1 PinPoint: An Asynchronous Time Based Location Determination System Moustafa Youssef, Adel Youssef, Chuck Rieger, UdayaShankar, Ashok Agrawala Presented by Sofia Nikitaki

2 Motivation  Location-aware applications  Software-based Standard protocols  Rapidly deployable No calibration  Energy-efficient Number of messages

3 Review  Motivation  Related Work  PinPoint Technology  PinPoint Hardware  Results  Conclusions

4 Time-based Localization  Problem Clock synchronization  Solutions 1.Use synchronized clocks: GPS Expensive 2.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

5 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

6 PinPoint Technology  3 Phases Measurement phase Information Exchange phase Computation Phase

7 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

8 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

9 PinPoint TechnologyGlobal O(n) messages for n nodes Let  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

10 PinPoint Technology: Information Exchange Phase  node  message containing its receive timestamp for messages transmitted by other nodes  O(n) messages

11 PinPoint Technology: Computation Phase  node computes spatial coordinates clock attributes of every other node  Redundant information used to reduce errors  No communication takes place  Accuracy: few feet  Synchronized clocks

12 Calculations for Node Pair A and B  Drift ratio  Propagation delay  Remote clock reading

13 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: b b d  B b is in the order of hundred parts in a million

14 PinPoint Hardware

15 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

16 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)

17 Distance Vs Reported Clock Ticks  4-6 feet accuracy using 3 ns clock

18 Indoors Evaluation-First Testbed  4.18 feet average error  8.42 feet at 95%  37 feet range

19 Indoors Evaluation-Second Testbed  All locations NLOS  4.95 feet average error 11 feet at 95%  74 feet range

20 Outdoors Evaluation  6.85 feet average error  feet at 95%  146 feet range

21 Mobility Evaluation  Can track the user in realtime  PinPoint exchange cycle is much faster than user mobility rat

22 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 ( m)

23 Thank You