ATM Annex 1 Definition

The vertical distance of a level, a point or an object considered as a
point, measured from a specified datum.

• Height

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The designated geographical location of an aerodrome

• Aerodrome Reference Point
  

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Authorization for an aircraft to proceed under conditions specified by
an air traffic control unit.

• Air traffic control clearance

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A means of communication between controller and pilot, using data
link for ATC communications

• Controller-pilot data link
  communications (CPDLC)

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A generic term meaning variously, flight information service, alerting
service, air traffic advisory service, air traffic control service (area
control service, approach control service or aerodrome control
service).

• Air traffic service

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An aerodrome to which an aircraft may proceed when it becomes
either impossible or inadvisable to proceed to or to land at the
aerodrome of intended landing where the necessary services and
facilities are available, where aircraft performance requirements can
be met and which is operational at the expected time of use.

• Alternate aerodrome

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The vertical distance of a level, a point or an object considered as a
point, measured from mean sea level (MSL).

• Altitude

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A defined area, on a land aerodrome, intended to accommodate
aircraft for purposes of loading and unloading passengers, mail or
cargo, fuelling, parking or maintenance.

• Apron

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A surveillance technique in which aircraft automatically provide, via a
data link, data derived from on-board navigation and position-fixing
systems, including aircraft identification, four-dimensional position
and additional data as appropriate.

• Automatic dependent
  surveillance (ADS)

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City, town, or settlement substantially used for residential industrial,
commercial or recreational purposes.

• Congested area

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