Nissanka Bodhi Priyantha Computer Science, Massachusetts Institute of Technology RTLab. Seolyoung, Jeong Dissertation, MIT, June 2005

Cricket System Architecture Distance Estimation in Cricket Location Estimation Techniques Mobile-assisted Localization Anchor-Free Localization In-door Navigation Research

Location-awareness will be a key feature of many future mobile applications Many scenarios in pervasive computing  Navigation  Resource discovery  Embedded applications, sensor systems  Monitoring and control applications Cricket focuses mainly on indoor deployment and applications

Must determine:  Spaces: Good boundary detection is important  Position: With respect to arbitrary inertial frame  Orientation: Relative to fixed-point in frame Must operate well indoors Preserve user privacy: don’t track users Must be easy to deploy and administer Must facilitate innovation in applications Low energy consumption

 No central beacon control or location database  Active beacon + Passive listener Beacon Listener info = “a1” info = “a2” Estimate distances to infer location

 The listener measure the time gap between the receipt of RF and the ultrasonic signals  Velocity of ultra sound << Velocity of RF   V us ≈ 344 m/s, V rf ≈ 3 x 10 8 m/s RF data : 433MHz (space name) Beacon Listener Ultrasound (pulse)

 Beacon transmissions are uncoordinated  Ultrasonic signals reflect heavily  Ultrasonic signals are pulses (no data)  These make the correlation problem hard and can lead to incorrect distance estimates Beacon A Beacon B t RF BRF AUS B US A Incorrect distance Listener

Carrier-sense + randomized transmissions  Reduce chances of concurrent beaconing  Erroneous estimates do not repeat Hidden terminal effect problem 

RF range > 2 x US range Ensures that if listener can hear ultrasound, corresponding RF will also be heard Time interval before the arrival of the ultrasonic signal  received more than one RF ranging message  discard the messages and the US signal t RF AUS A RF BUS B

“Space id” = name of space Deploy a pair of beacons at equal distances away from each open boundary Closest beacon is always in the same space as the listener

distance = d i – e i (e i : measurement error) initial coordinates (x 0, y 0, z 0 ) The position estimation error < 10cm

Mobile device Beacons on ceiling Orientation relative to B  B Beacons on ceiling Z X Y (x1,y1,z1) (x0,y0,z0) (x2,y2,z2) (x3,y3,z3) (parallel to horizontal plane) (on horizontal plane) (x4,y4,z4)  : angle formed by the heading direction

Assume: Device rests on horizontal plane Method: Use multiple ultrasonic sensors; calculate rotation using measured distances d1, d2, z Two terms need to be estimated: ① d2 - d1 ② z/d (by estimating coordinates) d1 d2 z  Beacon S2 S1 d L, where

Problem : for reasonable values of parameters (d, z), (d2 - d1) must have 5mm accuracy Observation: The differential distance (d2-d1) is reflected as a phase difference between the signals received at two sensors d2 d1  = 2  (d2 – d1)/ t L Beacon Solution  Estimate phase difference between ultrasonic waveforms! (λ : wavelength of the signal)

δd < λ/2 to unambiguously determine  L >= |d 1 – d 2 | = |δd|  L < λ/2 40KHz ultrasonic waveform at a temperature of 25 ℃ and 50% humidity  λ/2 = 4.35 mm Cannot place two sensors less than 0.5cm apart  Sensors are not tiny enough!!!  Placing sensors close together produces inaccurate measurements

 Estimate 2 phase differences to find unique solution for (d2-d1)  Can do this when L 12 and L 23 are relatively-prime multiples of   Accuracy increases! d1 t L 12 = 3  d2 d3 L 23 = 4  Beacon S1 S2 S3

Two locations B1, B2  same θ solution : using two sets of non- collinear receiver-triplets to break the symmetry

Beacons on ceiling at known coordinates (x i, y i, 0) B vt 1 vt 2 vt 3 vt 4 unknown coordinate (x, y, z) Four equations, four unknowns (x, y, z, v) Velocity of sound varies with temperature, humidity Can be “eliminated” (or calculated!) Coordinate System used in Cricket Z X Y 

Accurate to 3  for  30 , 5  for  40  Error increases at larger angles

Sensor Module Ultrasound Sensor Bank 1.25 cm x 4.5 cm Ultrasonic sensor RF antenna Ultrasonic sensor RF module (rcv) Atmel processor Listener RS232 i/f

Cricket provides location information for mobile, pervasive computing applications  Space  Position  Orientation Flexible and programmable infrastructure Deployment and management facilities Starting to be used by other research groups