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PinPoint: An Asynchronous Time Based Location Determination System

Moustafa Youssef, Adel Youssef, Chuck Rieger, UdayaShankar, Ashok Agrawala

Presented by Sofia Nikitaki

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Motivation

Location-aware applications Software-based

Standard protocols Rapidly deployable

No calibration Energy-efficient

Number of messages

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Review

Motivation Related Work PinPoint Technology PinPoint Hardware Results Conclusions

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Time-based Localization Problem

Clock synchronization Solutions1. Use synchronized clocks: GPS

Expensive2. Use echoing (round trip time measured by same

clock) Time measurement is not precise Affected by processing time

3. Use two different signals: Cricket RF for synchronization Ultrasound for ranging

4. PinPint

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The PinPoint Technology

Time of arrival based Software only solution: can work with

standard protocols (WiFi, WiMax) Efficient (constant number of

messages per node to locate all other nodes

Works both indoors and outdoors

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PinPoint Technology

3 Phases Measurement phase Information Exchange phase Computation Phase

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PinPoint Technology: Measurement Phase

node message (ID, transmit) records the receive timestamp

(the messages sent by other nodes) All messages are one way and broadcast

Inter-node distances, hence spatial layout (from trilateration)

Inter-node clock drifts and offsets, hence ability to carry out synchronous action with other nodes

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Clock Model Node’s clock is assumed to have drift

stable over short periods. Clock time τ is related to the real time t by

Where: a, b constant, measurement phase. B drift rate (no worse than 100 parts per

million) t is measured with a nanosecond resolution

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PinPoint TechnologyGlobal O(n) messages for n

nodesLet ta1, tb1: tx, rx ts of

first A msg tb2, ta2: tx, rx ts of

first B msg ta3, tb3: tx, rx ts of

second A msg ta4, tb4: tx, rx ts of

second B msg

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PinPoint Technology:Information Exchange Phase

node message containing its receive timestamp for

messages transmitted by other nodes

O(n) messages

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PinPoint Technology:Computation Phase

node computes spatial coordinates clock attributes of every other node

Redundant information used to reduce errorsNo communication takes place

Accuracy: few feet Synchronized clocks

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Calculations for Node Pair A and B

Drift ratio

Propagation delay

Remote clock reading

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Youssef14PinPoint Estimators Clock model

Each node has its offset (a)and drift rate (b) from the global time

t= b(a+ t) local_time= drift_rate* (offset+

global_time) Distance estimate: bbd Bb is in the order of hundred parts in a

million

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PinPoint Hardware

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PinPoint Hardware Altera Cyclone 1C20 FPGA

development kit computation processor and 3 ns

timestamping clock Maxim 2820 radio with Maxim

2242 RF power amp communication modules for

transmitting the signal used for timestamping

MaxStream 9xStream radio modem Information exchange

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PP2: Timestamping 2.4 GHz QPSK

modulation PP2 baseband

signal

Received signal processed as follows Zero-crossings detected and time stamped Many more zero-crossings detected than transmitted (noise, multi-

path) Choose “longest chain” of zero-crossings as true signal Do least-squares fit to obtain arrival time of signal Works well indoors (detects first signal in multi-path)

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Distance Vs Reported Clock Ticks

4-6 feet accuracy using 3 ns clock

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Indoors Evaluation-First Testbed

4.18 feet average error 8.42 feet at 95% 37 feet range

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Indoors Evaluation-Second Testbed

All locations NLOS 4.95 feet average

error11 feet at 95% 74 feet range

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Outdoors Evaluation

6.85 feet average error 13.03 feet at 95% 146 feet range

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Mobility Evaluation

Can track the user in realtime PinPoint exchange cycle is much

faster than user mobility rat

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Conclutions

A time-based ranging technology asynchronous clocks no echoing constant number of messages per node

Can synchronize clocks Can work with standard protocols Does not require calibration 4-6 feet accuracy (1.8288 m)

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Thank You