AIS Class A vs Class B: which and why

AIS (Automatic Identification System) transponders broadcast MMSI, position, course, speed, and vessel info to other AIS-equipped vessels and shore stations. Two classes matter for yachts. Class A: mandatory on SOLAS vessels (commercial 300 GT and up, all passenger ships), 12.5W transmit power, higher reporting rate (2-10 seconds moving, 3 minutes anchored), transmits vessel name, dimensions, cargo type, destination, ETA. Not required on recreational vessels but some large yachts (over 150 ft) choose Class A for visibility to commercial traffic. Class B: designed for recreational, 2W transmit power (Class B/SO 5W newer standard), reports every 30 seconds moving, 3 minutes anchored. Transmits MMSI, position, course, speed, vessel name and dimensions. Cost: Class B ~$800-1500, Class A ~$3000-5000 plus mandatory annual inspection. Rule for recreational: Class B is enough unless you're above 150 ft or crossing shipping lanes routinely, commercial ships filter out Class B on some plotters when workload gets high, so Class A stays more visible in busy waters. Every AIS needs a Class 2 SBAS GPS input (or built-in) and a marine VHF antenna. AIS receiver-only (no transmit) is legal without an MMSI and useful for situational awareness, but you're invisible to others. Program the MMSI ONCE, like DSC, it's a one-shot lock unless you factory-reset via a dealer.