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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.

When defining the Digital NOTAM encoding rules, it shall be kept in mind that the objective of the NOTAM digitisation is to enable the direct use of the data by software applications which support the ATM processes. Digitisation comes with a cost and there might be counter-productive to split into individual components information elements that are intended only for human consumption and that have to always be merged back before being used. Over-digitisation has to be avoided. For example, it is possible using the AIXM model to encode as individual data items phone numbers used for contacting an airport authority, in case of a prior permission requirement. However, it is highly unlikely that such data would be used by a computer to automatically dial the phone number. Therefore, in the current scenarios, such phone numbers are included in the free text Notes associated with the event.

As a general principle, the encoding rules have been developed in order to support the following range of applications:

  • flight planning;
  • pilot briefing and in-flight awareness;
  • calculation of engine-out procedures (obstacle data);
  • graphical presentation of the airspace status for VFR/IFR activities;
  • graphical presentation of the aerodrome surface and services status;

AIXM 5.1 Baseline Data


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=TEMPDELTA for the corresponding Navaid and including a property operationalStatus=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 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 the AIXM 5.1 data.

The BASELINE data can exist locally or in a remote reference database. Some {{Event Scenarios}} explicitly indicate that certain data has to be taken from the corresponding feature BASELINE. Although it is theoretically possible to manually input such information, when needed, although 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 format.

Solution Completeness


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 be also 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}}, in structured format.

Automatic text NOTAM generation


As a general principle, the encoding of the Digital NOTAM shall occur first and the application that supports the encoding shall be able to automatically generate the corresponding text NOTAM message, which complies with the International Civil Aviation Organisation (ICAO) requirements. The NOTAM shall also comply with eventual regional guidelines, such as contained in the European Operating Procedures for AIS Dynamic Data (OPADD).

The operator should not be allowed to modify the automatically generated NOTAM text, as this would introduce a risk for discrepancies between the digital data and the content of the NOTAM. However, some event scenarios may require operator input in order to decide for which airports a NOTAM needs to be generated. NOTAM specific values, such as series, number, country codes, etc. might need to be pre-configured in the application and eventually selected by the operator from a list of allowable values.

In addition, developers of Digital NOTAM encoding application shall be aware that in particular situations the operator might need to adjust the content of the Q line (code, purpose, area of influence, etc.) in order to ensure a certain processing for that NOTAM for pre-flight purpose. Therefore, it might be necessary to allow modifications in the items A and Q of an automatically generated text NOTAM. Manual intervention in the other NOTAM items is strongly discouraged.

NOTAM content versus digital data


As the information is becoming more complex, it becomes increasingly difficult to provide all the details in the NOTAM text. For example, providing the details of a new 'polygon' obstacle through NOTAM text would require a very long and very complex E fields. The NOTAM messages were not intended to contain such detailed geometrical information. The introduction of Digital NOTAMs will offer a solution to this problem. Only the information necessary for a human to identify the event will continue to be published in the NOTAM text. The very detailed geometrical information could in provided by the AIXM encoding only.

Even more, certain information updates could be made available just as digital data, if it is decided that a NOTAM does not need to be issued. For example, the GNSS outages could be encoded as digital events. However, if it is decided that they are not needed as NOTAM messages, they could either stay just as digital data or could be published for human consumption in another format, such as a table on a Web site, a map, etc.

Graphical presentation of Digital NOTAM


One of the major advantages of Digital NOTAM (as compared with the current text NOTAM) is that it enables the automatic creation of more precise graphical representations of the event. In the case of an ad-hoc restricted area, a navigation warning, a temporary obstacle, a taxiway closure, etc. the real location and geometry of the feature shall be presented graphically (right side of the following image) and not the radius of influence (left side of the following image, as provided by the coordinates/radius block in item Q of an ICAO NOTAM).               

                  Text NOTAM  (centre/radius)                              Digital NOTAM (real geometry)

Graphical visualisation of Digital NOTAM usually requires data which goes beyond strictly the features affected by the event. For example, a taxiway closure Digital NOTAM will provide the information about the closure status, eventual exceptions and schedules of the closure. In order to graphically display this closure on an airport map, the geometry of that taxiway is necessary. This might not be available in the message that contains the Event data and might require querying a source of reference database. A system providing Digital NOTAM data to consumer application might include the provision of ‘reference data’, to support applications such as graphical visualisation. Such data might be included directly in the Digital NOTAM messages (as an option) or be provided as a separate service.

Event filtering capabilities


In AIXM, it was not possible to have a unified handling of the locations of features because many aeronautical features have specific geometries or even no geometry at all. An airspace is a collection of polygons, an airport has a reference point but also a "boundary" polygon, an instrument approach procedure has a complex 3D flying trajectory, while services typically do not have any geometry of their own, etc.

Obviously, this complicates the spatial queries. However, a spatial query alone will probably not satisfy the future Digital NOTAM client applications. The text NOTAM messages of today and their Q Line already enable spatial filtering. The result is what is called a "narrow route bulletin". However, the current pre-flight information bulletins contain, on average, 50% NOTAM messages that are irrelevant because it is currently not possible filter out all information that is not pertinent for the affected aircraft or flight, for example. Filtering out such information requires interpreting the feature properties affected by the event. In the future, more intelligent queries will be needed in order to better filter the information that is presented to the pilot, beyond the basic spatial queries. It is possible that each AIXM feature type will need its own selection criteria.

Pure spatial filtering might still be applied as an initial pre-selection of the Events that need to be transmitted to a client. Therefore, the AIXM Event Schema needs to support the provision of a basic "bounding box" geometry, to be used in those scenarios that require basic spatial filtering. This is already supported in the model, because the Event is an AIXM feature and therefore has a bounding box as any other GML feature. Details about the calculation of the {{Bounding Box}} size are provided in a specific data encoding rule.

Data synchronisation requirements


Missing a pertinent NOTAM can cause a significant safety hazard and missing a Digital NOTAM is no different. To prevent such risks, the text NOTAM production and distribution systems have mechanisms (sequential numbering, checklists, etc.) that enable a user to check if they are in possession of all applicable NOTAM messages. Similar mechanisms must be put in place when working with Digital NOTAMs. However, these mechanisms are allowed to be different from the mechanisms used for the synchronisation of NOTAM text message sets. The requirements may also be different, as Digital NOTAM synchronisation requires both:

This could be supported through reports that contain the list of all valid TimeSlices - sequence number and correction number for each feature, identified by UUID or by a SNAPSHOT TimeSlice containing feature identifying properties (natural key). An AIXM BasicMessage could be used for this purpose.

  • the synchronisation of the BASELINE data, without which the TEMPDELTA TimeSlices cannot be processed correctly;
  • the synchronisation of the PERMDELTA and TEMPDELTA data itself;

Another aspect that requires the attention of Digital NOTAM receivers is the risk of duplicate information, in particular for data situated in the vicinity of two or more areas of responsibility. This duplication already exists today in the form of different NOTAM messages issued by different States for the same facility or situation. With Digital NOTAMs, this risk to happen as well because of potential difficulties in coordinating the data publication between neighbouring States, also taking into consideration the need for rapid data dissemination. With Digital NOTAMs, the risk should be reduced at least when it concerns updates to existing facilities, as the TEMPDELTA is expected to be issued by the same authority who has issued the BASELINE data. However, for situations when a new temporary item is announced (such as a temporary obstacle in the vicinity of the boundary), there is still a risk of lack of coordination. This may result in duplicate data being issued. Therefore, Digital NOTAM recipients should foresee appropriate actions for dealing with both potentially duplicate PERMDELTA and BASELINE data.

Event dependencies


There exist many operational dependencies between situations that can affect various aeronautical features. For example:

  • navaid outages can trigger the unavailability of certain routes or procedures;
  • the creation or the activation of restricted areas can trigger the closure of certain route segments or the deactivation of other areas;
  • the displacement of a runway threshold can trigger changes in the runway declared distances and landing minima;
  • etc.

In the current version, no work has been done yet in order to identify the rules that control such dependencies between events. Discussions in the Digital NOTAM Focus Group have enabled to identify the following aspects in relation with dependencies between events:

  • First, a question of principle - should the identification of dependencies be done by the data originator or by the end user? For example, in case of a navaid outage, should the Digital NOTAM originator also issue notifications about the unavailability of certain procedures/routes? Or should this be left for the end user, based on the technical/operational capabilities of the aircraft/crews involved?

  • It is expected that Digital NOTAM will enable to more efficiently identify situations where an event in a neighbouring State has consequences on the availability of routes or procedures in their area of responsibility. Today, such dependencies are difficult to identify as they would require a manual monitoring of the NOTAM issued by neighbouring States. With digital data, such monitoring and alerts could be easier and less expensive to put in place.


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