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.
| '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 |
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.