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The Cricket Compass for Context-Aware Mobile Applications Nissanka Priyantha, Allen Miu, Hari Balakrishnan, Seth The Cricket Compass for Context-Aware Mobile Applications Nissanka Priyantha, Allen Miu, Hari Balakrishnan, Seth Teller MIT Laboratory for Computer Science http: //nms. lcs. mit. edu/

Cricket Location System • Original Version [mobicom 00] – Location information: room, floor, building, Cricket Location System • Original Version [mobicom 00] – Location information: room, floor, building, etc. • New extensions – The Cricket Compass – Position information • (x, y, z) coordinates within a space – Orientation information • direction at which device faces Mobile device (x, y, z)

You Are Here… Great, now what? ! You are here You Are Here… Great, now what? ! You are here

Point-and-Use Application Point-and-Use Application

Orientation is important! Orientation is a building block that supports a wide variety of Orientation is important! Orientation is a building block that supports a wide variety of mobile applications

Design Goals • Compact, integrated, self-contained • Should not rely on motion to determine Design Goals • Compact, integrated, self-contained • Should not rely on motion to determine heading (as in GPS navigation systems) • Robust under a variety of indoor conditions • Low infrastructure cost; easy to deploy • Enough accuracy for mobile applications (5 o accuracy)

The Cricket Compass Architecture (x 1, y 1, z 1) Y (x 3, y The Cricket Compass Architecture (x 1, y 1, z 1) Y (x 3, y 3, z 3) (x 0, y 0, z 0) X Z vt 0 vt 1 Cricket listener with RF and ultrasonic sensors Beacons on ceiling vt 3 (x 2, y 2, z 2) RF + Ultrasonic Pulse vt 2 Mobile device ( x, y, z) vt 3 to solve for unknown speed of sound

Definition of Orientation (x 1, y 1, z 1) Y (x 3, y 3, Definition of Orientation (x 1, y 1, z 1) Y (x 3, y 3, z 3) (x 0, y 0, z 0) B Beacons on (x 2, y 2, z 2) ceiling X Z Orientation relative to B (on horizontal plane) Mobile device (on horizontal plane)

Approach: Use Differential Distance to Determine Orientation Beacon Assume: Device rests on horizontal plane Approach: Use Differential Distance to Determine Orientation Beacon Assume: Device rests on horizontal plane Method: Use multiple ultrasonic sensors; calculate rotation using measured distances d 1, d 2, z sin = (d 2 - d 1) / sqrt (1 - z 2/d 2) where d = (d 1+d 2)/2 d d 1 Need to measure: a) (d 2 - d 1) b) z/d S 1 d 2 L S 2 z

Problem: Measuring (d 2 – d 1) directly requires very high precision! Beacon • Problem: Measuring (d 2 – d 1) directly requires very high precision! Beacon • Consider a typical situation – Let L = 5 cm, d = 2 m, z = 1 m, = 10º – (d 2 – d 1) = 0. 6 cm d • Impossible to measure d 1, d 2 with such precision d 1 d 2 – Comparable with the wavelength of ultrasound ( = 0. 87 cm) S 1 L S 2 z

Solution: Differential Distance (d 2 d 1) from Phase Difference ( ) • Observation: Solution: Differential Distance (d 2 d 1) from Phase Difference ( ) • Observation: The differential distance (d 2 -d 1) is reflected as a phase difference between the signals received at two sensors Estimate phase difference between ultrasonic waveforms to find (d 2 -d 1)! Beacon = 2 p (d 2 – d 1)/l d 1 d 2 S 1 t S 2 t

Problem: Two Sensors Are Inadequate • Phase difference is periodic ambiguous solutions • We Problem: Two Sensors Are Inadequate • Phase difference is periodic ambiguous solutions • We don’t know the sign of the phase difference to differentiate between positive and negative angles • Cannot place two sensors less than 0. 5 apart – Sensors are not tiny enough!!! – Placing sensors close together produces inaccurate measurements

Solution: Use Three Sensors! • Beacon • d 1 d 2 d 3 • Solution: Use Three Sensors! • Beacon • d 1 d 2 d 3 • S 3 S 2 S 1 L 12 = 3 l/2 Estimate 2 phase differences to find unique solution for (d 2 -d 1) Can do this when L 12 and L 23 are relatively-prime multiples of l/2 Accuracy increases! L 23 = 4 l/2 t

Cricket Compass v 1 Prototype Ultrasound Sensor Bank 1. 25 cm x 4. 5 Cricket Compass v 1 Prototype Ultrasound Sensor Bank 1. 25 cm x 4. 5 cm Sensor Module RF module (xmit) RF antenna Ultrasonic transmitter Beacon

Angle Estimation Measurements • • Accurate to 3 for 30 , 5 for 40 Angle Estimation Measurements • • Accurate to 3 for 30 , 5 for 40 Error increases at larger angles

Cricket Compass Hardware • Improves accuracy • Disambiguates in [ - , ] Amplifiers, Cricket Compass Hardware • Improves accuracy • Disambiguates in [ - , ] Amplifiers, Wave shaping, and Selection Circuits RF RX Microcontroller RS 232 Driver

Conclusion The Cricket Compass provides accurate position and orientation information for indoor mobile applications Conclusion The Cricket Compass provides accurate position and orientation information for indoor mobile applications – Orientation information is useful – Novel techniques for precise position and phase difference estimation to obtain orientation information – Prototype implementation with multiple ultrasonic sensors Orientation accurate to within 3 -5 degrees http: //nms. lcs. mit. edu/cricket/

Considerations • Beacon placement – At least one beacon within range – Avoid degenerate Considerations • Beacon placement – At least one beacon within range – Avoid degenerate configuration (not in a circle) • Ultrasonic reflections – Use filtering algorithms to discard bad samples • Configuring beacon coordinates – Auto-configuration, auto-calibration

Current Orientation Systems Are Not Adequate for Indoor Use • Magnetic based sensors (magnetic Current Orientation Systems Are Not Adequate for Indoor Use • Magnetic based sensors (magnetic compass, magnetic motion trackers) – suffers from ferromagnetic interference commonly found indoors • Inertial sensors (accelerometers, gyroscopes) – used in sensor fusion to achieve high accuracy – require motion to determine heading – suffer from cumulative errors • Other systems require: – Extensive wiring: expensive & hard to deploy – Multiple active transmitters worn by the user: obtrusive, inconvenient, not scalable

Point in the direction of the Service… Not at the Service • Orientation information Point in the direction of the Service… Not at the Service • Orientation information provides a geometric primitive that is general and useful among a variety of “direction-aware” applications, e. g. – In-building navigation – Point and Shoot User Interfaces • Line-of-sight systems are limited – awkward to use, not robust – do not support navigation Orientation information is useful for context-aware mobile applications!

Is orientation necessary? • Direction-aware applications could be implemented using “TV remotes!” • But Is orientation necessary? • Direction-aware applications could be implemented using “TV remotes!” • But orientation information is useful – Application-specific semantics are possible – Convenient for navigation applications – Eliminates the need for a line of sight to target

System Model Cricket (x, y, z, )… Service Discovery Database Services, Other users System Model Cricket (x, y, z, )… Service Discovery Database Services, Other users

System Model Cricket (x, y, z, )… printer@(x, y, z, ) Service Discovery Database System Model Cricket (x, y, z, )… [email protected](x, y, z, ) Service Discovery Database [email protected](x, y, z, )… [email protected](x, y, z, ) Services

Differential Distance From Phase Difference • Observation: The differential distance (d 2 -d 1) Differential Distance From Phase Difference • Observation: The differential distance (d 2 -d 1) is reflected as a phase difference between the signals received at two sensors Ultrasound signal first hits sensor S 1 Beacon d 1 d 2 S 1 S 2 t

Differential Distance From Phase Difference • Observation: The differential distance (d 2 -d 1) Differential Distance From Phase Difference • Observation: The differential distance (d 2 -d 1) is reflected as a phase difference between the signals received at two sensors The same signal then hits sensor S 2 Beacon d 1 d 2 S 1 S 2 t

Where am I? (Active map) Where am I? (Active map)

Deployment Deployment

Differential Distance From Phase Difference • Observation: The differential distance is reflected as a Differential Distance From Phase Difference • Observation: The differential distance is reflected as a phase difference between the signals received at two receivers Estimate phase difference between ultrasonic waveforms to find (d 2 -d 1)! Beacon = 2 p v t/ l = 2 p (d 2 – d 1)/l d 1 d 2 R 1 t R 2 t <= L/v, where v is velocity of sound t

Ambiguous Solutions: Example • • We know: t, t’ <= L/v Let L = Ambiguous Solutions: Example • • We know: t, t’ <= L/v Let L = Observed time difference is t Possible time differences are t and t’ Beacon L/v t t t t’

Requirements • Navigational information – Space • address, room number – Position • coordinate, Requirements • Navigational information – Space • address, room number – Position • coordinate, with respect to a given origin in a space – Orientation • angle, with respect to a given fixed point in a space • Low cost, low power • Completely wireless – Deployable in existing buildings • Scalable • Autonomous – Mobile device determines its own location

Ambiguous Solutions: Example • We know: t <= L/v • Let L = /2 Ambiguous Solutions: Example • We know: t <= L/v • Let L = /2 In this case, we can find a unique solution Beacon L/v t t