A Testbed for Sensor Service Networks and the Fusion SOS: towards plug & measure in sensor networks for environmental monitoring with OGC standards

Fraunhofer IITB has realized a testbed for sensors and services in order to trial the architecture and specifications developed in the EU Project SANY (Sensors Anywhere) based on going work of OGC (in particular the Sensor Web Enablement suite of standards). At the sensor network level, an ad hoc wireless network (ZigBee) is complemented by simulated sensor nodes. The nodes measure properties such as temperature, humidity, luminance and acceleration. The testbed is designed for experiments in a wide range of scenarios, such as mobile sensors traversing several networks, and in scalability. The simulation is implemented as an application in the product LabVIEW (National Instruments) which has the dual task of configuring the ZigBee nodes. New sensor nodes (either real or simulated) are recognized automatically and registered in one of 3 OGC Sensor Observation Servers (SOS). The sensor values are then inserted into a SOS as they arise by measurement or simulation. The available network resources (observed features, sensors, services) are registered in a catalogue server along with meta-data to support resource discovery by client applications. Clients can find, for example, information sources for a given region and observable phenomenon of interest. Fraunhofer IITB has realized a special SOS in the testbed, called a Fusion SOS. An OGC Sensor Planning Service (SPS) parameterizes and tasks the Fusion SOS to aggregate or fuse sensor data from several SOS. The fusion task queries the catalogue for available SOS of the required type and then conducts a selected procedure to produce a spatial or spatial-temporal interpolation. The interpolation result is a so-called coverage, a function defined on a space-time grid of sampling points. The procedure takes the inaccuracy of the raw sensor data into account. The spatial-temporal uncertainty of the fusion result is specified using uncertML, an XML schema developed by the INTAMAP project to describe the statistics of uncertain data. The fusion procedure is described, just as for the underlying sensors, with the OGC sensor model language SensorML. In this way, the fusion procedure can be treated as a sensor, but with the important characteristic that its result is a coverage. The coverage can be visualized using a Map & Diagram service of the SANY partner ETH Zürich. The procedures developed to date are variants of the Bayesian Maximum Entropy method that is able to consider soft sensor data (e.g. the sensor value lies in an interval) and additional phenomenological knowledge in the form of models. If additional sensors or SOS enter the testbed, the Fusion SOS discovers these new resources with the aid of the catalogue and incorporates the new data sources automatically into the fusion procedure. The self-describing information plays an essential role in this plug & measure capability. The Fusion SOS is implemented on the platform WebGenesis, an information management server of Fraunhofer IITB. The information management server has the information categories features of interest (sampling grids), procedures and results with associated meta-data to support searching. All intermediate files produced by the fusion procedure are uploaded together with the fusion result to the WebGenesis information management server. This ensures a reproducible trace of the processing steps.