The percentage of time that an aid, or system of aids, is performing a required function under stated conditions. Non-availability can be caused by scheduled and/or unscheduled interruptions.
– Signal availability. The availability of a radio signal in a specified coverage area.
– System availability. The availability of a system to a user, including signal availability and the performance of the user’s receiver.
Source: Nick Ward, IALA e-Nav committee vicechair, March 2009
International Association of Marine Aids to Navigation and Lighthouse Authorities - AISM