Tracking issues in the Wireless sensor Network
Presented By
Vinay Kumar Singh
Date: 23-11-05
Outline
Introduction.
Localization.
Tracking
Tracking
 Specify target’s past trajectory.
 Predict future position of target.
Collaborative tracking
 Individual or a small group of sensors to track target in
neighboring region.
 Hand-off to the next group of sensors where the target is
heading to next.
Multiple target tracking.
Tracking Approaches
Outdoors
1.GPS.
Indoors
1.Active Badge (cellular proximity, infrared badges, central
server).
2.Active BAT ( ultrasound-based, more accurate location Based;
more accurate location identification).
3.Cricket (ultrasound emitters and object receivers, objects selflocalize).
4. RADAR (IEEE802.11 based, uses signal strength and S/N
ratio to deduce 2D position of wireless devices indoors).
5. Laser Range Finder.
6. Electro Motive Force Method.
7. Smart Floor method.
8. Motion Star
Location Technique
Triangulation

Lateration
1. Direct
2. Time-of-Flight(e.g.GPS,Active Bat location system,Cricket,Bluesoft etc.)
3. Attenuation. (e.g. Spot on Ad hoc location system).

Angulations (e.g. ominidirectional Ranging)
Scene Analysis (e.g. RADAR location system)
Proximity



Monitoring wireless cellular access points. (e.g. Active Badge system)
Observing automatic ID systems. (e.g. E-Toll system, EPC etc)
Detecting physical contact. (e.g. Capacitive field detection, Smart
Floor method)
Triangulation
y
(x3, y3)
c
a
(x,y)
b
(x1,y1)
(x2,y2)
(0,0)
(x-x1)2 + (y-y1)2 = a2
(x-x2)2 + (y-y2)2 = b2
(x-x3)2 + (y-y3)2 = c2
x
Angulations
Distance Measurement Technologies Comparisons
Ultrasonic time-of-flight
 Common frequencies 25 – 40KHz, range few meters (or tens of meters),
avg. case accuracy ~ 2-5 cm, lobe-shaped beam angle in most of the cases
 Wide-band ultrasonic transducers also available, mostly in prototype
phases
Acoustic ToF
 Range – tens of meters, accuracy =10cm
RF Time-of-flight
 Ubinet UWB claims = ~ 6 inches
Acoustic angle of arrival
 Average accuracy = ~ 5 degrees (e.g. acoustic beam former, MIT Cricket)
Received Signal Strength Indicator
 Motes: Accuracy 2-3 m, Range = ~ 10m
 802.11: Accuracy = ~30m
Laser Time-of-Flight Range Measurement
 Range =~ 200, accuracy =~ 2cm very directional
RFID and Infrared Sensors – many different technologies
 Mostly used as a proximity metric
Possible Implementations/ Computation
Models
Computing Nodes
1. Centralized
2. Locally Centralized
3. (Fully) Distributed
Only one node computes
Some of unknown nodes compute
Every unknown node computes
Each approach may be appropriate for a different application.
Centralized approaches require routing and leader election.
Fully distributed approach does not have this requirement.
GPS(Global Positioning System)
For outdoor use, we have the Global Positioning System
(GPS).
GPS basics:
GPS determines the distance by measuring the time it takes
a signal to propagate from satellite to receiver
Need to have very good synchronization of clocks
Satellite clock is atomic
Need to know satellite location
Receive signal from three satellites to determine location
Need a fourth satellite to estimate elevation and for
accuracy
Satellite GPS accuracy is getting reasonable (10-20 meters)
BTW, there is intended noise
 Why? Don’t want weapons
GPS Requirement
GPS Constellation
 24 satellites (Space Vehicles or SVs)
 20,200km altitude (12 hour orbit period)
 6 orbital planes (55° inclination)
 4 satellites in each plane
GPS Satellite Details
 Manufactured by Rockwell International,
later by Lockheed M&S
 ~1900 lbs (in orbit)
 2.2m body, 7m with solar panels
 7-10 year expected lifetime
GPS problems
GPS doesn’t work indoors because the satellite
signal is weak or reflected which means lowers
accuracy.
Indoor location systems is an active research area.
Ideal location sensor in indoor environments have
the following properties:
 Provide fine-grain spatial information at a high
update rate.
 Unobtrusive, cheap, scalable and robust
Cricket: System Architecture
Deploy actively transmitting beacons on walls and/or
ceilings, and attach listeners to host devices (handhelds,
laptops, etc.)
A beacon is a small device attached to some location
within the geographic space it advertises.
Configure beacons with space Identifiers, and optionally
with position coordinates
How Cricket work?
Each beacon periodically broadcasts its space identifier and position
coordinates on a radio frequency (RF) channel.
Each beacon also broadcasts an ultrasonic pulse at
The same time as the RF message Listeners that have line-of-sight
connectivity to the beacon and are within the ultrasonic range will receive this
pulse.
RF travels about 10 6 times faster than ultrasound, the listener calculate Time
difference of arrival between the start of the RF message from a beacon and
the corresponding ultrasonic pulse.
RADAR
RADAR attempts to use common off-the-shelf
components. For example, 802.11b base stations.
Basically, RADAR makes use of WLAN technology.
RADAR assumes that the access points (AP)s provide
overlapping coverage over area of interest.
The user carries a mobile device which helps in
determining location e.g. laptop, palmtop, badge.
Practical signal strength model.
Radio Propagation Model
RADAR Approach
It is a RF based Indoor location tracking system.
It provides the information based on various base station
range overlapping areas and their signal measurement.
It combines empirical measurements with signal
propagation modeling to determine user location.
RADAR uses signal strength information gathered at
multiple receiver locations to triangulate the user’s
coordinates.
Triangulation is done using both empirically-determined
and theoretically computed signal strength information.
Active Badge
First indoor badge system
Based on infrared technology
Each locatable wears a badge
Emits a unique ID periodically
Server collects data from fixed sensors (base stations)
System provides symbolic absolute location information
Sunlight and fluorescent light interfere with infrared
Infrared limits cell sizes to small- or medium-sized room.
Users wear infrared badges Badge emits GUID every 10 seconds
Active Bat system
Based on ultrasound
Locatable carry Active Bat tags
Request/Response protocol





Controller sends request via short-range radio
Bat replies with ultrasonic pulse
Controller resets ceiling sensors via wired network
Ceiling sensor measures distance using time from reset to
ultrasonic pulse arrival
Estimated distance
Active Bat System
Radio transceiver, controlling logic and an ultrasonic transducer.
Periodically transmits a radio message containing a single identifier
(corresponds to a Bat unit).
Placed at known points on the ceiling of the rooms to be instrumented.
Receivers are connected by a wired daisy-chain network.
Receivers monitor the incoming ultrasound and record the time of arrival
for any bat signal.
Acoustic Target Tracking Operation
Acoustic Target Tracking
Motion Star
Virtual reality and motion capture
Fixed antenna generates
Axial DC magnetic-field pulses
Receiving antennas measure
Field pulse in three orthogonal
Axes (combined with earth magnetic field)
• Pro: Accurate resolution of 1mm, 1ms, and 0.1°
• Cons: implementation costs, object tethered to control unit, sensors
must remain within 1-3m of transmitter, sensitive to metallic objects
Motion Star Wireless
(Magnetic pulse transmitting antennas
receiving antennas and Controller)
Smart Floor method
Embedded Pressure Sensors


Capture Footfalls.
Data used for position
Tracking and pedestrian recognition
Unobtrusive system
Does not require people to carry any device or tag
Poor scalability and high incremental cost
Many users in one room?
Spot-On
Implement ad hoc Lateration with low-cost tags
Ad-hoc location sensing is a fusion of concepts from object location
tracking and the theories of ad-hoc networking
Spot-On tags use radio signal strength information (RSSI) as a distance
estimator to perform ad-hoc Lateration.
E911
FCC is requiring wireless phone providers to locate any phone that
makes an E911 call
Different approaches
 proximity
 angulations with phased antenna arrays
 GPS-enabled handsets
Leads to numerous new consumer services
Easy Living




Keeps track of devices etc. in a room
Uses real time 3D cameras for vision positioning.
Monitoring from the Internet to control lights, audio video,
watch television.
lots of processing power used to analyze frames captured,
difficult to maintain accuracy, since vision struggles with
analysis accuracy
Comparison
Modern applications
Physical security
 Detecting intruders
Medical
 Patients in a hospital
Habitat monitoring
 Wildlife, plants
Environmental
 Tracking forest fires, pollution
Smart buildings
Air traffic control
Surveillance
Required in most
applications:
Location of the sensor
Target tracking problem
Problem statement
 A varying number of targets
 Arise at random in space and time
 Move with continuous motions
 Persist for a random time and possibly disappear
 Positions of targets are sampled at random intervals
 Measurements are noisy and
 Detection probability < 1.0
 False alarms
Goal: detect, alert, and track for each target
Tracking Challenges
Data dissemination and storage
Localization.
Resource allocation and control
Operating under uncertainty
Real-time constraints
Data fusion (measurement interpretation)
Multiple target disambiguation
Track modeling, continuity and prediction
Target identification and classification
Conclusion
Which one of these approaches is better?
Difficult to compare error rate.
RF is not robust, ultrasound systems are better but only if ceiling
mounted.
Lots of start-up cost with Active Bats; same with Cricket.
RADAR is relatively inexpensive in terms of hardware but
extremely time-consuming to do calibration.
RADAR needs network cards.
References
P. Bahl, V. Padmanabhan, "RADAR: An In-Building RF-based User
Location and Tracking System" IEEE INFOCOM 2000, vol. 2, pp.
775-784.
Nissanka B. Priyantha, Anit Chakraborty and Hari Balakrishnan, " The
Cricket Location-Support System " Proc. 6th ACM MOBICOM, A
ugust 2000, pp. 32-43.
Andy Hopper, Pete Steggles, Andy Ward, Paul Webster, " The
Anatomy of a Context-Aware Applica tion " Proceedings of the 5th
Annual ACM/IEEE International Conference on Mobile Computing
and Networking (Mobicom '99), Seattle, Washington, USA, August
1999.
Special Notes: Special thanks goes to MIT for a presentation that has
great pictures.
Location Sensing Techniques Jerey Hightower and Gaetano Borriello
UW-CSE-01-07-01 University of Washington, Computer Science and
Engineering