Industry Insider - interestingly, the blurb on those beacons highlights that the AIS function is line of sight only - your line of sight from sea level (ie MOB) is extremely limited, especially in a big sea. These beacons don't transmit on 406Mhz so they don't have the satellite capability that they seem to imply and 121.5 isn't monitored by satellite any more, just used for homing by SAR.