an MMSI (Maritime Mobile Service Identity) — a unique 9-digit number that identifies your vessel. Your MMSI is also used for DSC (Digital Selective Calling) distress alerts on your VHF radio.
Register your MMSI free at BoatUS or Sea Tow (U.S. recreational vessels not requiring a Ship Station License). If you hold an FCC Ship Station License, register through the FCC directly.
One MMSI per vessel. Do not use the same MMSI on multiple boats or multiple devices.
AIS vs Radar: Do You Need Both?
AIS and radar serve different but complementary purposes:
- AIS shows you: vessels that are broadcasting — gives identity, name, speed, destination data
- Radar shows you: everything with a radar return — land, buoys, rain, vessels with or without AIS
- Key gap in AIS: small boats, kayaks, and older vessels may not carry AIS — radar catches what AIS misses
- Key gap in radar: no identity data — you see a blip, not a name or heading intention
Best practice offshore: Use both. AIS for traffic awareness and CPA monitoring; radar for buoys, land, and non-AIS targets.
AIS and the VHF Antenna
AIS transponders connect to a VHF antenna — either sharing your existing VHF antenna via a splitter, or using a dedicated AIS antenna. A dedicated AIS antenna is preferred because:
- No splitter insertion loss (splitters reduce signal strength on both radio and AIS)
- You can position the AIS antenna optimally for horizon coverage
- Your VHF radio and AIS transponder operate simultaneously without compromise
Shakespeare and Digital Yacht both make dedicated AIS combo antennas that cover VHF and AIS bands.
Practical Tips for Recreational Boaters
- Turn it on before departure — AIS needs time to acquire GPS fix and begin transmitting
- Set CPA/TCPA alarms — most chart plotters allow alerts when a vessel will come within a set distance in a set time
- Don't rely on AIS alone — maintain a proper visual and radar watch
- Check MarineTraffic.com to verify your vessel appears correctly after installation
- Update voyage data — enter your destination and POB before offshore passages
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This content is for general informational purposes only. AIS is a supplementary tool and does not replace radar, a proper watch, or other collision-avoidance practices. Follow COLREGS at all times. In emergencies, contact the Coast Guard on VHF Channel 16 or call 911. Sonark Marine is a retailer and does not provide professional maritime certification, safety training, or emergency response services.
Quick summary: AIS (Automatic Identification System) broadcasts your vessel's position, speed, and heading — and receives the same data from other AIS-equipped vessels. It's the most powerful collision-avoidance and situational-awareness tool available to recreational boaters.