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AIXM Model Overview

Although PANS-AIM only requires the facility coverage to be coded, also other limitations of a navaid may be provided in the scope of an AIP data set.

Angle Sectors and Horizontal Components

For coding the horizontal extent of a sector, seven attributes will be used:

• The CircleSector.fromArc attribute is used to define the beginning of angle of the sector.
• The CircleSector.toArc attribute is used to define the ending of angle of the sector.
• The CircleSector.arcDirection attribute is then used to indicate if the direction fromArc to the toArc is clock-wise ('CWA') or counter-clockwise ('CCA').
• The CircleSector.angleDirectionReference attribute is used to define the direction with regard to a reference point, i.e. if the angles have to be read 'FROM' or 'TO' the navaid.
• In addition, the CircleSector.type attribute is used for indicating the type of the angles: magnetic bearing, true bearing, VOR radial, etc.

For the sake data harmonisation it is recommended to encode the angles of the sector volumes in clockwise direction and from the navaid.

In order to complete the horizontal extend of the sector a CircleSector.outerDistance has to be defined. In addtion also a CircleSector.innerDistance will be defined. In case the inner distance is at the position of the Navaid it shall be coded equal-to '0'. The UomDistanceType class provides a set of values that can be used for the coding of the unit of measurements such as 'NM' or 'KM'.

If the inner limit of a limitation is '0', i.e. begins at the Navaid, the innerDistance attribute should be encoded like that and not be left "uncoded".

The example of an AIP shows the coding of the inner and outer distance:

The following three cases and associated figures show the way in which these attributes are used to describe various sector shapes:

Case 1: A simple sector is described by the fromAngle and toAngle

fromAngle defines the angle that describes the leading edge of the sector - the edge at which the sector volume begins. toAngle defines the angle to the end of the sector volume. The arcDirection is relevant in order to define the sector volume.

Two horizontal distances are defined, outerDistance and innerDistance, each defining the radius of a circle, centred on the Navaid. The circle defined by innerDistance is the edge at which the limitation starts [when moving away from the Navaid]. outerDistance describes the edge at which the limitation stops. Therefore the two values together describe the extent of the limitation in terms of distance away from the Navaid.

Case 2: Full circle limitation

Where the Navaid limitation is defined for the entire area surrounding a Navaid, between two lateral distances from the Navaid, the limitation is defined in a full circle centred on the Navaid.

In order to describe this situation fromAngle is set equal-to '0' and toAngle is set equal-to '360'.

Case 3: Limitation defined at a single angle.

When a limitation is defined at a single angle this is modelled as a zero width sector. The innerDistance and outerDistance attribute values have identical use as in the previous cases.

In order to define a zero width sector toAngle and fromAngle are both set to the angle of the limitation.

The figure below illustrates the Horizontal limits group applied to three different instances of limitations (e.g. Coverage), one from 0 NM to 80 NM within a angle sector from 025 to 202.5 degrees, a second one from 0 NM to 60 NM within a angle sector from 202.5 to 337.5 degrees and a third one from 0 NM to 40 NM within an angle sector from 337.5 to 025 degrees.

Vertical Components

For the coding of the vertical extent of a sector four attributes may be used:

The CircleSector.upperLimit attribute is used for the uppermost altitude or level that is included in the sector.

The CircleSector.lowerLimit attribute is used for the lowermost altitude or level that is included in the sector.

The UomDistanceVerticalType class provides a set of values that can be used for the coding of the unit of measurement for the two vertical limits, such as feet ('FT') or flight level ('FL').

The two corresponding ...LimitReference attributes define the reference surface used for the value of vertical limits. For example, Mean Sea Level ('MSL'), Ground ('SFC'), etc.

For more coding details and rules about vertical limits see topic Vertical Limits.

The figure below illustrates the upper and lower limits applied to two different instances of limitations (e.g. Coverage) having different horizontal limits.

In the example provided, one coverage goes from SFC up to 3000 FT (with a limitation range from 0 to 30 NM) and a second one from 3000 FT up to FL 500 (with a limitation range from 0 to 60NM).

Coding Rules for Facility Coverage

Coding Examples

The example below illustrates the encoding of the coverage (DOC) of the VOR Tango (note that the TACAN has other limitations):

The figure below illustrated how the limitations of the VOR Tango look like in regard to the horizontal and vertical limits (drawing is not to scale):

¹ As not specified in the source data, the lower limit and its reference may also not be coded.

**As not specified in the source data, the inner limit may also be left blank.

Coding examples can also be found in the Sample AIP Data Set (DONLON):