Localization of mobile devices

1
Localization of mobile devices

• Xian Zhong
• March 10, 2003

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Overview

• Introduction
• Location Sensor Technologies
• Selected Systems
• GPS
• ORL Ultrasonic Location System
• - The Cricket Location-Support System

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Introduction

• Background
• - wide use of sensor networks
• - Each sensor is self-sufficient to sense its
  environment, perform simple computation and
  communicate with its peers and observers
• -Some sensors are unaware of their position
  and required to be localized
• Context-aware Applications

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Context Awareness

• What is context?
• - Who
• - What
• - When
• - Where
• - How
• Context-aware applications need to know the
  location of users and equipment, and the
  capabilities of the equipment and networking
  infrastructure

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What is Location?

• Absolute position on geoids
• e.g. GPS
• Location relative to fixed beacons
• e.g. LORAN
• Location relative to a starting point
• e.g. inertial platforms
• Most applications
• location relative to other people or objects,
  whether moving or stationary, or the location
  within a building or an area

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Location Sensor Technologies

• Electromagnetic Trackers
• High accuracy and resolution, expensive
• Optical Trackers
• Robust, high accuracy and resolution, expensive
  and mechanical complex
• Radio Position Systems (Such as GPS)
• Successful in the wide area, but ineffective in
  buildings, only offer modest location accuracy
• Video Image (Such as the MIT Smart Rooms
  project)
• Location information can be derived from analysis
  of video images, cheap hardware but large
  computer processing

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GPS

• History
• When 1973 start, 1978-1994 test
• Who Why
• U.S. Department of Defense wanted the military
  to have a super precise form of worldwide
  positioning
• Missiles can hit enemy missile silos but you
  need to know where you are launching from
• US subs needed to know quickly where they were
• After 12B, the result was the GPS system!

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GPS

• Approach
• man-made stars" as reference points to calculate
  positions accurate to a matter of meters
• with advanced forms of GPS you can make
  measurements to better than a centimeter
• it's like giving every square meter on the planet
  a unique address!

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GPS System Architecture
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GPS System Architecture

• Constellation of 24 NAVSTAR satellites made by
  Rockwell
• Altitude 10,900 nautical miles
• Weight 1900 lbs (in orbit)
• Size17 ft with solar panels extended
• Orbital Period 12 hours
• Orbital Plane 55 degrees to equitorial plane
• Planned Lifespan 7.5 years

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GPS System Architecture

• Ground Stations, aka Control Segment
• The USAF monitor the GPS satellites, checking
  both their operational health and their exact
  position in space
• the master ground station transmits corrections
  for the satellite's ephemeris constants and clock
  offsets back to the satellites themselves
• the satellites can then incorporate these updates
  in the signals they send to GPS receivers.
• Five monitor stations
• Hawaii, Ascension Island, Diego Garcia,
  Kwajalein, and Colorado Springs.

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GPS Signals in Detail

• Carriers
• Pseudo-random Codes
• two types of pseudo-random code
• the C/A (Coarse Acquisition) code
• it modulates the L1 carrier
• each satellite has a unique pseudo-random code
• the C/A code is the basis for civilian GPS use

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GPS Signals in Detail (contd.)

• the P (Precise) code
• It repeats on a seven day cycle and modulates
  both the L1 and L2 carriers at a 10MHz rate
• this code is intended for military users and can
  be encrypted and called "Y"
• Navigation message
• a low frequency signal added to the L1 codes that
  gives information about the satellite's orbits,
  their clock corrections and other system status

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How GPS Works

• The basis of GPS is trilateration" from
  satellites. (popularly but wrongly called
  triangulation)
• To trilaterate," a GPS receiver measures
  distance using the travel time of radio signals.
• To measure travel time, GPS needs very accurate
  timing which it achieves with some tricks.
• Along with distance, you need to know exactly
  where the satellites are in space. High orbits
  and careful monitoring are the secret.
• Finally you must correct for any delays the
  signal experiences as it travels through the
  atmosphere.

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Earth-Centered Earth-Fixed X, Y, Z Coordinates
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Geodetic Coordinates (Latitude, Longitude, Height)
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Trilateration

• GPS receiver measures distances from satellites
• Distance from satellite 1 11000 miles
• we must be on the surface of a sphere of radius
  11000 miles, centered at satellite 1
• Distance from satellite 2 12000 miles
• we are also on the surface of a sphere of radius
  12000 miles, centered at satellite 2
• i.e. on the circle where the two spheres intersect

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Trilateration (contd.)

• Distance from satellite 3 13000 miles
• we are also on the surface of a sphere of radius
  13000 miles, centered at satellite 3
• i.e. on the two points where this sphere and the
  circle intersect
• could use a fourth measurement, but usually one
  of the point is ridiculous (far from earth, or
  moving with high velocity) and can be rejected
• but fourth measurement useful for another reason!

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Measuring Distances from Satellites

• By timing how long it takes for a signal sent
  from the satellite to arrive at the receiver
• we already know the speed of light
• Timing problem is tricky
• the times are going to be awfully short
• need some really precise clocks
• on satellite side, atomic clocks provide almost
  perfectly stable and accurate timing
• what about on the receiver side?
• atomic clocks too expensive!
• Assuming precise clocks, how do we measure travel
  times?

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Measuring Travel Times from Satellites

• Each satellite transmits a unique pseudo-random
  code, a copy of which is created in real time in
  the user-set receiver by the internal electronics
• The receiver then gradually time-shifts its
  internal code until it corresponds to the
  received code--an event called lock-on.
• Once locked on to a satellite, the receiver can
  determine the exact timing of the received signal
  in reference to its own internal clock

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Measuring Travel Times from Satellites (contd.)

• If that clock were perfectly synchronized with
  the satellite's atomic clocks, the distance to
  each satellite could be determined by subtracting
  a known transmission time from the calculated
  receive time
• in real GPS receivers, the internal clock is not
  quite accurate enough
• an inaccuracy of a mere microsecond corresponds
  to a 300-meter error
• The clock bias error can be determined by
  locking on to four satellites, and solving for X,
  Y, and Z coordinates, and the clock bias error

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Extra Satellite Measurement to Eliminate Clock
Errors

• Three perfect measurements can locate a point in
  3D
• Four imperfect measurements can do the same thing
• Pseudo-ranges measurements that has not been
  corrected for error
• If there is error in receiver clock, the fourth
  measurement will not intersect with the first
  three
• Receiver looks for a single correction factor
  that will result in all the four imperfect
  measurements to intersect at a single point
• With the correction factor determined, the
  receiver can then apply the correction to all
  measurements from then on.
• and from then on its clock is synced to universal
  time.
• this correction process would have to be repeated
  constantly to make sure the receiver's clocks
  stay synced
• Any decent GPS receiver will need to have at
  least four channels so that it can make the four
  measurements simultaneously

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Where are the Satellites?

• For the trilateration to work we not only need to
  know distance, we also need to know exactly where
  the satellites are
• Each GPS satellite has a very precise orbit,
  11000 miles up in space, according to the GPS
  master Plan
• GPS Master Plan
• spacing of the satellites are arranged so that a
  minimum of five satellites are in view from every
  point on the globe

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Where are the Satellites (contd.)?

• GPS satellite orbits are constantly monitored by
  the DoD
• check for "ephemeris errors" caused by
  gravitational pulls from the moon and sun and by
  the pressure of solar radiation on the satellites
  
• satellites exact position is relayed back to it,
  and is then included in the timing signal
  broadcast by it
• On the ground all GPS receivers have an almanac
  programmed into their computers that tells them
  where in the sky each satellite is, moment by
  moment

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GPS Technology Status

• Standard Positioning Service (SPS) C/A code with
  SA
• Horizontal accuracy of 100 m (95) 30m without
  SA
• Vertical accuracy of 156 m (95)
• UTC time transfer accuracy 340 ns (95 )
• Precise Positioning Service (PPS) P code
• Horizontal accuracy of 22 m (95)
• Vertical accuracy of 27.7 m (95)
• UTC time transfer accuracy 200 ns (95 )

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GPS Technology Status (contd.)

• Differential GPS
• Horizontal accuracy of 2 m
• Vertical accuracy of 3 m
• Requires a differential base station within 100
  km
• Real Time Kinematic GPS
• Horizontal accuracy of 2 cm
• Vertical accuracy of 3 cm
• Requires a differential base station within 10-20
  km

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GPS Technology Status (contd.)

• The size and price of GPS receivers is shrinking
• Worlds smallest commercial GPS receiver
  (www.u-blox.ch)
• Differential GPS receivers are inexpensive
  (100-250)
• Differential GPS available in all coastal areas
• Real Time Kinematic GPS receivers are expensive
• GPS needs line-of-sight to satellites
• does not work indoors, in urban canyons, forests
  etc.

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So, we need indoors location system
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ORL Ultrasonic Location System

• Measurements are made of time-of-flight of sound
  pulses from an ultrasonic transmitter to
  receivers placed at known positions around it.
• Transmitter-receiver distances can be calculated
  from the pulse transit times.
• A small wireless transmitter is attached to every
  object that is to be located

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Distance Calculation

• For each receiver, the interval Tp between the
  start of the sampling window and the peak signal
  time represents the sum of several individual
  periods

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Position Calculation (4 spheres)
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Position Calculation (Contd)
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Position Calculation (Contd)
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Position Calculation (Contd)

• In the ORL system all the receivers lie in the
  plane of the ceiling, and the transmitters must
  be below the ceiling. This allows calculation of
  transmitter positions using only three distances
  rather than the four required in the general
  case.
• Occasionally, however, the direct path may be
  blocked, and the first received signal peak will
  be due to a reflected pulse. In this case, the
  measured transmitter-receiver distance will be
  greater than true distance.
• The difference between two transmitter-receiver
  distances cannot be greater than the distance
  between the receivers.

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Applications

• The teleporting system Redirect an X-window
  system environment to different computer
  displays. We can use location data to present a
  users familiar desktop on a screen that face
  them whenever they enter a room.
• Nearest printer service offered to users of
  portable computers. Tags placed on the computer
  and printers report their positions, and the
  computer is automatically configured to use the
  nearest available printer as it is moved around a
  building.

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The Cricket Location-Support System (to be contd.)
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Bibliography

• "A New Location Technique for the Active Office",
  Andy Ward, Alan Jones, Andy Hopper, IEEE Personal
  Communications, Vol. 4, No.5, October 1997, pp.
  42-47.
• Special Issue on Global Positioning
  System,Proceedings of the IEEE, Vol.87, NO.1,
  January 1999
• The Cricket Location-support System,Nissanka B.
  Priyantha, Anit Chakraborty, and
  HariBalakrishnan, MIT Laboratory for Computer
  Science, Cambridge, MA 02139
• The Global Positioning System, I.A.Getting,
  IEEE Spectrum, Vol.30, December 1993
• Adaptive Beacon Placement, N.Bulusu, H.John,
  E.Deborah

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Bibliography (contd.)

• The Active Badge Location System, Want, R.,
  Hopper, A.,Falcao, V., And Gibbons, J.,ACM
  Transactions on Information Systems 10, 1
  (January 1992), 91-102.
• The Cricket Compass for Context-Aware Mobile
  Applications,Nissanka B. Priyantha, Allen k.L.
  Miu, Hari Balakrishnan, and Seth Teller, MIT
  Laboratory for Computer Science
• PowerPoint--Location Sensing for Context-Aware
  Applications,Mani Srivastava,UCLA EE
  Department, mbs_at_ee.ucla.edu
• PowerPoint Localization, Huei-Jiun JU(Laura)
  Yichen Liu, UCLA-EE Department