Baseline Data - ICAO Digital Data Sets


The Digital NOTAM encoding is based on the general temporality rules of AIXM 5, 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 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.

The 40th Amendment to the ICAO Annex 15 and the new PANS-AIM specify four digital data sets that are considered as the baseline data for the coding of digital NOTAM:

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 is available in AIXM 5 format.

AIXM version


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:

Digitisation level to match the intended use


Digital NOTAM will be provided as AIXM 5 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:

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:

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 data source. As a minimum, all text NOTAM shall be provided as "text notes" of the AIXM 5 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.

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 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.

Use of upper and lower case text


The Event schema allows the automatic generated NOTAM text to be coded and provided as part of the Digital NOTAM data set. As for its distribution it is possible to use more advanced channels than AFTN, there is no constraint for this text to be only in upper case. As demonstrated by several human factors studies, the use of the usual "sentence case" mix of lower and upper case improves the readability of the information. This is particularly important for Pre-flight Information Bulletin (PIB) applications. Therefore, the Digital NOTAM Specification provides rules for generating the NOTAM text using lower case in general, while upper case is used only in specific cases, such as start of a sentence, place names, certain abbreviations and acronyms, etc.For NOTAM transmission over AFTN, the conversion into full upper case will still have to be done, but it is straight-forward.

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 can be presented graphically. 

The Q line of an ICAO NOTAM message includes a geographical position and a radius, which were originally intended as NOTAM filtering parameters, for selecting NOTAM geographically. This is commonly exploited in order to represent the NOTAM graphically, as a circle. For airport NOTAM, the circle is typically centered on the Airport Reference Point (ARP) location and has a radius of 5 NM. Also, many NOTAM are issued with a maximum radius of 999 NM in order to ensure that they appear in all PIB. These aspects limit significantly the use of this location/radius as graphical representation of a NOTAM.

A Digital NOTAM can be represented graphically much more precisely, showing the extent of the area or the feature affected. This is exemplified in the following two pictures, which compare the display of a restricted area as a circle (based on the NOTAM Q line) versus the real geometry of the airspace (as made possible by Digital NOTAM).

                  Text NOTAM  (center/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.

The graphical capabilities offered by Digital NOTAM can be exploited in the provision of digitally enhanced Preflight Information Bulletin services and products. Such a specification was developed through the SESAR Research projects.

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.

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:

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: