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Life Under your Feet:A Wireless Soil Ecology Sensor Network

R. Musaloiu-E†., A. Terzis† , K. Szlavecz‡ , A. Szalay*, J. Cogan*, J. Gray◊† Computer Science Department, JHU‡ Earth and Planetary Sciences Department, JHU* Physics Department, JHU◊Microsoft Research

In the Beginning…

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• The Cast• An ecologist: Katalin Szlavecz• An undergrad student working

in a summer project: Joshua Cogan

• The Stage• The urban forest near the JHU

campus• Off-the-self soil moisture and

temperature sensors• A few Mica2 motes

• The Goal• Test whether WSNs can be

used to collect soil monitoring data at spatial and temporal scales larger than that of data loggers

First Step

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• The sensors were thoroughly tested in the lab

• 2 out of 6 sensors misbehaved

• Relatively precise (+/-0.5°C)

• … but offset of 1.5°C from a NIST approved thermocouple.

• Next step: teach the motes to play their part.

Second Step

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• More actors:• CS faculty (Andreas Terzis)• CS grad student (Razvan Musaloiu-E.)• DB experts: Alex Szalay and Jim Gray

• More setup:• MicaZ motes• MSSQL• Skyserver DB software

• The goal• Build an end-to-end data collection

system

System requirements

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• Measurement fidelity• All the raw measurements should be collected and

persistently stored• Measurement accuracy and precision

• Temperature data error < 0.5 C, volumetric moisture data error < 1%.

• Fusion with external sources• Comparing measurements with external data sources is

crucial• Additional Requirements

• Sampling Frequency: minutes • Experiment Duration: years• Deployment Size: networks should be deployed in ways

that capture the heterogeneity of land use

System Architecture

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Sensor Node Hardware

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• Sensors:• Watermark soil moisture• Irrometer soil thermistor• Onboard Temperature and Light

• MicaZ motes• Waterproof cases, cables, connectors, ...

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Mote Software

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Mote Base

1s2min

(b)(a)

...

Mote

Bulk Phase

Sending

Base

...

Receiving

data message

status messagedata request

Send−and−Wait Phase

• Sensors are sampled every minute• Raw measurements are stored in flash

• Storage overrun in > 20 days• Data retrieved using NACK-based sliding window protocol• Each mote sends every 2 min a status messages to base-

station: (#meas, bat voltage)• Web-accessible real-time monitoring

Database Design

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• DB Design derived from experiment and WSN setup

• Self-documenting framework

• Raw data follow a multi-stage validation process

• Stored data available through Web pages and Web Services

• Datacube for analysis• 7M datapoints collected

Calibration workflow

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Temperature SensorCalibration

Temperature SensorCalibration

SoilTemperature

SoilTemperature

Water ContentVolumetric

Water ContentVolumetric

Soil Water Potential->Volumetric ConversionSoil Water Potential->Volumetric Conversion

VoltageVoltage

VoltageVoltage

VoltageVoltage

Moisture sensorA/D units

Moisture sensorA/D units

Reference voltageA/D units

Reference voltageA/D units

Temperature sensorA/D units

Temperature sensorA/D units

CPU clockCPU clock

Air TemperatureA/D units

Air TemperatureA/D units

Light IntensityA/D units

Light IntensityA/D units

TemperatureConversion

TemperatureConversion

Air TemperatureCelsius

Air TemperatureCelsius

UTC DateTimeUTC DateTime

ResistanceResistance

ResistanceResistance

Mote Resistor Calibration

Mote Resistor Calibration

Moisture SensorCalibration

Moisture SensorCalibration

Water DeficitSoil Matrix Potential

Water DeficitSoil Matrix Potential

Calibrationsin the Lab

Calibrationsin the Lab

Deployment Site

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• 10 motes deployed on a slanted grid, ~2m apart • 147 days of deployment• Basestation within direct radio range

• Some links had unexpectedly poor quality• Had to collect some of the data using laptop

2m

8m

2m

Ecology Results (Temperature)

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Ecology Results (Soil Moisture)

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Energy consumption

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• Motes go to sleep when not collecting data/sending reports

• Easy to calculate average current draw

• Powered by AA alkaline batteries

• Linear discharge curve• Possible to predict node

lifetime

Data Transmission Performance

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Good Link Lossy Link

Problems we encountered

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• Waterproofing the boxes• Moisture sensor behavior in

the wild• Power consumption• Data provenance• Reprogramming

Conclusions

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• Sensor Networks are still expensive in time, money and know-how

• Need for data analysis tools• Need for increased confidence in collected

measurements• End-to-End system that transforms raw

measurements to scientifically significant data

Questions?

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• For more information http://www.lifeunderyourfeet.org/

Life Under your Feet:A Wireless Soil Ecology Sensor NetworkIn the Beginning…First StepSecond StepSystem requirementsSystem ArchitectureSensor Node HardwareMote SoftwareDatabase DesignCalibration workflowDeployment SiteEcology Results (Temperature)Ecology Results (Soil Moisture)Energy consumptionData Transmission PerformanceProblems we encounteredConclusionsQuestions?