Technology Behind
Location Awareness
CST 594- Mobile Computing
Team members
Agastheswar Suribhatla Eshwari Mente
Global Positioning System
Location Estimation with 802.11
Cellular Based System
Improving Location Accuracy
Location Based Applications and
• 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
• 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
o Transit
• Precision of few hundred meters
• Used Doppler Shift of radio signals by polar orbiting satellites to get
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 frequency
o PRN codes could be detected even at low power density
GPS Architecture
GPS 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 clocks
o Responsible for monitoring satellite positions , clock corrections,
satellite orbit updates
• GPS receivers
o Determine position by tracking at least 4, but commonly up to 12
GPS Algorithm
• Ri (distance between receiver
and satellite)=
(xi − x2) + ( yi − y)2 + (zi − z)2 -b
•  ,  ,  location of satellite i
• b – clock bias
• GPS accuracy is function of
o Error in ranging estimates
o 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 errors
o Ionosphere and tropospheric delays
o 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 802.11
o 802.11 radios have drivers that allow devices to scan nearby devices
(access points) which return unique IDs and MAC addresses
o AS access points have limited range it can know vicinity of access points
o Access points send beacon frames to alert clients their presence
• Client devices can detect access points in two
o Passively listening on 802.11 channels for beacon frames
o Initiate scan by sending requests which access points reply
Location based on 802.11
• 802.11 takes advantages of two properties observed
by clients
o 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 ways
o Device location can be determined by modeling the
propagation of signal
• Privacy of 802.11 location systems can be excellent
as it is implemented with
o Passive radio reception
o Client side computation
Cellular Based Location
• Development of location systems based on mobile
phase was driven by US Federal Communication
• The main purpose was to locate mobile phones to
assist phones to deliver emergency services
• Later they it was also used for
Location Based services
Recommendation systems
Mobile Phone Location based
Location based systems can be grouped to four
• Cell Id based approach
• Radio propagation Modeling
• Assisted GPS
• Surveying techniques based on finger printing
Mobile Phone Location based
• Cell Id based approach
o Mobile Phone base station has number of directional antennas that
define sector of coverage or cell , each of which is assigned cell ID
o Location is determined based on the cell ID of the service providing
o It is Implemented at Network side so it works for all mobile devices
o Accuracy ranges from 150m to 30 km
• Radio Propagation Modeling
o Radio Modeling systems for GSM and CDMA are based on Time of
flight measurement and uses Time difference of Arrival triangulation
Mobile Phone Location based
• Radio Propagation Modeling
o In TDOA position is determined by intersecting hyperbolic lines
derived taking difference between time measurements of base
Mobile Phone Location based
• Assisted GPS
o Hybrid technique which combines GPS and Mobile Phone network
o Due to the disadvantages of using GPS like at least four satellites, clear
view of sky it doesn’t work in indoors
o AGPS supplements GPS receiver on phone with secondary reference of
GPS receiver with clear view of sky at known location
o As it has supplementary data it can work with as few as one satellite
• Signal Strength finger printing
o It involves mapping in which radio map is constructed by collecting
fingerprints at multiple locations
o 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
Another error reducing technique is to constrain coordinate
based location estimate using natural boundaries in physical
eg GPS based in-car navigation systems.
o This technique has been applied to indoor location
o Employs infrared and ultrasonic beacons.
o Limitation: Does not accommodate unconstrained
movement in open spaces.
Map Showing Indoor Location Estimate
Fg: 1
Fg: 2
Fig 1: Uses Infrared(small squares) and Ultrasonic (circles) beacons to
locate the user.
Fig 2: Schematics of multistory building
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
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.
• 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
Location based application and
1. Navigation And Way- Finding:
Most of the systems use GSP for location
Newer devices have the ability to continually
download real time traffic data to avoid
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
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 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
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 locationenhancing 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
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