Technology Behind Location Awareness

CST 594- Mobile Computing

Team membersAgastheswar Suribhatla Eshwari Mente

Overview• Global Positioning System• Location Estimation with 802.11• Cellular Based System• Improving Location Accuracy• Location Based Applications and

Services• Challenges and Opportunities

Global Positioning System

• GPS is satellite navigation system which provides location and time information

• NAVSTAR GPS is most widely used GPS in Location technology

• GPS products and services expected to reach 140 billion euros by 2015

• GPS can scale to unlimited number of users preserving user privacy

• Accurate GPS Location requires at least four satellites• GPS cannot penetrate through walls, solid and water

GPS Origins• GPS is synthesis of three location bases technologies

o Transit• Precision of few hundred meters• Used Doppler Shift of radio signals by polar orbiting satellites to get location

o Timation• Precise time synchronization between points on earth• Used orbital clocks which enables GPS tight synchronization

• US Air Force Project 621B o Technique used determining the distance from satellite using Pseudorandom

noise (PRN)o PRN can be decoded when multiple satellites transmit at same frequencyo PRN codes could be detected even at low power density

GPS ArchitectureGPS Architecture has three distinct parts• Constellation of earth Orbital satellites

o Constellation consists of 31 satellites organized into 6 non geometric circular orbits

o Full GPS coverage requires 24 additional satellites act as spare

• Ground stations and clockso Responsible for monitoring satellite positions , clock corrections,

satellite orbit updates

• GPS receiverso Determine position by tracking at least 4, but commonly up to 12

satellites

GPS Algorithm• Ri (distance between receiver

and satellite)= -b

• , , location of satellite i • b – clock bias• GPS accuracy is function of

o Error in ranging estimateso Error in satellite geometry

Differential GPS• Differential GPS is enhanced GPS that provides

improved location accuracy from 15 m to 10 cm • Differential GPS take advantage of

o Satellite clock and coordinate errorso Ionosphere and tropospheric delayso Exhibits temporal and spatial co-relation

• DGPS coordinates multiple GPS receivers that track same satellite

• DGPS broadcast the difference between the positions indicated by the satellite systems and the known fixed positions

Location based on 802.11

• Estimating Location based on 802.11 was introduced in 2000 by Bal et al

• Working of 802.11o 802.11 radios have drivers that allow devices to scan nearby devices

(access points) which return unique IDs and MAC addresseso AS access points have limited range it can know vicinity of access pointso Access points send beacon frames to alert clients their presence

• Client devices can detect access points in two wayso Passively listening on 802.11 channels for beacon frameso Initiate scan by sending requests which access points reply

Location based on 802.11

• 802.11 takes advantages of two properties observed by clientso Spatial variability

• Due to short range and way signals are obstructed and blocked, strength with which access point is observed varies spatially

o Temporal Consistency• If given place is good to establish a connection then it would be a good

place even in next few minutes, days and months

• Based on two properties a map of radio environment by collecting location tagged radio fingerprints which is used perform localization of mobile devices

Signal Strength Modeling and Privacy considerations• The Principle that all systems take advantage is

o Radio signals propagate in predictable wayso Device location can be determined by modeling the

propagation of signal• Privacy of 802.11 location systems can be excellent

as it is implemented witho Passive radio receptiono Client side computation

Cellular Based Location

• Development of location systems based on mobile phase was driven by US Federal Communication commission

• The main purpose was to locate mobile phones to assist phones to deliver emergency services

• Later they it was also used foro Location Based serviceso Advertisemento Recommendation systemso Gaming

Mobile Phone Location based Systems

Location based systems can be grouped to four categories • Cell Id based approach• Radio propagation Modeling• Assisted GPS• Surveying techniques based on finger printing

Mobile Phone Location based Systems

• Cell Id based approacho Mobile Phone base station has number of directional antennas that

define sector of coverage or cell , each of which is assigned cell IDo Location is determined based on the cell ID of the service providing

stationo It is Implemented at Network side so it works for all mobile deviceso Accuracy ranges from 150m to 30 km

• Radio Propagation Modelingo Radio Modeling systems for GSM and CDMA are based on Time of

flight measurement and uses Time difference of Arrival triangulation method

Mobile Phone Location based Systems

• Radio Propagation Modelingo In TDOA position is determined by intersecting hyperbolic lines

derived taking difference between time measurements of base stations

Mobile Phone Location based Systems

• Assisted GPSo Hybrid technique which combines GPS and Mobile Phone networko Due to the disadvantages of using GPS like at least four satellites, clear

view of sky it doesn’t work in indoorso AGPS supplements GPS receiver on phone with secondary reference of

GPS receiver with clear view of sky at known locationo As it has supplementary data it can work with as few as one satellite

• Signal Strength finger printingo It involves mapping in which radio map is constructed by collecting

fingerprints at multiple locationso Once mapping is complete the client can lookup for the closet match

to current measurement to measurement in mapping phase

Improving localization accuracy

o Location estimates of a systems are noisy and error prone.o Smoothing and averaging lessen the impact of these errors

on the accuracy. Various ways accuracy improvement techniques:

1. Smoothing - Damp out errors and improves accuracy• Using product of (GPS, WiFi, GSM)• Spatial smoothing technique. (K-mean clustering

algorithm ) • Temporal smoothing technique.• Hidden Markov model

Snapping:

Another error reducing technique is to constrain coordinatebased location estimate using natural boundaries in physicalenvironment.

eg GPS based in-car navigation systems.

o This technique has been applied to indoor location systems.

o Employs infrared and ultrasonic beacons.o Limitation: Does not accommodate unconstrained

movement in open spaces.

Map Showing Indoor Location Estimate

Fig 1: Uses Infrared(small squares) and Ultrasonic (circles) beacons to locate the user.Fig 2: Schematics of multistory building

Fg: 1 Fg: 2

3. Fusing And Tracking: Uses two or more different types of sensor observation to

determine location of the device. Bayes filter perform state estimate on a dynamic system when

there is uncertainty due to noise in the evidence being collected.

Bayes filters are used to estimate the chance of a device being in a given location having seen the sequence of sensor reading (E1, E2,E3..)

Graphical Representation

Accuracy of a location system using particle filter fusion of infrared beacon and ultrasonic ranging observation.

Cont..• Probabilistic framework like Kalman filters and

particle filters offer the ability to fuse readings across sensor technologies and in-corporate motion models which improve accuracy significantly.

• When carefully implemented, these probabilistic techniques are suitable for mobile phones and PDA platforms.

Location based application and services

1. Navigation And Way- Finding: Most of the systems use GSP for location estimation.Newer devices have the ability to continually download real time traffic data to avoid congestion.

2. Asset tracking

• Printed tags and RFID tags

Real- time fleet management maps.

3. Emergency response

One of the highest value application of location technology is to aid in assisting individual who may be lost, hurt or damage.

Assert tags capable of providing real time location information.

4. Geo fencing• Allows to user to define important locations like

home, school and receive SMS alerts when the device reaches one of the place.

• Newer models use both GPS and cell tower based location.

• Location technologies are proposed as a way to help secure 802.11 networks.

5. Location based content and search

o Most devices and applications enable users to

both retrieve content based on location and produce content tagged with location.

6. Social Networking • Helps people to interact, understand social

patterns, and meet new people.

9. Health And Wellness:

• Popular Google Map Mashups ,GMap Pedometer let people visualize and share their walking route based on GPS trace.

• Not accurate, not based on the location, it used the change in the signal strength of GSM cell towers to predict user’s walking.

• User can connect to PC and download the data, maps, graphs and reports.

Report from mationbased.com

Challenges 1. Mobile devices deployed are not equipped with

a location system. Fortunately, there are free commercial libraries

that perform wide area location estimation without addition hardware.

2. Developers facing difficulties in designing location-enhancing interactions and testing them in various of condition.Topiary allows rich location based interaction to be designed, tested and quickly iterated.

3. Most importantly Privacy.

Accuracy and technologies

Accuracy and technologies

Thank you