单项选择题 （）means a technique using digital codes which enables a radio station to establish contact with, and transfer information to another station or group of stations and complying with the relevant recommendations of the CCIR.

单项选择题 （）means navigational and meteorological warnings, meteorological forecasts and other urgent related messages broadcast to ships.

单项选择题 （）is designed to send an automatic distress alert when a ship sinks suddenly.