Positioning

1
Positioning

• Positioning Principles and Applications
• GPS Positioning
• GPS and Cellular Network Methods
• Wireless In-door Positioning

2
Positioning Principles and Applications

• Location-sensing system is designed to obtain
  data about the physical location of an object gt
  location-dependent services
• Indoor Vs. outdoor only
• Coordinates (x, y) Vs. location ID only
• 2D (x, y) Vs. 3D (x, y, z) (mostly 2-D currently)
• 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 (LAP)
• LAP How to divide the service area into
  sub-areas for location management
• Spatial indexing to facilitate the search of
  spatial objects, i.e., shortest path searching
• How to maintain the validity of the location data
  of moving objects? Temporal consistency and
  update scheduling
• Privacy issue who decide when to perform
  positioning and the access to location information

3
Positioning Principles and Applications

• Positioning a signaling and timing problem
• Sender and receivers communicate with each other
• Based on the received signal strength, the
  receivers/sender may be able to estimate the
  distance between them
• Two points and a distance form a circle
• One point and an angle form a line
• Multiple of them can form an intersection
• A higher level controller to perform the
  calculation
• However, the precision is affected by the
  accuracy of the clocks of the involved units,
  the errors introduced from the environment due
  the problem in mobile communication (i.e.,
  multi-path propagation)

angle
Point
Point
distance
Point
4
Basic Calculation Techniques
L
L
a
b
a
b
P1
P2
P1
P2

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

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

The calculation seems quite simple. But in
practice, it is not so easy
5
Signaling and Timing Problems

• Signals problem mobile communication problems
  affected by interference, multi-path propagation,
  the environment and movements of the objects
  (both sender and receiver)
• Timing problems the clocks of the senders and
  the receiver may not be the same and may have
  different drift rate (advancement rate)
• Light speed in propagation gt an error of 1µsec
  could lead to a difference of 300m (light speed,
  app. 300,000 km/s)
• The clocks need to be accurate, i.e., starting
  with the same reading and have the same drift
  rate
• How to synchronize the clocks?
• Need to estimate the transmission delay between
  the time server and the local clock of the sender
  
• Usually not possible with the users, why?

6
dr dt x c
If the clocks are perfect, then we can estimate
the r from two senders to determine the position
of the user. However, uncertainty in clock
readings introduces uncertainty in
positioning. The uncertainty in positioning can
be minimized by getting positioning data from
more senders
7
Some Positioning 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
8
Global Positioning System (GPS)

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

9
Global Positioning System (GPS)

• Space segment
• Consists of the GPS satellites. These space
  vehicles (SVs) move on fixed and pre-defined
  orbits and send radio signals from space to be
  received by GPS receivers
• 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
• Multiple SVs are required for positioning
• SVs uses same frequency for sending position data
  which are encoded using a unique code called the
  pseudo random noise (PRN). The coding is known to
  the receiver

10
www.colorado.edu/geography/gcraft/notes/gps/gps.ht
ml
11
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 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

12
www.colorado.edu/geography/gcraft/notes/gps/gps.ht
ml
13
GPS SV
GPS SV
GPS SV
Data from SV
Data to SV
Data from SV
Control Station Computation and load data to SVs
Receiver Computation of the data from multiple SVs
14
Global Positioning System (GPS)

• User segment
• The GPS User segment consists of the GPS
  receivers and the user community
• GPS selects a set of SVs for receiving data. It
  converts 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)
• What is a reference position? The position with
  known coordinates
• Note the receivers do not send any data to SVs
  and the SVs do not know the positions of the
  receivers. It is a one way communication like a
  radio receiver

15
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 will be more accurate if the data
  are from more satellites

16
www.colorado.edu/geography/gcraft/notes/gps/gps.ht
m
17
GPS Services

• Precise Positioning Service (PPS)
• Authorized users with cryptographic equipment and
  keys and specially equipped receivers use the
  Precise Positioning System. U. S. and Allied
  military, certain U. S. Government agencies, and
  selected civil users specifically approved by the
  U. S. Government, can use the PPS
• Accuracy 22 meter Horizontal accuracy and 27.7
  meter vertical accuracy
• Standard Positioning Service (SPS)
• Civil users worldwide use the SPS without charge
  or restrictions. Most receivers are capable of
  receiving and using the SPS signal
• The SPS accuracy is intentionally degraded by the
  DOD
• Accuracy100 meter horizontal accuracy and 156
  meter vertical accuracy
• To improve the accuracy, a reference beacon may
  be used and the system is called differential GPS
  (DGPS)

18
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)
• Considerations
• Accuracy (depends on the calculation methods and
  how to elimination the noises resulting from
  mobile communication)
• Network cost (the amount of data required for
  communication)
• Response time (the number of rounds of
  communication)
• Processing cost at the front-end and back-end
• In-door Vs out-door (penetration and
  communication range of signals)

19
Network-Based Methods

• Cells of Origin (COO)
• The most primitive and cheapest method
• In a cellular network, each mobile station is
  associated with a base station
• Each mobile phone is associated with a cell ID
  assigned by the system
• The ID of a mobile station is recorded in the
  location database (LD) in location update
• Searching the LD of an mobile station to get the
  last updated location of the mobile station
• The accuracy depends on the size of a cell (i.e.,
  from 10 to 0.1 km) and the update frequency of
  the mobile station
• Location management (location update vs.
  uncertainty in location)
• The mobile phone operators maintain the location
  information of mobile phones for call connection
• In order to minimize the number of location
  updates, it may group several interconnected
  cells into a location area. Only when an object
  moves out an LA, an update will be generated

20
Angle of Arrival (AOA)

• Mobile station sends radio signals to be received
  by near-by base stations
• The base stations which are installed with
  directional antenna determine the direction of
  the signal from the mobile station
• The base stations send the calculated direction
  data to a controller
• The controller calculates the position of the
  mobile station based on the direction data from
  multiple base stations
• The calculation cannot be done too often because
  of additional processing cost and additional time
  delay. Normally it is on request
• No need to upgrade the mobile station (in
  HW/SW)gt a network-based method
• How to determine what are the set of base
  stations to measure the direction data from a
  mobile station? It can be determined by the
  controller in the initial step

21
Angle of Arrival (AOA)
22
Time Difference of Arrival (TDOA)

• Similar to AOA but using uplink time instead of
  AOA in calculation
• What is uplink time?
• TDOA also is called uplink time of arrival (UTOA)
• Mobile stations 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 received from the
  base stations, the controller calculates the
  position of the mobile station
• A set of arcs are created to determine the
  location of the mobile station
• The mobile location centre uses triangulation to
  calculate the location of the mobile station
• This requires synchronization of the clocks at
  different base stations and the mobile station.
  How? By the controller (mobile location centre)
• www.trueposition.com

23
Time Difference of Arrival (TDOA)
24
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
• How do the base stations know that they need to
  send positioning data? What will be the
  positioning procedure? Additional time delay and
  communication overhead

25
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
• GPS installed at a fixed position to obtain
  positioning data from a set of selected SVs.
• Calculate the error in positioning obtained from
  the GPS receiver
• User at an unknown position obtains positioning
  data from the same set of SVs
• What are the assumptions of this method?
• Need to select the same set of SVs for positioning

26
Enhanced GPS

• 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

27
Comparison
28
Wireless Indoor Positioning

• Infrared beacons
• Radio beacons
• Ultrasound systems
• Wireless LAN (LAN segment)

29
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
• Limitation of infrared require line of sight
• Cannot be used for finding the exact location.
  Difficult to calculate the position using AOA or
  TDOA. Why?

30
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
Scope problem may be resolved by calculating the
signals received from multiple receivers using
triangulation technique
31
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
• Radio beacons emit radio signals to be received
  by the receiver which can then calculate its
  location if it knows the position of the beacons
• 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

32
Ultrasound System

• The speed of ultrasound is slow than EM 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

33
References

• DS Ch16 (sections 16.4.3 and 16.4.4)
• Jochen Schiller and Agnes Voisaro, Location-Based
  Services, Morgan Kaufmann, ch. 7 (ebook in the
  CityU library)
• http//www.trueposition.com/positioning.php