Tracking Prasun Dewan Department of Computer Science University of North Carolina

2 Triangulation n Need to solve for x, y, z u Assume orientation not an issue n Need distance to three points with known coordinates u Can solve for x, y, z

3 Issues n What are the three known points? n How to determine distances? n Expense n Privacy

4 GPS n Satellites are known points u Their current location known 24 hrs in advance upto accuracy of a few meters F Used for tuning? u They also broadcast their position n Measure time takes for signal to each receiver u Signal frequency MHz and MHz n Code division multiple access to eliminate interference n Time of flight of signal gives distance

5 Clock Synchronization n Clocks of satellites synchronized n Clock of receiver not synchronized n Offset same for all satellites n One more variable n Need four satellites

6 Excerpt from Hopper’s Slides n Start of excerpt

7 Sentient Computing Ubiquitous Computing vision Computing devices everywhere Access to applications anywhere Whatever is on hand is available Sentient Computing vision Ubiquitous Computing made context- aware Physical context used for automatic control Sensors and space are part of computing systems

8 Programming With Space The components Notions and representations of physical space Data and computational models Sensor information User interface the real world

9 Components for Programming With Space Devices Platforms Sensors Networks +Architecture Conduits

10 Components for Programming With Space Devices Platforms Sensors +Architecture

11 Sensors: Location Information n Containment u GSM, UMTS, broadband radio u Active badge n Proximity u Bluetooth, IrDA u PICOnet n Co-ordinate u GPS u Active bat

12 Sensors: Location Information n Containment u GSM, UMTS, Broadband Radio u Active Badge n Proximity u Bluetooth, IrDA u PICOnet n Co-ordinate u GPS u Active bat

13 Containment: Active Badge Infra-Red Network 10 meter range diffuse room-scale location

14 Sensors: Location Information n Containment u GSM, UMTS, Broadband Radio u Active Badge n Proximity u Bluetooth, IrDA u PICOnet n Co-ordinate u GPS u Active Bat

15 Sensors: Location Information n Containment u GSM, UMTS, broadband radio u Active badge n Proximity u Bluetooth, IrDA u PICOnet n Co-ordinate u GPS u Active bat

16 Ultrasonic Location System Mobile transmitter (Bat) Fixed receivers Ceiling Active Bats Ultrasonic transponder Measure pulse time-of-flight Radio synchronised

17 DSP Ceiling Array 25,000 MIPS to cover AT&T Laboratories Cambridge!

18 Components for Programming With Space Devices Platforms Sensors Networks +Architecture Conduits

19 Telephone 318 Computer “Pumpkin” Computer “Papaya” Person “Mike” Person “Pete” Representing the Real World n Model real world as collection of objects Computer “Plantain” Person “Andy” Follow-me Phonebook Mobile Desktop Telephone 241 Telephone 217 CTI switch Resource monitor Keyboard monitor Location service Applications Software objects Sensors u Objects maintain state using sensor data u Applications query relevant sets of objects

20 Data Model Visualisation

21 Spatial Monitoring Vague spatial facts formalised as geometric containment and overlapping relationships between spaces X M ‘X is holding the microphone M’ ‘X can be seen by camera B but not by camera A’ A B X

22 Spatial Indexing Generates all positive/negative overlapping or containment events throughput (‘000 updates s -1 ) population (‘000) non-overlapping spaces overlapping spaces

23 Putting It All Together Move user’s desktop to screen in front of them Visible A Visible B Visible C Callbacks Registration +ve Containment (Andy) -ve Overlapping (Andy) -ve Overlapping(Andy,”Visible B”) CLEAR DESKTOP FROM B -ve Overlapping(Andy,”Visible A”) CLEAR DESKTOP FROM A +ve Containment(Andy,”Visible B”) MACHINE B: NOT IN USE MOVE DESKTOP TO B +ve Containment(Andy,”Visible C”) MACHINE C: IN USE NO ACTION

24 Example Applications Corporate memory Record me / what’s around me Annotate multimedia stream Camera field-of-view Flat display Composite display “Plonk-and-play” systems Spatial configuration determines logical configuration No need to know device IDs Automatic personalisation

25 Sentient Computing: New User Interfaces  Non-user interfaces!  Objects and people are cursors in the real-world of icons   Aural and visual feedback

Nissanka B. PriyanthaAnit Chakraborty Hari Balakrishnan MIT Lab for Computer Science The Cricket Location- Support System

27 Motivation n Emergence of pervasive computing environments n Context-aware applications u Location-dependent behavior n User and service mobility u Navigation via active maps u Resource discovery Cricket provides applications information about geographic spaces they are in

28 Design Goals n Preserve user privacy n Operate inside buildings n Recognize spaces, not just physical position u Good boundary detection is important n Easy to administer and deploy u Decentralized architecture and control n Low cost and power consumption

29 Traditional Approach Controller/ Location database Base stations ID = u Transceivers Centralized architecture User-privacy issues High deployment cost ID = u ?

30 Cricket Architecture Beacon Listener Space A Space B Space C I am at C Decentralized, no tracking, low cost Think of it as an “inverted BAT”!

31 Determining Distance n A beacon transmits an RF and an ultrasonic signal simultaneously u RF carries location data, ultrasound is a narrow pulse u Velocity of ultra sound << velocity of RF RF data (location name) Beacon Listener Ultrasound (pulse) The listener measures the time gap between the receipt of RF and ultrasonic signals –A time gap of x ms roughly corresponds to a distance of x feet from beacon

32 Uncoordinated Beacons n Multiple beacon transmissions are uncoordinated n Different beacon transmissions can interfere u Causing inaccurate distance measurements at the listener Beacon A Beacon B timeRF BRF AUS B US A Incorrect distance

Handling Spurious Interactions n Combination of three different techniques: u Bounding stray signal interference u Preventing repeated interactions via randomization u Listener inference algorithms

34 Bounding Stray Signal Interference n RF range > ultrasonic range u Ensures an accompanied RF signal with ultrasound t RF AUS A

35 t S/b r/v (max) S - size of space string b - RF bit rate r - ultrasound range v - velocity of ultrasound Bounding Stray Signal Interference (RF transmission time) (Max. RF US separation at the listener) S r b v

36 Bounding Stray Signal Interference Envelop ultrasound by RF Interfering ultrasound causes RF signals to collide Listener does a block parity error check –The reading is discarded t RF AUS A RF BUS B

37 Preventing Repeated Interactions n Randomize beacon transmissions: loop: pick r ~ Uniform[T 1, T 2 ]; delay(r); xmit_beacon(RF,US); n Erroneous estimates do not repeat Optimal choice of T 1 and T 2 can be calculated analytically u Trade-off between latency and collision probability

Inference Algorithms n MinMode u Determine mode for each beacon u Select the one with the minimum mode n MinMean u Calculate the mean distance for each beacon u Select the one with the minimum value n Majority (actually, “plurality”) u Select the beacon with most number of readings u Roughly corresponds to strongest radio signal

Inference Algorithms Distance (feet) Frequency A B AB Actual distance (feet)68 Mode (feet)68 Mean (feet) Number of samples710

40 Closest Beacon May Not Reflect Correct Space I am at B Room ARoom B

41 Correct Beacon Positioning Room ARoom B xx I am at A Position beacons to detect the boundary Multiple beacons per space are possible

42 Implementation n Cricket beacon and listener LocationManager provides an API to applications Integrated with intentional naming system for resource discovery

43 Implementation n Cricket beacon and listener LocationManager provides an API to applications Integrated with intentional naming system for resource discovery Micro- controller RF US Micro- controller RF US RS232

44 Static listener performance Interference L2 L1 Immunity to interference –Four beacons within each others range –Two RF interference sources Boundary detection ability –L1 only two feet away from boundary I1I2 L10.0% L20.3%0.4% I1 I2 % readings due to interference of RF from I1 and I2 with ultrasound from beacons Room B Room C Room A

45 Inference Algorithm Error Rates

46 Mobile listener performance Room ARoom B Room C

47 Comparisons Bat Active badge RADARCricket Track user location? Yes No, if client has signal map No Deployment considerations Centralized controller + matrix of sensors Centralized database + wired IR sensors RF signal mapping and good radios Space naming convention Position accuracy Few cmRoom-wide ~2 feet for spatial resolution Attribute System

48 Summary n Cricket provides information about geographic spaces to applications u Location-support, not tracking u Decentralized operation and administration n Passive listeners and no explicit beacon coordination u Requires distributed algorithms for beacon transmission and listener inference n Implemented and works!

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51 u Decentralized

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53 u Preserves user privacy u Good granularity u Component cost U.S. $10

54 Beacon positioning Imaginary boundaries Multiple beacons per location Location X X1X2 X3 Imaginary Boundary

55 Future work n Dynamic transmission rate with carrier- sense for collision avoidance. n Dynamic ultrasonic sensitivity. n Improved location accuracy. n Integration with other technologies such as Blue Tooth.

56 Related work n Bat n Pinpoint n Active badge n Radar

57 Inference algorithms n Compared three algorithms u Minimum mode u Minimum arithmetic mean u Majority

58 Minimizing errors. n Proper ultrasonic range ensures overlapping RF and ultrasonic signals u RF data 7 bytes at 1 kb/s bit rate u RF signal duration 49 ms u Selected ultrasonic range = 30ft < 49 ft u Signal separation < 49 ms

59 Minimizing errors. n Interfering ultrasound causes RF signals to collide n Listener does a block parity error check u The reading is discarded

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