The Automatic Identification System was designed for commercial shipping. Every vessel over 300 gross tons is legally required to carry it. A Class A AIS transponder on a container ship broadcasts the ship's name, MMSI number, call sign, position, course, speed, rate of turn, destination, and estimated arrival — every 2 seconds when maneuvering, every 10 seconds at speed. That data is received by every AIS-equipped vessel within VHF range, displayed on their chartplotter, and simultaneously captured by shore-based AIS receivers and retransmitted over the internet in near real-time.
For a recreational sailor, the practical question is simpler: ships are watching AIS displays, not the horizon. If you are not on their AIS display, the watch officer does not know you exist. A 70,000-ton bulk carrier doing 18 knots cannot stop in less than three miles. By the time the lookout spots your white sail against a confused sea, the geometry may already be decided.
Class A, Class B, and Class B+
Class A is the commercial standard — required on ships over 300 GT, mandated by SOLAS. It transmits at 12.5 watts, updates every 2–10 seconds depending on speed and maneuver state, and is substantially more expensive than the recreational alternatives. You will not install a Class A unit on your sailboat.
Class B is the recreational standard. It transmits at 2 watts on a 30-second update cycle when underway and is adequate for most coastal and offshore sailing. The Vesper Cortex M1 is the current benchmark unit — it combines VHF radio, AIS transponder, and anchor watch in a single box with a well-designed phone app for monitoring. The Garmin VHF 215i AIS integrates seamlessly into a Garmin chartplotter network. Either will serve you for coastal passages and the majority of offshore sailing.
Class B+ (also called SO-TDMA or SOTDMA-capable Class B) represents a meaningful upgrade. Class B+ transmits at 5 watts rather than 2W, uses a more sophisticated time-slot assignment algorithm that gives it priority in congested AIS environments (major ports, shipping lanes), and updates at a rate closer to Class A — every 5 seconds when maneuvering at speed. The Garmin AIS 800 and Vesper Watchmate Vision2 are Class B+ units. For offshore passages through commercial shipping lanes or in and out of major ports, the additional transmit power and update rate are genuine advantages. For coastal daysailing, the difference is marginal.
Receive-Only AIS: The Wrong Choice Offshore
A receive-only AIS antenna — the Vesper SR160 or a simple antenna connected to a chartplotter's AIS input — shows you where commercial vessels are without transmitting your position. The cost is $150–$300. The appeal is obvious.
The problem is that it makes the collision avoidance calculation one-sided. When a ship's AIS system calculates TCPA (Time to Closest Point of Approach) and CPA (Closest Point of Approach), it does the math for every target it can see. If you are not transmitting, you are not in their calculation. The watch officer's screen shows a clean CPA for every vessel nearby — except yours, which is invisible.
On a daysail in a body of water with good radar coverage and light commercial traffic, receive-only AIS is a reasonable compromise. On an offshore passage, in fog, at night, or in any shipping lane where the margin for error is small, it is the wrong choice. A Class B transponder costs $400. That is not a large number relative to the safety it provides.
MMSI Registration
MMSI stands for Maritime Mobile Service Identity — a nine-digit number that uniquely identifies your vessel across all maritime communications systems, including DSC-equipped VHF radios and AIS transponders. The number is free and takes five minutes to obtain.
US recreational boaters have three registration options. The FCC issues MMSI numbers as part of the ship station license process, which is required if you carry SSB/HF radio or operate in international waters. BoatUS (boatus.com) issues MMSI numbers for free with no membership required. Sea Tow issues MMSI numbers to members. The BoatUS database is the most widely used for coastal US sailing and integrates with the Coast Guard's SAR systems.
When you register, you link your MMSI to your vessel name, home port, call sign, and emergency contact information. If your DSC distress button is pressed or your AIS transponder is involved in a collision avoidance incident, the MMSI gives the Coast Guard and other vessels your identity immediately.
One MMSI per vessel. If you replace your radio or transponder, the MMSI goes with the vessel, not the hardware. Update the registration if you sell the boat.
What the Data Looks Like on a Chartplotter
On a Garmin, Simrad, Raymarine, or B&G chartplotter with AIS input, each AIS target appears as a triangle pointing in the direction of travel. The size of the triangle indicates vessel type (or simply indicates the target is AIS-equipped). Tapping or clicking the target pulls up the vessel's broadcast data: name, call sign, MMSI, type, length, beam, course over ground, speed over ground, and destination if filed.
More useful than the static data is the dynamic display. Most chartplotters show a predicted track line extending from each target — the dotted line where the vessel will be in the next few minutes if it maintains course and speed. Your own predicted track is overlaid on the same display. Where these tracks intersect, or where they come within a defined distance, the chartplotter generates a CPA/TCPA alarm.
The default alarm thresholds on most chartplotters are conservative — they will alarm at 2nm CPA, which generates nuisance alarms in busy coastal waters. Experienced sailors tighten this to 0.5nm CPA and 10 minutes TCPA, which represents actual collision risk rather than navigational awareness. Experiment with your own thresholds on a clear day before you need to make decisions in the dark.
Collision Avoidance: TCPA and CPA
TCPA (Time to Closest Point of Approach) and CPA (Closest Point of Approach) are calculated continuously from the relative motion of your vessel and each target. CPA tells you how close the target will pass if both vessels maintain current course and speed. TCPA tells you when that closest point occurs.
The COLREGS (International Regulations for Preventing Collisions at Sea) govern who gives way, but the calculation always starts with CPA/TCPA awareness. A target with a CPA of 2nm and a TCPA of 45 minutes is background information. A target with a CPA of 0.3nm and a TCPA of 8 minutes requires immediate attention — either a course change, a call on Channel 16, or both.
AIS makes this calculation automatic and continuous. Before AIS, the offshore sailor tracked relative bearings manually — if the bearing to an approaching ship remains constant while the distance decreases, collision is the outcome. AIS does this math for every target simultaneously and alarms when the thresholds are met. This is a genuine safety improvement. It does not eliminate the need to understand the underlying geometry.
Why AIS Doesn't Replace Radar
AIS covers vessels required to carry it: commercial ships over 300 GT, most ferries, fishing vessels over 300 GT. It does not cover the 40-foot wooden lobster boat running from a Maine harbor at 4 am. It does not cover the fishing nets spread across 200 meters of water with only a small float at each end. It does not cover growlers — small ice chunks barely above the waterline — in northern waters.
GRP (fiberglass) and wood are poor radar targets even with a transponder. A small vessel heeled over in chop with a wet sail presents a minimal radar cross-section. An unlit vessel running without AIS in a busy shipping lane is simply invisible to AIS — and may be nearly invisible to radar depending on its construction and aspect.
The practical integration is this: AIS is your primary tool for tracking commercial shipping. Radar is your primary tool for detecting everything else, including squall lines, precipitation, and targets that are not broadcasting. In fog especially, radar remains essential because it shows you the physical world, not the electronic reports from vessels willing to report.
Integration with Chartplotter and Crew Tablets
A Class B transponder connects to your chartplotter via NMEA 2000 or NMEA 0183. On an N2K network, the AIS data flows directly to any display on the backbone — chartplotter, multifunction display, radar overlay. The integration is largely plug-and-play on modern equipment from the same manufacturer ecosystem.
For crew monitoring on passage, AIS apps on tablets and phones add a useful layer. PredictWind's Offshore app includes an AIS layer over chart data and allows the crew on watch to monitor traffic without hovering over the chartplotter. MarineTraffic, primarily a vessel-tracking service, has an app that uses internet connectivity to show all AIS traffic in an area — useful in port or within cellular range, not useful 200 miles offshore where you have no data connection.
The Vesper Cortex M1 runs its own app over the boat's wifi network, allowing any crew member with a phone to see AIS traffic, set anchor alarms, and monitor the watch without being at the helm. On a shorthanded offshore passage, that capability is worth the premium over a basic transponder.