node-based tracking using received signal strength indication · node-based tracking using received...
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Node-Based Tracking Using Received Signal Strength Indication Thomas DubaniewiczLead Research Engineer NIOSH Pittsburgh Research Laboratory
The findings and conclusions in this presentation have not been formally disseminated by the National Institute for Occupational Safety and Health and
should not be construed to represent any agency determination or policy. Mention of company names or products does not constitute endorsement by
the National Institute for Occupational Safety and Health.
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Overview
• Node-Based Tracking– Handset = Tracking Device– Node = Reader
• Received Signal Strength Indication (RSSI)
• Central Server Considerations Before and During Emergencies
• NIOSH Wireless Mesh Communication & Tracking Contract
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Node-Based TrackingHandset = Tracking DeviceNode = Reader
NodeHandset signalBackhaul link
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Handset Network Connectivity
• Unless powered off, communication handsets used with node-based networks will transmit periodically to maintain network connectivity
• The miner does not need to key the handset for tracking purposes
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Received Signal Strength Indication (RSSI)
• RSSI measurements are arbitrary indications of the signal power
• Relationship to actual signal power and range of reported RSSI values are vendor specific
• Supported by Zigbee (IEEE 802.15.4) and WiFi (IEEE 802.11) standards
• A similar metric also supported by WiFi is Received Channel Power Indicator (RCPI), with defined levels of power accuracy and resolution
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Tracking via RSSI
• Takes advantage of line-of-sight signal power fade over distance
• Anything that affects the received signal strength influences the accuracy: – Around coal pillars– Through stoppings– Body absorption– Equipment in the entries– Antenna orientation– Multipath constructive or destructive interference
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RSSI Error Correction Techniques
• There are ways to improve the accuracy, including:– Time averaging of the received signal– Considering the rate of travel– Mapping movement to the mine map– Time-of-arrival
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Tracking – Radio Contact With One Node• Location accuracy based on radius around node
(omni directional antenna)• Worst case accuracy is maximum radio range from
the handset to the node (radius from node)
Node
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Tracking Between Two Nodes
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Tracking Between Two Nodes
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Tracking Between Two Nodes
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RSSI Error Test
• Transmitters spaced every 100 ft• Miner walks along entry for 500 ft
100 ft
Transmitters
ReceiverMiners’ Path
• True position and RSSI estimated position recorded at regular intervals along path
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Comparison of Actual and RSSI Calculated Displacement as a Miner Walks 500 Feet Down an Entry
-100
0
100
200
300
400
500
600
0 50 100 150 200 250 300
Time (s)
Dis
tanc
e (ft
)
Calculated Measured Calculated Trend
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RSSI Tracking Error Distribution (Example)
Error Distribution
0
5
10
15
20
25
0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200 210 220
Error (ft)
No.
of s
ampl
es
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RSSI Tracking Error Distribution (Example)
Probability Distribution (Cumulative)
0%10%20%30%40%50%60%70%80%90%
100%
0 20 40 60 80 100 120 140 160 180 200
Error (feet)
Prob
abili
ty
90% Probability
Median Error
Maximum Error
Probabilistic nature of radio measurements is inherent to radio location systems
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Node-Based TrackingHandset = Tracking DeviceNode = Reader
NodeHandset signalBackhaul link
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Node failure puts miner in radio contact with one node Here?
Or Here? Recording last known location before event can help
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Central Server• Calculates and displays
handset location• Node placement on display
must be updated frequently• Last known location before
emergency will be saved per MINER Act
• Detailed network status must be easily accessible post-event
Dispatch Display
Network Display
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NIOSH Contract: Wireless Mesh Mine Communication and Tracking System
• L3 Communications Global Security & Engineering Solutions
• ICG-Wolf Run Mining Co. Sentinel mine