The Digital NOTAM encoding is based on the general temporality rules of AIXM 5.1, as detailed in the Temporality Concept document. Thus, most Digital NOTAM encodings will produce TEMPDELTA TimeSlices for the affected AIXM features. For example, a temporary navaid out of service event will be encoded as a new TimeSlice with interpretation equal-to 'TEMPDELTA' for the corresponding Navaid and including a property operationalStatus with value 'UNSERVICEABLE'.
Therefore, a pre-requisite for any Digital NOTAM application is the availability of the corresponding static data, in the form of AIXM 5.1(.1) BASELINE TimeSlices. In the example given above, it is required that the Navaid BASELINE TimeSlice is available both to the application that will generate the Digital NOTAM encoding and to the clients who will receive and use the Digital NOTAM data. The BASELINE data can exist locally or in a remote reference database.
- AIP Data Set - providing baseline data for the coding of events that affect points, navaids, routes, airspace, en-route holding, airport, runways and related data such as services;
- Airport Mapping data sets - providing baseline data for the coding of events that affect teh the airport movement area and other airport surfaces;
- Obstacle data sets - providing baseline data for the coding of temporary obstacles changes, such as light outages;
- Instrument Flight Procedures data sets - providing baseline data for the coding of Digital NOTAM that affect SID, STAR and Instrument Approach Procedures.
The dependency on the availability of static (baseline) data in a specific data set is indicated for each Digital NOTAM event scenario concerned. Although it is theoretically possible to manually create the baseline data when needed, such an approach would significantly increase the risk of data encoding errors and reduce the quality of the Digital NOTAM data. Therefore, it is considered a critical prerequisite that all BASELINE data necessary for the encoding of the Event Scenarios described in this Specification be available in AIXM 5.1(.1) formatis available in AIXM 5 format.
In general, the Digital NOTAM Specification does not need to be linked with a particular AIXM 5 version. Therefore, the term "AIXM 5" is used in this specification where discussing general principles. However, the coding and decoding rules for each particular scenario and the Event schema need to be linked to a particular AIXM version in order to avoid inconsistencies, which might occur due to attributes and list of values that change in new AIXM versions. Therefore, each edition of the Digital NOTAM Specification indicates a reference AIXM version:
- the 1st edition of the Digital NOTAM Specification was developed using AIXM 5.1;
- the 2nd edition is based in the AIXM version 5.1.1, which is also the reference for the provision of the baseline Digital Data Sets discussed previously.
Digitisation level to match the intended use
Digital NOTAM will be provided as AIXM 5 .1 data sets. The logical data model of AIXM defines features and properties that enable the provision as separate data items of numerical values, coded values and textual elements. Therefore, many of the information items contains in a NOTAM message can also be provided as individual data elements, such as: names/identifiers of the item affected, operational status, schedules, levels, etc.
In order to balance the implementation costs with the expected benefits, an incremental approach is likely to be followed by a large majority of the Digital NOTAM adopters. This raises a potential difficulty, as data users might need to consult two data sources:
- an AIXM 5 .1 data source for those events that are already provided as digital NOTAM;
- a text NOTAM data source for the rest of the NOTAM, which are not already available in digital AIXM 5 .1 format.
To avoid this difficulty, the Digital NOTAM Specification needs to support from the beginning a complete solution. The NOTAMs that are not yet provided as fully structured digital encodings shall also be made available through the AIXM 5 .1 data source. As a minimum, all text NOTAM shall be provided as "text notes" of the AIXM 5 .1 feature concerned. This is supported by the AIXM Event Schema, with a dedicated NOTAM object, which can also be used as data source for text in NOTAM briefing applications.