air traffic control unit(ATC unit) means a generic term meaning variously, area control centre, approach control unit, or aerodrome control tower;
data link servicemeans a set of related air traffic management transactions, supported by air–ground data link communications, which have a clearly defined operational goal and which begin and end on an operational event;
offset carrier operationmeans a case where the designated operational coverage cannot be ensured by a single ground transmitter, and where the signals from two or more ground transmitters are offset from the nominal channel centre frequency in order to minimise interference problems.