1. Nissanka Bodhi Priyantha Computer Science, Massachusetts Institute of Technology 2011. 10. 14. RTLab. Seolyoung, Jeong Dissertation, MIT, June 2005

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

3. 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

4. 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

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

6.  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)

7.  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

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

9. 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

10. “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

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

12. 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

13. 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

14. 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)

15. 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

16.  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

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

18. 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 

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

20. 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

22. 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