Introduction and Background

PANS-AIM ENR 4.1 "Radio navigation aids — en-route" and AD 2.19/AD 3.18 "Radio navigation and landing aids" require for the AIP data set the following:

Facility coverage shall be indicated in the remarks column.

PANS-AIM only requires the facility coverage to be provided as part of the remark. Hence, it is sufficient to provide that information as an NavaidEquipment.annotation. However, AIXM 5 also provides the possibility to code that information in a more accurate form.

Page Table of Content

AIXM Model Overview

The diagram below shows the AIXM classes, including the relevant data types and code lists that are relevant for this topic.

The RadioFrequencyArea class is used to code any kind of geographical usage limitations of a navaid equipments or service frequencies, such as coverage, unusability, etc. In particular, it may be used to code the service area of a radio navigation or landing aid. The RadioFrequencyArea is not related to the Navaid but to the NavaidEquipment class, as each navaid equipment that a navaid is composed of may have a different service area or coverage. Even a single navaid equipment may have different limitations for different RadioFrequencyArea.singnalType, e.g. for a TACAN the limitation for the 'AZIMUTH' may be different than that for the 'DISTANCE'.

A navaid equipment may have different types of limitations. The code list of the RadioFrequencyArea.type attribute provides the following values to define the type of the navaid limitation.

CodeRadioFrequencyAreaType	Definition
'COV'	Actual coverage range (flight checked). Equivalent to Operational Service Volume.
'T_COV'	Theoretical coverage range (not flight checked). Equivalent to Standard Service Volume.
'SCL'	Scalloping.
'RHG'	Roughness (signal roughness experienced).
'UNREL'	Unreliability.
'RES'	Restricted (use is restricted to defined sectors).
'UUS'	Un-usability (use is not permitted in the defined sectors).
'OUT'	Out of tolerance.
'ESV'	Extended Service Volume (Coverage)
'OTHER'	Other

AIXM 5.1(.1) issue_032_CodeRadioFrequencyAreaType

The definitions of COV and T-COV are wrong. Hence, avoid coding this values.

There is no clear ICAO definition of the "actual coverage" term, while the "Operational Service Volume" seems to have a precise FAA definition. Therefore, this assumption that COV and OSV mean the same risks being incorrect. T_COV: This definition also makes an assumption for which there is no clear basis in ICAO or in other specifications.

Workaround AIXM 5.1(.1): use OTHER, e.g.

OTHER:SV (for service volume) - for DME, aligned with ED-77

OTHER:SSV (for standard service volume)

OTHER:DOC (for designated operational coverage)

OTHER:COV (when the data provider indicates simply 'coverage' without further details.

etc.

Status AIXM 5.2: see CCB AIXM-376

Navaid service areas are modelled as volumes of airspace described as circle sectors, centred on the NavaidEquipment in which certain limitations in the navaid performance are defined. The CircleSector class is defined by describing a start angle, stop angle, inner distance, outer distance and upper/lower altitude. Each instance of the limitation associated with a Navaid describes a single sector and with a single type of limitation. A service area may consist of more than one circle sector.

An additional possibility is to define the extent of the navaid limitation (e.g. service area, coverage, etc.) using GML elements related to the Surface class.

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 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):

type	lowerLimitReference¹	Upper Limit	upperLimitReference	innerLimit	outerLimit	arcDirection	angleDirectionReference	FromArc	ToArc
COV	SFC	500 FL	STD	0 NM **	80 NM	CWA	FROM	045	135
COV	SFC	500 FL	STD	0 NM **	60 NM	CWA	FROM	135	045
UUS	SFC	3200 FT	MSL	20 NM	not coded	CWA	FROM	125	145
UUS	SFC	5200 FT	MSL	30NM	not coded	CWA	FROM	125	145
UUS	SFC	7200 FT	MSL	40 NM	not coded	CWA	FROM	125	145

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 AIP Data Set - Specimen (DONLON):

No.

Description

XPath Expression

VOR-EX-01

DME-EX-01

VOR/DME facility coverage (full circle)

//aixm:RadioFrequencyAreaTimeSlice [@gml:id ='RFA_VOR_BOR'] |

//aixm:RadioFrequencyAreaTimeSlice [@gml:id ='RFA_DME_BOR']

NDB-EX-01

NDB (unknown facility coverage)

//aixm:NDBTimeSlice [@gml:id ='NDB_DON']

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