Location Sensing and Positioning

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Location Sensing and Positioning

• Location Sensing
• GPS Positioning
• GPS and Cellular Network Methods
• Wireless In-door Positioning

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Location-Sensing

• Location-sensing system is designed to obtain
  data about the physical location of an object
• Indoor Vs. outdoor only
• Coordinates (x, y) Vs. location ID only
• 2D (x, y) Vs. 3D (x, y, z)
• What can be done if we have the location
  information of moving objects?
• How to management the location information?
  Update processing, indexing and location area
  planning
• How to maintain the validity of the location data
  for moving objects? Temporal consistency and
  update scheduling
• Privacy issue who decide when to perform
  positioning and the access to location
  information
• The location information of buildings and
  properties can be used for building GIS models

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Some location-sensing technologies
Type
Mechanism
Limitations
Accuracy
Type of location data
Privacy
GPS
Multilateration
Outdoors
110m
Absolute geographic
Yes
from satellite
only (satellite
coordinates (latitude,
radio sources
visibility)
longitude, altitude)
Radio
Broadcasts from
Areas with
10m1km
Proximity to known
Yes
beaconing
wireless base
wireless
entity (usually semantic)
stations (GSM,
coverage
802.11, Bluetooth)
Active Bat
Multilateration
Ceiling
10cm
Relative (room)
Bat identity
from radio and
coordinates.
disclosed
mounted
ultrasound
sensors
Ultra Wide
Multilateration
Receiver in
15cm
Relative (room)
Tag identity
Band
from reception of
stallations
coordinates
disclosed
radio pulses
Active
Infrared sensing
Sunlight or
Room size
Proximity to known
Badge
badge
fluorescent
entity (usually semantic)
identity
light
disclosed
Automatic
RFID, Near Field
Reader
1cm10m
Proximity to known
Tag identity
identification
Communication,
installations
entity (usually semantic)
disclosed
tag
visual tag (e.g.
barcode)
Easy Living
Vision,
Camera
Variable
Relative (room)
No
triangulation
installations
coordinates
Fr. Schiller
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What are Triangulation Trilateration?
L
L
a
b
a
b
P1
P2
P1
P2

• Given
• Two fixed points
• The angle from the two points

• Given
• Two fixed points
• The distance from the two points

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Global Positioning System (GPS)

• Funded by DoD of US government initially designed
  for providing positioning services for military
  purposes all around the world
• Civilian and commercial applications include
  fleet management, flight and navy navigation,
  finding stolen vehicles and people navigation,
  and positioning for hiking and adventure
• It is mainly for outdoor positioning since the
  radio signals receipted from the satellites will
  be very weak indoor
• The GPS system consists of three main components
• Space segments
• Control segments
• User segments
• It uses multilateration for determining the
  object position (3D coordinates)

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Global Positioning System (GPS)

• Space segment
• Consists of the GPS satellites. These space
  vehicles (SVs) move in fixed orbits and send
  radio signals from space
• The GPS constellation consists of 24 satellites
  that orbit the earth in 12 hours
• There are six orbital planes (with nominally four
  SVs in each), equally spaced (60 degrees apart)
• This constellation provides the users with
  between five and eight SVs visible from any point
  on the earth

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Global Positioning System (GPS)

• Control Segment (fixed stations on earth)
• The control segment consists of a system of
  tracking stations located around the world
• The monitor stations measure signals from the SVs
  which are incorporated into the orbital models
  for each satellites
• The models compute precise orbital data (time Vs.
  location) and SV clock corrections for each
  satellite
• The master control station uploads the data to
  the SVs
• The SVs then send subsets of the orbital data to
  GPS receivers over radio signals

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Global Positioning System (GPS)

• User segment
• The GPS User Segment consists of the GPS
  receivers and the user community
• GPS receivers convert coded SV signals into
  position, velocity, and time estimates. Four
  satellites are required to compute the four
  dimensions of X, Y, Z (position) and Time
• The data from different SVs are encoded with
  different pseudo random noise to prevent
  interferences from different SVs
• Precise positioning is possible using GPS
  receivers at reference locations providing
  corrections and relative positioning data for
  remote receivers (differential GPS)

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Triangulation/Trilateration Technique

• Consider the GPS receiver is placed on one point
  on an imaginary sphere of radius equal to the
  distance between satellite A and the receiver on
  the ground
• The same receiver is also a point on the another
  imaginary sphere with another satellite B at its
  centre
• The GPS receiver is somewhere on the circle
  formed by the interaction line of these two
  spheres
• With the measurement from the third satellite C,
  the position of the receiver is reduced to just 2
  points on the circles, and one of which is
  imaginary and can be eliminated
• Thus, the distance measured from the three
  satellites can determine the position of the GPS
  receiver on the earth surface
• The distance is calculated from the speed of the
  radio signals and the time taken for the signal
  to the earth surface
• The calculation is more accurate if the data are
  from more satellites

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Other Positioning Techniques

• Positioning using mobile stations and mobile
  networks
• Can be for outdoor and indoor
• Using mobile network mobile station
• Cell of origin
• Angle of arrival (AOA)
• Time Difference of Arrival (TDOA)
• Enhanced-observed time difference (E-OTD)
• Assisted GPS (A-GPS)

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Network-Based Methods

• Cells of Origin (COO)
• The most primitive and cheapest method
• In a cellular network, each mobile terminal is
  associated with a base station
• Each mobile phone is associated with a cell ID
  assigned by the system
• The ID of a mobile terminal is recorded in the
  location database (LD) in location update
• Searching the LD of an mobile terminal to get the
  last updated location of the mobile terminal
• The accuracy depends on the size of a cell (i.e.,
  from 10 to 0.1 km)

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Angle of Arrival
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Network-Based Methods

• Time Difference of Arrival (TDOA)
• TDOA also is called uplink time of arrival (UTOA)
• Mobile terminals send signals to base stations
  (or called location management unit (LMU))
• The transmission time to each base station is
  sent to mobile location centre
• Based on the transmission time, base stations
  calculate the distance of the mobile terminal
  from the base stations
• A set of arcs are created to determine the
  location of the mobile terminals
• The mobile location centre uses triangulation to
  calculate the location of the mobile terminal
• This requires synchronization of the clocks at
  different base stations
• www.trueposition.com

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Time Difference of Arrival (TDOA)
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Handset-centric Methods

• Enhanced-observed time difference (E-OTD)
• Similar to TDOA but is handset positioning rather
  network-based (upgrade of handset)
• Handset takes signal data from surrounding base
  stations to measure the difference in time it
  takes to reach the handset
• Calculation is then performed at the handset to
  obtain the location
• Need to know the locations of the base stations
• The handset needs additional computation power
  and memory for calculation

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Enhanced GPS

• Differential GPS
• D-GPS uses relative position to correct position
  estimates and can have an accuracy up to 1m
• To correct bias errors at one location, a
  reference receiver, or base station, computes
  corrections for each satellite signal
• What are the assumptions of this method?
• Assisted GPS (A-GPS)
• A technology combining cellular network
  positioning and GPS
• A wide area differential GPS network is set up
  with receivers that operate continuously
• The network is connected to a GSM network
• When a mobile device request a position,
  assistance data from the reference network is
  transmitted to the location server enhance the
  performance to accelerate the positioning process
• http//www.snaptrack.com/

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Wireless Indoor Positioning

• Infrared beacons
• Radio beacons
• Ultrasound systems
• Wireless LAN

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Infrared Beacons

• Active badge by Ollivetti
• Each user carries a small infrared transmitter,
  the active badge
• The badge sends infrared signal of approximate
  0.1s every 15 sec containing the unique ID of the
  badge
• Infrared sensors are installed in the building to
  detect the ID signals
• The sensors are connected to the location
  management database
• Simple, low cost and consume small amount of
  energy
• Give the location area (i.e., within a room) of
  an object

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Locating an active bat within a room
Scope problem
1. Base station sends timing signal
to ultrasound receivers and radio
signal to bat simultaneously
3. Ultrasound receivers
4. Base station computes distances
report times of flight of
to ultrasound receivers from
times of flight, and thus position
ultrasound pulse
2. Active bat
of bat
emits ultrasound signal
on receipt of radio signal
Fr. Dollimore
Communication range Vs. the precision in
location Scope problem may be resolved by
calculating the signals received from multiple
receivers using triangulation technique
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Radio Beacons

• Active badge cannot tell the exact location of a
  moving object
• Radio signals can penetrate the wall and can be
  used for positioning to find out the current
  exact location
• Multiple signal streams can even provide 3D
  location based on the strength of signal streams
  received by multiple sensors
• The calculation can use the time-of-arrival
  method
• Problem timing issue and signal strength is
  affected by the environment (time
  synchronization, multiple sensors and training)
• SpotOn project (University of Washington) can
  achieve an accuracy up to 3m

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Ultrasound System

• The speed of ultrasound is slow than electronic
  radio
• Ultrasound positioning system active bat can
  achieve an accuracy up to 10cm
• The calculation is still based on time-of-arrival
  to multiple sensors
• The bat sends ultrasound upon the requests from
  the positioning server

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References

• DS Ch16 (except 16.3, 16.5)
• Ch 5 Andrew Jagoe, Mobile Location Services,
  Prentice Hall, 2003
• Jochen Schiller and Agnes Voisaro, Location-Based
  Services, Morgan Kaufmann, ch. 7
• http//www.colorado.edu/geography/gcraft/notes/gps
  /gps_f.html