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Fidelity and Yield in a Volcano Monitoring Sensor Network
WSN Seminar 2008Napoleon E. Cornejo
Werner-Allen, Lorincz, Johnson, Lees, Welsh
Introduction
Volcan Reventador in Ecuador16 nodes19-days229 events (quakes, eruptions, etc)
Evaluation Parameters
RobustnessEvent detection accuracyData Transfer PerformanceTiming accuracyData Fidelity
System ArchitectureTMote Sky
Seismometer(s)Omnidirectionalmicrophone
Tiny OSFreeWave Radio ModemsGPS receiverTwo RefTek data loggers (ground truth)
Field DeploymentNode separation 200-400m~3 km diamaterNodes self-organize into multihop routing treeNodes select parent with strongest link quality indicatorCommands propagated with flooding protocol (drip)No data aggregation
Network Robustness
Deluge reboot failure – affects all nodesPower outages in base station –laptop/modem failOne node antenna destroyed by volcano
Event Detection and Data Collection
60 sec of last data from outstanding eventNode event
Difference between two input signals exceeds a threshold
Global Event30% of nodes report event in 10 sec window
Fetch Data collection Protocol
Event Detector Accuracy5% accuracy
Fetch algorithm takes several minutes per node and blocks (24% events)Downtimes (base, nodes, modems)EWMA parameters poorly chosenLow sensitive sensors
Data Collection
60 secs of data after event1 block = 256 bytes60 secs = 206 blocks of data (51.5 kb)1 MB Flash stores ~20min of data
Event Yield: fraction of nodes for which signal correctly downloadedNode Yield: Probability that event successfully downloaded from node
Factors affecting YieldDepth of node (routing)Some nodes record more data (250,251)Poor link with parent (node 209) or continuously changing parentsBug in node 250 – continuous unnecessary sampling
Fetching Latency
Median latency: 186 secsSome even with 444 secsDeeper -> LongerNode 214, 409 secs = 63 secs/hopFETCH designed for infrequent events
Timing AccuracyTiming is paramount to measure volcanic activity
At 100hz sampling, accuracy within 10msGPS receiver to synchronize nodesFTSP: Flooding Time Synchronization Protocol
GPS time, FTSP Global time, local timeEvey 10 secs, node records local and global time, sends msg to base station
Failures:FTSP: Message losses cause global time to driftTinyOS clock driver returning wrong local timeIncorrect reading from one node may corrupt other nodes
Time Rectification10ms error windowCorrelate global timestamp of each node to a common reference.Discard if differing by a threshold
|GTref – GT | > δδ = 1 sec7.8% filtered out
Linear regression to map times (last 5 mins)
Data FidelityVerified only in consistency: expected volcanic activityAcoustic wave propagation
~340 m/sOriginates in volcano vent
Seismic Wave propagation
Origin unknownCould verify shallow or deep events
Data Fidelity (Results)
Compare Node 213 and RVEN signals. 56m apart.Wave traveling aprox 1.5 km/secExpected time lag 37ms+ 10ms accepted error window
Conclusions & LessonsTiming accuracy is pivotal in volcanic monitoring
Wave propagationEvent characterization
Thorough pre-deployment testing is necessaryFetch protocolDeluge protocol failuresTinyOS local time bug
WSN have great potential in this field96% node uptimeLarge telemetry distances3D tomographic characterization