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