fidelity and yield in a volcano monitoring sensor network · fidelity and yield in a volcano...
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Fidelity and Yield in a Volcano Monitoring Sensor Network
WSN Seminar 2008Napoleon E. Cornejo
Werner-Allen, Lorincz, Johnson, Lees, Welsh
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Introduction
Volcan Reventador in Ecuador16 nodes19-days229 events (quakes, eruptions, etc)
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Evaluation Parameters
RobustnessEvent detection accuracyData Transfer PerformanceTiming accuracyData Fidelity
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System ArchitectureTMote Sky
Seismometer(s)Omnidirectionalmicrophone
Tiny OSFreeWave Radio ModemsGPS receiverTwo RefTek data loggers (ground truth)
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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
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Network Robustness
Deluge reboot failure – affects all nodesPower outages in base station –laptop/modem failOne node antenna destroyed by volcano
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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
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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
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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
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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
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Fetching Latency
Median latency: 186 secsSome even with 444 secsDeeper -> LongerNode 214, 409 secs = 63 secs/hopFETCH designed for infrequent events
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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
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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)
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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
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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
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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