International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Volume XXXIX-B4, 2012
XXII ISPRS Congress, 25 August — 01 September 2012, Melbourne, Australia
minutes delay before service B makes sensor data available
online. In this case, the updating behaviour of service B may
not be suitable to be data sources as near real-time applications.
Service A Service B
Average time difference 30,679 5.433.863
(millisecond)
Number of unnecessary 0 24
requests
Total number of feedings 21 26
Table 1. Experimental results
4. CONSLUSIONS AND FUTURE WORK
We have presented a hybrid push-pull system to retrieve sensor
data in a near real-time manner. The proposed system first uses
the query aggregator to aggregate user queries and filter out
unnecessary requests. Then the adaptive feeder component
detects the updating frequency of OGC sensor web services and
retrieves sensor data with the aggregated requests in a timely
manner. As shown in the experimental results, our proposed
system can retrieve sensor data in a timely manner if the service
makes data available online as soon as it is measured. On the
other hand, if the service buffers or calibrates sensor data before
making them available online, the proposed system will
periodically request data with the detected sampling frequency
with the trade-off of redundant requests.
As we can see from the experimental results, the performance of
the proposed system is highly related to the updating behaviour
of sensor web service. Therefore, one of our future works is to
simulate sensor web services with different data updating
behaviours. The other future direction is the integration of
sensor data from different sensor web service. The current
sensor web services are heterogeneous in terms of protocol,
syntactic, and semantic. Users need to first find the services that
host the data they are interested in. However, with the growing
number of sensor web services, this discovery process becomes
a challenging task. Therefore, how to integrate sensor data to
provide a coherent view of sensor web is also one of our future
works.
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