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.

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 self- localize). 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

Tracking Approaches

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

(x3, y3)

(x1,y1)

(x2,y2)

y

x

a b

c

(x-x1)2 + (y-y1)2 = a2 (x-x2)2 + (y-y2)2 = b2

(0,0)

(x,y)

(x-x3)2 + (y-y3)2 = c2

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 IndicatorMotes: Accuracy 2-3 m, Range = ~ 10m802.11: Accuracy = ~30m

Laser Time-of-Flight Range Measurement Range =~ 200, accuracy =~ 2cm very directional

RFID and Infrared Sensors – many different technologiesMostly used as a proximity metric

Possible Implementations/ Computation Models

2. Locally Centralized Some of unknown nodes compute

1. CentralizedOnly one node computes

3. (Fully) DistributedEvery unknown node computes

Computing Nodes

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 receiverNeed to have very good synchronization of clocksSatellite clock is atomicNeed to know satellite locationReceive signal from three satellites to determine locationNeed a fourth satellite to estimate elevation and for accuracySatellite 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 atThe 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 ultrasoundLocatable carry Active Bat tagsRequest/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 captureFixed antenna generatesAxial DC magnetic-field pulsesReceiving antennas measureField pulse in three orthogonalAxes (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 buildingsAir traffic controlSurveillance

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

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.

Conclusion

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