What Is AIS? A Beginner’s Guide for Recreational Boaters
AIS (Automatic Identification System) helps you see AIS-equipped vessels and (with a transceiver) helps them see you. It's situational awareness—not a replacement for radar or a proper lookout.
Class A vs Class B AIS
AIS comes in two main classes with different capabilities and requirements:
Class A AIS
- Required for: Commercial vessels over 300 gross tons, passenger vessels, international voyages
- Update Rate: Every 2-10 seconds (depending on speed)
- Transmit Power: 12.5 watts
- Range: 20-30+ nautical miles
- Data Transmitted: Vessel name, MMSI, position, course, speed, dimensions, destination, ETA
- Display: Dedicated screen with full keyboard
- Cost: $2,000-$5,000+
Class B AIS
- Common on: Recreational boats, small commercial vessels (voluntary)
- Update Rate: Every 30 seconds (slower than Class A)
- Transmit Power: 2 watts (SO) or 5 watts (CS)
- Range: 10-15 nautical miles (less than Class A)
- Data Transmitted: Vessel name, MMSI, position, course, speed
- Display: Usually integrates with chartplotter/MFD
- Cost: $300-$1,200
Class B is the standard choice. It provides excellent situational awareness at a reasonable cost and meets the needs of most pleasure craft.
Receiver vs Transceiver
You can either just receive AIS data from other vessels, or transmit your own information as well:
AIS Receiver (Receive Only)
- Function: Displays other vessels on your plotter/display
- You transmit: Nothing (you're invisible to AIS)
- No MMSI required: You don't need registration
- Use case: Awareness of commercial traffic, big ships, ferries
- Cost: $150-$400
- Best for: Small boats, daysailers, budget-conscious boaters
AIS Transceiver (Transmit + Receive)
- Function: See others AND broadcast your position
- You transmit: Your vessel info to everyone with AIS
- MMSI required: Must register with FCC (US) or local authority
- Use case: Offshore cruising, sailing, being seen by ships
- Cost: $400-$1,200 (Class B)
- Best for: Cruisers, offshore sailors, night sailing, fog-prone areas
Which Should You Choose?
Choose a Receiver if:
- You primarily boat in busy harbors and want awareness of commercial traffic
- You're on a budget
- You mostly do daysailing or stay in sight of land
- Your boat is easily visible (large, brightly colored)
Choose a Transceiver if:
- You sail offshore or cross shipping lanes
- You navigate at night or in fog
- You want to be seen by commercial vessels (they're watching AIS, not always radar)
- You cruise long distances or do overnight passages
- Your boat is small and might not show up well on ship radar
A receiver tells you where the big ships are. A transceiver tells the big ships where YOU are. If you're sailing at night in a 35-foot sailboat crossing a shipping lane, you want that freighter's officer on watch to see you on his AIS display.
CPA / TCPA - Collision Avoidance
Most AIS displays calculate two critical values to help you avoid collisions:
CPA (Closest Point of Approach)
CPA is the minimum distance that will exist between your vessel and another vessel if both maintain current course and speed.
Example:
- Target vessel is 3 miles away
- CPA shows 0.2 nm (about 1,200 feet)
- This means if neither of you changes course, you'll pass within 0.2 nautical miles of each other
TCPA (Time to Closest Point of Approach)
TCPA tells you how many minutes until you reach that CPA distance.
Example:
- CPA: 0.2 nm
- TCPA: 8 minutes
- You have 8 minutes to decide if 0.2 nm separation is safe, or if you need to alter course
Setting Alarms
Most plotters let you set CPA/TCPA alarms:
- CPA Alarm: "Alert me if any vessel will come within 0.5 nm"
- TCPA Alarm: "Alert me if collision risk is within 10 minutes"
CPA/TCPA assume both vessels maintain course and speed. If either vessel changes course, the calculations update in real-time. Always verify with visual confirmation and radar when available.
Typical Safe Settings
- Coastal/Offshore: CPA 1.0 nm, TCPA 15-20 minutes
- Harbor/Confined Waters: CPA 0.3-0.5 nm, TCPA 5-10 minutes
- Open Ocean: CPA 2.0 nm, TCPA 30 minutes
Installation Tips
Proper installation is critical for AIS performance. Here's what you need to know:
1. Antenna Placement
VHF antenna location matters:
- AIS operates on VHF marine band (161.975 MHz and 162.025 MHz)
- Mount antenna as high as practical for maximum range
- Line-of-sight communication—higher is better
- Keep antenna clear of radar arches, spreaders, and metal structures
2. Antenna Options
Option A: Dedicated AIS Antenna
- Best performance
- No interference with VHF radio
- Mount on mast, arch, or radar tower
- Typical cost: $100-$300
Option B: VHF/AIS Splitter
- Share VHF radio antenna with AIS
- Saves space and cost
- May reduce VHF radio range slightly
- Good for boats with limited mounting options
- Splitter cost: $80-$150
3. GPS Antenna
AIS transceivers require GPS for position data:
- Most units have built-in GPS
- External GPS antenna may improve accuracy
- Mount GPS antenna with clear sky view
- Avoid mounting near radar or other RF sources
4. Power Requirements
- Voltage: 12V DC (typical)
-
Current Draw:
- Receiver: 0.2-0.4A
- Class B Transceiver: 0.5-1.5A
- Use appropriate wire gauge (typically 16 AWG or heavier)
- Install inline fuse (3-5A for most units)
- Connect directly to battery or distribution panel
5. Integration with Chartplotter
AIS data can be displayed on your existing chartplotter via:
- NMEA 2000: Plug-and-play network connection
- NMEA 0183: Serial connection (older plotters)
- Wi-Fi: Some units broadcast via wireless
- USB: Connect to PC/tablet navigation software
If you're installing a transceiver, have your MMSI number ready BEFORE installation. The unit needs to be programmed with your MMSI, and most require professional programming or cannot be changed once set.
AIS Best Practices
1. AIS is NOT a Replacement for Radar or Lookout
Common misconception: "I have AIS, so I don't need radar."
Wrong. Here's why:
- Not all vessels have AIS (small boats, kayaks, debris, buoys)
- Weather systems and land masses don't appear on AIS
- AIS can fail or be turned off
- Visual lookout is required by COLREGS (Rule 5)
⚠️ SAFETY RULE
Use AIS as ONE tool in a layered approach:
- Visual lookout (eyes, binoculars)
- Radar (if equipped)
- AIS (supplemental awareness)
- VHF radio (communication)
Never rely on any single system. AIS enhances safety but doesn't replace fundamental seamanship.
2. Verify Your MMSI Registration
If you have a transceiver, your MMSI (Maritime Mobile Service Identity) MUST be:
- Registered with FCC (US boats) or appropriate authority
- Programmed correctly in your AIS unit
- Linked to current vessel and owner information
- Updated if you sell the boat or change ownership
3. Keep Vessel Information Current
Your AIS broadcasts vessel details. Keep them accurate:
- Vessel Name: Match your registration
- Call Sign: FCC-issued call sign
- Vessel Type: Sailing, pleasure craft, fishing, etc.
- Dimensions: Length, beam (important for bridge clearances)
- Destination: Update when on passages (Class A only)
4. Monitor AIS Continuously When Underway
- Have AIS display visible on chartplotter at all times
- Set appropriate CPA/TCPA alarms for your environment
- Check targets every 5-10 minutes, even in open water
- Increase monitoring frequency in traffic or restricted visibility
5. Understand AIS Limitations
Range Limitations:
- VHF is line-of-sight: ~10-15 nm for Class B
- Antenna height affects range dramatically
- Land masses and weather can block signals
Update Rate:
- Class B updates every 30 seconds (vs 2-10 sec for Class A)
- Fast-moving vessels may appear to "jump" on your display
- Use radar for precise tracking of nearby fast targets
Not All Vessels Transmit:
- Small recreational boats often don't have AIS
- Fishing boats may turn off AIS to hide fishing spots
- Military vessels often operate dark (no AIS)
- Kayaks, paddle boards, swimmers won't show up
6. Use AIS with VHF for Communication
AIS gives you vessel name and MMSI—use them:
- "Vessel [NAME], this is [YOUR VESSEL], do you copy?"
- Select vessel on AIS display, initiate DSC call if available
- Discuss passing arrangements in congested waters
- Never assume the other vessel sees you—confirm via radio
7. Test Your System Regularly
- Verify you're transmitting (if transceiver) by checking on another vessel's AIS or online AIS tracking
- Check GPS accuracy (should be within 10 meters)
- Verify antenna connections are tight and corrosion-free
- Update firmware when available from manufacturer
Use free websites like MarineTraffic.com or VesselFinder.com to verify your transceiver is broadcasting correctly. Search for your MMSI or vessel name while underway.
8. Privacy Considerations
If you're broadcasting AIS, you're publicly visible:
- Anyone can see your position, course, and speed in real-time
- Online AIS tracking services publish your movements
- Some cruisers turn off AIS when at anchor for privacy
- Consider the trade-off: safety vs privacy based on your situation
⚠️ IMPORTANT SAFETY DISCLAIMER
This content is for general informational purposes only and does not constitute professional maritime safety advice, navigation guidance, or emergency response instructions. Always consult official navigation rules (COLREGS), maintain proper lookout, and use multiple systems (radar, visual, AIS) for collision avoidance.
AIS is a supplemental aid to navigation, not a replacement for situational awareness, seamanship, or compliance with navigation rules. Never rely solely on AIS for safe navigation.