10 location-based services

7/31/2019 10 Location-Based Services

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Department of Computer Science Institute for System Architecture, Chair for Computer Networks

Location-based Services

Mobile Communication and Mobile Computing

. .

http://www.rn.inf.tu-dresden.de


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Positionin and Trackin S stems

both outdoor and indoor s stems can be classified into

stand-alone infrastructure

dedicated infrastructure for positioning/tracking

e.g. satellite systems, infrared-based systems

integrated infrastructure ex s ng commun ca on n ras ruc ure a so use or

positioning/tracking

e. . cellular s stems WLAN-based s stems

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Positionin vs. Trackin

positioning (client-based

positioning)

device actively determines/calculates

+ location information under control oflocated device

device required

tracking (infrastructure-basedpositioning)

position determined by infrastructure,i.e. device is passively located

location information not under controlof located object

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Global Positionin S stem GPS

(1) Satellites 24 satellites on 6 orbits,

i.e. 4 satellites per orbit

additional satellites for

robustness height = 20.200 km

(medium earth orbit)

min. 5, max. 11 satellitesvisible from each point on

earth

hours, i.e. satellites are at thesame position each 24 hours

(3) User Devices GPS receiver

min. 12 channels, i.e.12 satellite signals can bereceived in parallel

[Roth, Ortsbezogene Anwendungen und Dienste]

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Satellite S stems

+ advantages

few environmental influences (e.g. weather)

high accuracy (in the range of meters)

disadvantages

high costs, since for global availability many satellites are required

enough signals have to be available for positioning

at least 4 signals (3 for positioning, 1 for time synchronization)

example: GPS

originally named NAVSTAR (Navigation System with Timing andRanging)

toda known as GPS Global Positionin S stem

consists of

(1) satellites

(3) user devices

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Cellular S stems - GSM

positioning based on GSMcell ID

available from Home

cell size 1 35 km

Mobile Positioning System

(MPS) by Ericsson to

few changes ofinfrastructure;

no c anges o user ev ces

Cell Global Identity (CGI)for identification of the cell

uses GSM cell ID

if sector-antennas used[Roth, Ortsbezogene Anwendungen und Dienste]

segment of circle6


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GSM based on FDMA and TDMA

certain time slot used forcommunication

exact timing required for signalsynchronization

time shift depends on

base station

device calculates distance

based on signal propagationtime

n orma on s use orpositioning in combination

with CGI Roth, Ortsbezo ene Anwendun en und Dienste7


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U link Time Of Arrival UL-TOA

can be used, if device is inrange of four base stations

measurement of signalpropagation time at

calculation of position

accuracy 50 150m

osition information

stored in MobilePositioning Center (MPC)

rac ng [Roth, Ortsbezogene Anwendungen und Dienste]

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Cellular S stems

inte rated a roach based on existin infrastructure

advantages

+ low costs

high availability

works indoor and outdoor

sa van ages

lower accuracy

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Indoor Positionin Trackin

satellite or cell s stems often not available in buildin s ornot accurate enough

therefore other approaches required:

(1) stand-alone infrastructure approaches infrared

radio

(2) integrated infrastructure approach WLAN

,

installation effort yields high costs

usuall users have to carr additional e ui ment bad e

most of the systems are scientific prototypes

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Active Bad e

stand-alone infrastructure based trackin s stem

based on infrared (IR)

users carry badge sending specific user ID

IR-receivers in the rooms receive those signals

position of user is tracked by central server

energy-efficient badges (sending short signals of

0,1 s each 15 s)

problems:

users position is positionof IR-beacon which

location

system

received the signal

user does not get positionactive badge

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infrared receiver infrared


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Wireless Indoor Positionin S stem WIPS

stand-alone infrastructure based ositionin s stem

based on infrared (IR)

beacons installed in the rooms sending unique ID

users badges receiving signals of local beacons

received beacon ID is sent to location server via WLAN

server maps received beacon ID to semantic location

which is sent back to the user

+ advantage:

users nows s own pos onlocation

system

integration of two wireless

techniques

IR receiver

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infrared beacons infrared WLAN


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Further S stems

Active Bat stand-alone infrastructure based tracking system

based on ultrasonic

which is sent to the location server for processing

accuracy up to 10 cm

SpotOn-

based on radio signals

radio signals are able to pervade walls compared to infrareds gna s

strength of radio signal depends not only on distance (thinkof walls, ) accuracy of 3 m for WIPS

compensation of errors if more beacons available

variant: using RFID-tags (user carries tag, reader in rooms)13


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WLAN-based Positionin

based on alread installed WLAN infrastructure(that was installed for communication purposes primarily) integrated infrastructure based positioning

measurement of signal strength of various access points

positioning by means of lateration

at least three access points have to be available

measurements are influenced by obstacles like walls,

idea: tagging the environment with radio tags

+ improved accuracy

startup phase required for reference measurements

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Radio Ma

measurement of si nal stren th at reference oints

positioning based on current measurement

accuracy depends on amount of reference points

environmental changes require new referencemeasurements

15cross points of measurement circle access points


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Ma ic Ma

developed by Humboldt University Berlin

based on WLAN, but could also use ZigBee, Bluetooth orRFID

hybrid approach based on measured signal strength andradio ma s

calculation algorithm called probabilistic graph mapping

accuracy: access points only 10m

more reference points 1m

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Ma ic Ma

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Com arison of S stems

Name Category Positioning/Tracking Mechanism Medium Accuracy

GPS Satellite Positioning TOA Radio 25 m

DGPS Satellite Positioning TOA Radio 3 m

ActiveBadge

Indoor Tracking COO IR Cell

n oor os on ng e

SpotOn Indoor Tracking Signal

strength

Radio 3 m

ActiveBat

Indoor Tracking TOA Ultrasonic 0.1 m

GSM Network Both Radio Cell, ,TOA

MPS Network Both COO, AOA,TOA

Radio 150 m

MagicMap

Network Positioning Signalstrength

Radio 3 m

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Wh Location Models?

think of Alice and Bob:

Where is Alice?

What is the distance between Alice and Bob?

n w c room s o How many bus stops are between Alice and the faculty of computer

science?

for answering those questions we need a model of ourenvironment location models

geometric coordinates geometric models (51.0298N 13.72896E)

symbolic coordinates symbolic models (HSZ TU-DD, Bergstr.64)

location models should model the following:

object position

topological relation connected to and contains

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Geometric Location Models

define places in form of coordinate tuples relative to referencecoordinate system e.g. World Geodetic System 1984 (WGS-84)

mapping between coordinates of different reference

North Pole coordinates usually do not have

a meanin for humans

calculation of distances,

overlappings and inclusions of

Equator

90 West 90 EastOrigin

geometric operations possible

topological relationPmcontains implicit available

topological relation connected

World geocentric coordinate system

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S mbolic Location Models

define laces in form of abstract s mbols

e.g. Cell ID in GSM, sensor identifiers in active badge

system, room and street names

no distance and other topological relations implicitlydefined

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S mbolic Models: Set-based Models

usage of symbolic names

e.g. a building with several floors and rooms can be modelled like this

L ={L , L , }

LFloor2={2.002, 2.003, , 2.067}

contains relation defined by intersection of sets ( {xyz} L = {xyz} ? )

overlappings can be identified ( L1 L2 ? )

nearest nei hbour and navi ation

connected to relation can be defined by modelling pairs of connected locations

also simple (only qualitative) notion of distance possible

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S mbolic Models: Gra h-based Models

symbolic coordinates define vertices V of graph G=(V, E)

connected to relation modelled by edges between vertices

simple distance by calculation of hops

weights for edges to model advanced distance notion

no ex licit contains definition

other: hybride models as a combination of geometric andsymbolic models

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Back to the Exam le uestions

How can the different model t es be used for answerinthe introduced questions?

absolute question relative question

model

coordinates?

Alice and Bob?

symbolic

model

In what room isBob

located at?

How many bus stops arebetween Alice and the facultyof com uter science?

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A lication Areas

rovision location-based information

museum, city guide, shopping guide, tourist guide

-

buddy-finder

service search ost office, restaurant nearb

navigation systems

tracking systems

mediation of incoming calls

a arm sys ems or c ren or e er y peop e

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Mobilis - Solution

Scenario: touristapplication

roup o our s sexploring a cityindividually

see each other on the,

reusable frameworkwith services formobile real-time

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Mobilis: Im lementation technolo ies

Client: Android and Java, further implementations with Java ME and Java SE

Communication: Extensible Messaging and Presence Protocol (XMPP)OpenStandard for real-time collaboration

Numerous extension protocols for presence, pub/sub, location, etc.

Server: OpenFireXMPP Server imp ementation ase on JavaBro erservices implemented as XMPP clients based on Java 27


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Summar

lar e set of ositionin s stems

satellite based, cellular, local/indoor

different types of location models hybrid approach is preferable

suppor s a ypes o quer es

enables mapping between geometric and symbolic coordinates

disadvantage: higher modelling effort, more data

location-based services widely adopted

e.g. navigation systems or tourist guides

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Some further readin s 1

Becker, C. & Duerr, F.: On location models for ubiquitous computing.Personal Ubiquitous Comput., Springer-Verlag, 2005, 9, 20-31

Dedes, George; Dempster, Andrew G.: Indoor GPS Positioning Challenges and Opportunities, 2005

Hightower, J.; Borriello, G. & Want, R.: SpotON: An Indoor 3D LocationSensing Technology Based on RF Signal Strength 2000

P.; Abowd, Gregory D.: TrackSense: Infrastructure Free Precise IndoorPositioning Using Projected Patterns, ETH Zrich, 2007

-, . , .,ISBN 978-0470-09231-6, 2005

MagicMap: http://www2.informatik.hu-berlin.de/rok/MagicMap/

c er, . o sar , . oca on- ase serv ces

Morgan Kaufmann, 2004 Want, R.; Hopper, A.; Falcao, V. & Gibbons, J. The active badge

oca on sys em rans. n . ys ., , , , -

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Some further readin s 2

WIPS Technical Documentation, Royal Institute of Technology,Schweden, www.online.kth.se/csd/projects/0012/

Yeh, Shun-yuan; Chang, Keng-hao; Wu, Chon-in; Okuda, Kenji; Chu,Hao-hua: GETA Sandals: Walk Away with Localization, National Taiwan

University, 2005

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10 location-based services

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