Single aircraft operator similarity rules
Single aircraft operator (AO) similarity rules are a set of rules for detecting similarities between two (or more) call signs within a single aircraft operator's schedule. These rules are applicable to all flights in all airspaces.
Local similarity rules
In some cases, the air navigation service providers (ANSPs) might put specific constraints on the composition of call signs.
For example, the ANSP might not be able to accept mixed alphanumeric call signs or might wish to avoid confusion with other data included within a radar label. These rules are applicable to a specific airspace and is therefore referred to as a ‘local’ rule. Depending on the specific circumstances, these rules are differ in terms of their importance.
Single call sign similarity rules
A similarity event does not have to involve two call signs. A call sign can be similar to different entities commonly broadcast within an ATC clearance (e.g. runway designator, a flight level or VHF frequency). While these similarity events can certainly cause confusion, they are generally considered less severe as they will not normally cause one aircraft to take the clearance intended for another. Because there is usually only one aircraft involved, recovery is easier. They are also more predictable for the pilot and/or controller as the entities concerned are always present. Depending upon the detail, single call sign similarity rules are either applicable to all flights or to a specific airspace.