Provision of Aircraft Position Data

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9.5.1 Dealing with the AIXM Temporality Model

In the OWS-7 testbed a workaround was applied to provide all data to the Event Service needed for applying filtering. In particular, the geometries of features were included in DNOTAMs containing delta timeslices, although the geometries themselves did not change. This is not conform to the intention of the AIXM Temporality Model and treating events published to an Event Service this way should be avoided as emphasized by the Digital NOTAM Event Specification. Still, Event Service instances should be able to do filtering and complex processing on any property of a feature to support more complex filtering. To overcome this issue an approach has been discussed within this testbed. It is described in the following sections.

9.5.1.1 Event Service Conformance Classes

Conformance classes define the level of information needed by an Event Service to apply appropriate filters. Three conformance classes have been defined for the Event Service: basic, snapshot and full-info. Basic means that an Event Service can only filter on the data provided by thin DNOTAM events following the AIXM Temporality Model. In particular, only the changed data is available for filtering. Full-info means that an Event Service has enrichment capabilities. The Event Service is aware of an appropriate WFS data store and requests all needed data for an AIXM feature of a thin DNOTAM. Snapshot is a conformance class in between the former two. An event publisher should provide snapshots of features to the Event Service who can then support subscriptions that access all properties of an AIXM feature at a given time though only at the time instant represented by that snapshot. The communication patterns for the “Snapshot” conformance class do not follow the AIXM Temporality Model as event publishers would provide information which does not change. But still there are use cases probably beyond AIXM where such Snapshots can be used in an Event Architecture. Within OWS-8 the Event Services either implemented the Full-info class or stuck to the Basic class.

9.5.1.2 WFS Support for Dynamic Features

To support the Full-info conformance class a WFS data store must support the retrieval of all information of an AIXM feature e.g. Snapshot. The support for snapshots has been discussed and implemented within this testbed. Details can be found in the OWS-8 Aviation: Guidance for Retrieving AIXM 5.1 data via an OGC WFS 2.0 Engineering Report OGC 11-073. Nevertheless, it has to be analyzed if a single snapshot is sufficient for such use cases. An event has a time period as its validTime. Hence, a single snapshot does not always represent every possible state of a feature for the time period of an event. An Event Service should therefore be able to request all states of the feature and based on these states decide if an Event matches certain filter criteria. A general solution is needed for such decisions e.g. an Event matches if there exists one state of the feature for which the criteria match.