An AMRD is a mobile station; operating at sea and transmitting independently of a ship station or a coast station. Two groups of AMRDs are identified (ITU document 5B/411-E November 2017):

• Group A: AMRDs that enhance the safety of navigation,
• Group B: AMRDs that do not enhance the safety of navigation

(AMRDs which deliver signals or information which do not concern the vessel can distract or mislead the navigator and degrade the safety of navigation).