## 3 New Generation Sensor Web Enablement
### Web Notification Service
#### SensorML Profile for Discovery  
The SensorML standard (Section 3.2.3) is the recommended encoding for sensor metadata within SWE. Due to the very broad range of use cases and sensor types that are supported by the SWE specifications, SensorML has been designed in a very flexible way. Thus, SensorML can be used for describing sensors ranging from fixed stationary devices such as weather stations to complex data acquisition systems for aerial images. For enabling sensor discovery this flexibility of SensorML induces certain challenges: SensorML defines only very few mandatory elements so that it is not ensured that all metadata elements necessary for enabling sensor discovery are available. Furthermore, SensorML allows different ways for encoding the same metadata items. Consequently it becomes difficult to automatically process SensorML documents and to index them in a sensor catalogue. As a solution, the SensorML Profile for Discovery has been developed [97]. Based on a number of \begin{table}
\begin{tabular}{p{85.4 pt}|p{284.5 pt}} \hline
**Specification** & **Description of change** \\ \hline SensorML Profile for Discovery & Profile of SensorML ensuring the presence of a minimum set of metadata that is necessary for allowing sensor discovery. \\ \hline SensorML-ebRIM Mapping & Mapping of SensorML elements into the ebRIM Catalogue information model in order to enable the management of sensor metadata by OGC Catalogues. \\ \hline SIR & Web service interface for managing sensor metadata; this includes the collection of sensor metadata, management of sensor status information as well as functionality for pushing sensor metadata into OGC Catalogues. \\ \hline SOR & Web service interface for accessing phenomenon definitions and for exploring semantic relationships between different phenomena. \\ \hline \end{tabular}
\end{table} 
Table 6: Major advancements introduced by discovery architecture. formalized rules (expressed in Schematron [98]), this profile defines a minimum set of metadata elements and their structure that need to be provided if a sensor shall become discoverable. SensorML-ebRIM-Mapping  
While SensorML is used in SWE to encode sensor metadata, catalogues used in conventional SDIs are based on different models (e.g., the ISO 191 xx standards or ebRIM [96]). Hence, existing OGC Catalogues are not directly able to handle SensorML encoded metadata. Instead, it is necessary to map SensorML based sensor metadata to the existing Catalogue information models so that the discovery of sensors using the OGC Catalogue becomes possible. An according mapping between SensorML and the ebRIM model is described in [99]. Based on the SensorML Profile for Discovery [97], this mapping describes how the different discovery relevant elements of SensorML can be put into an ebRIM model. Practically, this mapping allows deriving an XSLT [100] transformation that can be applied when inserting SensorML based metadata into OGC Catalogue instances. Sensor Instance Registry  
The SIR interface [101] provides functionality to collect, manage, transform and transfer sensor metadata. It is intended to close the gap between the SensorML based SWE world and conventional SDIs. In order to achieve this aim, the SIR provides functionality to:  
* collect sensor metadata (_i.e._, automatic harvesting of SensorML documents and manual insertion of sensor metadata)
* provide extended discovery functionality based on the metadata provided through SensorML
* manage status information of sensors (e.g., finding all sensors with a critical battery level or automatic notification if a sensor reaches a critical state)
* transform SensorML-based sensor metadata into conventional Catalogue information models and push the transformed metadata sets into OGC Catalogue instances  
In the future it is expected that the functionality of the SIR interface will partly be covered by other existing SWE services (e.g., SOS for retrieving sensor status information and the SES for filtering sensor status updates).