Node Density Independent Localization

1
Node Density Independent Localization

• Presented by Brano Kusy
• B.Kusy, M.Maroti, G.Balogh, P.Volgyesi, J.Sallai,
  A.Nadas and A.Ledeczi (Vanderbilt University,
  Nashville, TN)
• L.Meertens (Kestrel Institute, Palo Alto, CA)

2
Overview

• Introduction
• Radio Interferometric Ranging (RIPS)
• Improving RIPS
• Ambiguity of Interferometric Range
• Multipath
• Interleaving Ranging and Localization
• Scalability of RIPS in Time
• Maximum Number of Independent Interferometric
  Measurements
• Scheduling
• Evaluation
• Tracking Demo
• Conclusions

3
Introduction

• Ranging determine distances between nodes
• Localization find physical 3-D locations of
  nodes
• State of Art
• Time of flight - acoustic ranging sound tens
  of meters range 10cm accuracy
  ultrasound 5m range, 6cm accuracy
• RF Time of flight GPS anywhere outdoors
  accuracy is feet with DGPS
• UWB lt10m range 6 inches
  accuracy
• Radio signal strength - RSSI MS RADAR 35m
  range 3m accuracy
• Calamari 20m range 2m accuracy
• Range Free SPOTLIGHT 170m range 20cm
  accuracy
• Radio interference RIPS 170m range 4cm
  accuracy
• Contribution
• improved the range of RIPS from 20m to 170m
• improved RIPS ranging error distribution
• addressed scalability constraints

4
Radio Interferometric Ranging

• Interference superposition of two or more waves
  resulting in a new wave pattern
• Interferometry cross-correlates a signal from a
  single source recorded by 2 observers, used in
  geodesy, astronomy,
• Our novel technique - RIPS no processing power
  to correlate high freq radio signals in
  sensornets, instead we utilize two transmitters
  to produce low frequency interference signal
  directly

• Signal strength is not crucial no dependence on
  orientation, power level, hardware deviations

• Low freq envelope (of composite signal)
  inexpensive HW

• High carrier freq
• high accuracy

5
Radio Interferometric Ranging
fCD (dAD-dBDdBC-dAC) mod ?

• Senders (A, B) transmit simultaneously
• pure sinusoid waves
• high carrier freq (400 MHz)
• small freq difference (500 Hz)
• Receivers (C, D) measure radio interference
• sample RSSI (9 KHz)
• find beat frequency (500 Hz)
• measure phase offset of RSSI
• use 1 µs timesync to correlate phase offsets
• result (dAD-dBDdBC-dAC) mod ?
• dXY distance of X and Y
• ? wave length of carrier freq

6
Overview

• Introduction
• Radio Interferometric Ranging (RIPS)
• Improving RIPS
• Ambiguity of Interferometric Range
• Multipath
• Interleaving Ranging and Localization
• Scalability of RIPS in Time
• Maximum Number of Independent Interferometric
  Measurements
• Scheduling
• Evaluation
• Tracking Demo
• Conclusions

7
Ambiguity of Interferometric Ranges
Interferometric range (q-range) for 4 fixed nodes
A, B, C, D is a solution of the following
equation
fCD (dAD-dBDdBC-dAC) mod ?
interferometric range, or q-range
In general, infinitely many ranges solve this
equation. We eliminate incorrect solutions by
measuring phase offsets at multiple carrier
frequencies. We then solve the following system
of equations
f1CD (dAD-dBDdBC-dAC) mod ?1
...
fkCD (dAD-dBDdBC-dAC) mod ?k
8
Ambiguity of Interferometric Ranges

• Define the ranging problem as a frog jumping
  problem
• we have k frogs jumping along a line
• phase offsets the points where the frogs start
  jumping
• wavelengths lengths of the frogs jumps
• interferometric range r place where all frogs
  align

How do we solve the system of equations and what
are the ranging errors of our method?
9
Ambiguity of Interferometric Ranges

• Define the ranging problem as a frog jumping
  problem
• we have k frogs jumping along a line
• phase offsets the points where the frogs start
  jumping
• wavelengths lengths of the frogs jumps
• interferometric range r place where all frogs
  align

• phase-offset discrepancy a mean square error of
  fini?i from r

Extra solutions exist small number of wavelengths
to the left and right of r
f1
? 1
f2
? 2
f3
? 3
r
d
Error of type 1 is minimized if ? separation is
maximized.
10
Ambiguity of Interferometric Ranges

• Define the ranging problem as a frog jumping
  problem
• we have k frogs jumping along a line
• phase offsets the points where the frogs start
  jumping
• wavelengths lengths of the frogs jumps
• interferometric range r place where all frogs
  align
• phase-offset discrepancy a mean square error of
  fini?i from r

d
11
Ambiguity of Interferometric Ranges

• Define the ranging problem as a frog jumping
  problem
• we have k frogs jumping along a line
• phase offsets the points where the frogs start
  jumping
• wavelengths lengths of the frogs jumps
• interferometric range r place where all frogs
  align
• phase-offset discrepancy a mean square error of
  fini?i from r

d
Error of type 2 is minimized if f separation is
minimized.
12
Multipath Ground Reflection

• RIPS ranging error distribution observations
• gets significantly worse for distances above 25m,
  if nodes on the ground (campus)
• improves when lifting motes 1m above the ground
  (campus)
• is good for distances above 100m, even if nodes
  are positioned on the ground (field no trees,
  no buildings close by)

(3) suggests that multipath is at play here! But
why elevating the motes helps the problem?
13
Interleaving Ranging and Localization
Recap q-range r - solution of equation
fini?ir phase-offset discrepancy function -
alignment of points

• Our observations
• it rarely happens that all q-ranges are affected
  by multipath
• 30-40 of good (lt30cm error) q-ranges give us
  good (ltfew m error) locations
• even if q-range is affected by multipath,
  discrepancy function has local minimum at true
  range

• Our interleaved algorithm
• find all q-ranges
• localize using these q-ranges
• constrain q-range search with the intermediate
  locations

14
Evaluation setup 1

• UCB Richmond Field Station
• 50 node setup, 9m neighbor distance, 108m max
  distance
• moderate multipath present (trees, buildings
  close by)

• manually localized subset of 30 nodes have 5cm
  error
• 68 of measured ranges has lt10cm error

• GPS ground truth has few m error
• 68 of ranges has lt1m error
• better ground truth needed

15
Evaluation setup 2

• q-range error distribution
• 72 of ranges has lt30cm error

• rural area close to Nashville
• 16 node setup, deployed on the ground, 35m avg,
  170m max neighbor distance
• covering 12000m2 area (2 football fields)
• no multipath present, other than the ground
  reflections
• localization 4cm avg, 12cm max error

• error distribution after 3 iterations
• 98 of ranges has lt30cm error

16
Dirty Bomb Detection and Localization Demo

• RIPS was used in our tracking demo yesterday
• 12 XSM anchor motes used
• near real-time tracking
• 2 sec location refresh rate,
• 3 sec delay,
• high accuracy
• better than 1m error
• long range
• often larger than 100m ranges used for
  localization
• covered 80m x 90m area

• tracked 1 mote only
• theoretically, we can track arbitrarily many
  motes as only anchor motes need to transmit
• completely stealthy

17
Conclusions and Questions

• Introduced improved version of novel ranging
  technique for wireless sensor networks
• Large range, high accuracy, requires no extra HW,
  stealthy, ...
• Had successful tracking demo
• QUESTIONS ???