Localization

1
Localization

• Xian Zhong
• March 12, 2003

2
Overview

• Introduction
• Location Sensor Technologies
• Selected Systems
• GPS (Global Positioning System)
• ORL Ultrasonic Location System
• - The Cricket Location-Support System

3
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
• -Determining physical location of a sensor
  node is a critical service in these wireless
  sensor networks
• Context-aware Applications

4
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

5
What is Location?

• Absolute position on geoid
• Location relative to fixed beacons
• Location relative to a starting point
• Most applications
• location relative to other people or objects,
  whether moving or stationary, or the location
  within a building or an area

6
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
• Some new technologies are developing

7
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!

8
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!

9
GPS System Architecture
10
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

11
(No Transcript)
12
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.

13
(No Transcript)
14
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

15
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

16
(No Transcript)
17
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.

18
Earth-Centered Earth-Fixed X, Y, Z Coordinates
19
Geodetic Coordinates (Latitude, Longitude, Height)
20
Geodetic Coordinates

• The Cartesian coordinates, though convenient for
  calculations, are not practical for
  representations on maps
• Maps historically have used Geodetic coordinates
  (latitude, longitude, and height above a
  reference surface)
• Positions obtained from GPS can be converted into
  a local datum with an appropriate transformation

21
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

22
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
• the fourth measurement useful for another reason!

23
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
• -300,000kilometers/second
• 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?

24
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

25
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

26
Extra Satellite Measurement to Eliminate Clock
Errors

• Three perfect measurements can locate a point in
  3D
• Four imperfect measurements can do the same thing
• 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

27
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

28
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

29
Differential GPS

• Error in a measurement can be estimated if the
  receiver location is known
• These error estimates computed at a reference
  receiver, if made available to other GPS users in
  the area, would allow them to mitigate errors in
  their measurements.
• To be usable for navigation, such differential
  corrections have to be transmitted in real time
  over a radio link---DGPS

30
Differential GPS (contd.)

• DPGS can provide meter-level position estimates
  depending upon the closeness of the user to a
  reference station and the latency of the
  corrections transmitted over the radio link
• Such performance can meet the requirements of
  much of land transportation and maritime traffic
  DGPS services, both commercial and federally
  provided, are now widely available

31
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)

32
GPS Technology Status (contd.)

• Differential GPS
• Horizontal accuracy of 2 m
• Vertical accuracy of 3 m
• Requires a differential base station within 100
  km

33
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
• GPS needs line-of-sight to satellites
• does not work indoors, in urban canyons, forests
  etc.

34
we need indoor location system
35
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.

36
ORL Ultrasonic Location System Structure

• A small wireless transmitter is attached to every
  object that is to be located
• Consist of a microprocessor, a 418MHz radio
  transceiver, a Xilinx FPGA and a hemispherical
  array of five ultrasonic transducers
• Each prototype mobile device has a unique 16-bit
  address, is powered by two lithium cells, and
  measures 100mm60mm20mm

37
ORL Ultrasonic Location System Structure (contd.)

• A matrix of receiver elements is mounted on the
  ceiling of the room to be instrumented
• Each receiver has an ultrasonic detector, whose
  output is being digitized at 20KHz by an ADC
  which is controlled by a Xilinx FPGA, which can
  monitor the digitized signal levels.
• Receiver also are individually addressable and
  are connected in a daisy-chain to a controlling
  PC

38
ORL Ultrasonic Location System Structure (contd.)

• A controller connected to the PC transmit a radio
  message consisting of a preamble and 16-bit
  address in every 200ms
• The PC dictates which address is sent in each
  message
• The transceiver pick up the radio signals and
  decode it by the on-board FPGA
• The single addressed device broadcast an
  ultrasonic pulse
• The controlling PC sends a reset signal to
  receivers at the same time as each radio message
  is broadcast

39
ORL Ultrasonic Location System Structure
(contd.)

• The FPGAs on each receiver then monitor the
  digitized signals from the ultrasonic detector
  for 20ms, calculating the moment at which the
  received signals peak for the first time
• The short width of the ultrasonic pulse ensures
  that receivers detect a sharp signal peak
• The controlling PC then polls the receivers on
  the network, retrieving from them the time
  interval between the reset signal and detection
  of the first signal peak (if any signal was
  detected)

40
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

41
Position calculation
42
Position Calculation (Contd)
43
Position Calculation (Contd)
44
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.

45
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.

46
The Cricket Location-Support System

• Cricket Indoor Location System
• Support for mobile, indoor applications
• Location-aware scenarios
• Active maps
• Resource discovery and interaction
• Way-finding and navigation
• Stream redirection

47
Design Goals for Cricket

• Operates well indoors
• Different regions distinguishable
• Preserves user privacy (listeners are passive)
• Decentralized administration owner of space
  installs and configures beacons as needed
• Operates with low energy,
• Easy to deploy and administer
• Low cost, both H/W and installation
• Granularity This will really depend on beacon
  placement

48
Where am I?(Active Map)
49
Whats near me? Find me a (Resource Discovery)
Location by intent Print map on a color
printer. System response Locates nearby free
color printer Sends data there Tells you
where it is
50
Whats over there?(Interaction)
Viewfinder Point-and-use interface
51
How do I get to Haris office? (Navigation)
52
Traditional Approach

• Centralized architecture
• User-privacy issues
• High deployment cost

53
Cricket Architecture

• Decentralized, no tracking, low cost
• It does not attempt to calculate any kind of
  absolute position, but only the nearest beacon
  beacon

54
Metrics Terminology

• Precision how well can a listener detect a
  boundary (rate of correct detection)
• Granularity The smallest possible size for a
  detectable geographic region
• Objective is near 100 precision with a
  granularity of a few square feet

55
Determining Distance
Beacon
Ultrasound (pulse)
Listener

• A beacon transmits an RF and an ultrasonic signal
  simultaneously
• RF carries location data, ultrasound is a narrow
  pulse
• Velocity of ultra sound ltlt velocity of RF

• Beacon transmits simultaneous RF, ultrasound
• - RF carries location data, ultrasound is
  narrow pulse
• The listener measures the time gap between the
  receipt of RF and ultrasonic signals
• A time gap of x ms roughly corresponds to a
  distance of x feet from beacon

56
ProblemDetermining Space from Distance
Room A
Room B
I am at B
57
Solution Beacon Placement
Room A
Room B
x
x
I am at A

• Position beacons to detect the boundary
• Multiple beacons per space are possible

58
Problem Multiple Beacons
Beacon A
Beacon B
Incorrect distance
t
Listener
RF B
RF A
US B
US A

• Hard to correlate RF and ultrasound signals
• Beacon transmissions are uncoordinated
• Ultrasonic signals reflect heavily
• Ultrasonic signals are pulses (no data)
• Can lead to incorrect distance estimates

59
Solutions Avoiding Interference

• Limit stray signal interference
• Envelop all ultrasonic signals with RF
• Carrier-sense, randomized transmission
• Reduce chances of concurrent beaconing
• Listener inference algorithm
• Use distance samples to estimate location

60
Bounding Stray Signal Interference

• RF range gt ultrasonic range
• Ensures an accompanied RF signal with ultrasound

61
Bounding Stray Signal Interference

S - size of space string b - RF bit rate r -
ultrasound range v - velocity of ultrasound
(RF transmission time) (Max. RF US
separation
at the listener)
62
Bounding Stray Signal Interference

RF B
US B
RF A
US A
t

• Envelop ultrasound by RF
• Interfering ultrasound causes RF signals to
  collide
• Listener does a block parity error check
• The reading is discarded

63
Reducing Concurrent Beaconing

• Randomize beacon transmissions
• do while (true)
• pick r UniformT1, T2
• delay(r)
• xmit_beacon(RF,US)
• Optimal T1, T2 can be calculated analytically
• Trade-off latency against collision probability
• Erroneous estimates do not repeat

64
Inference Algorithms

• MinMode
• Determine mode for each beacon
• Select the one with the minimum mode
• MinMean
• Calculate the mean distance for each beacon
• Select the one with the minimum value
• Majority (actually, plurality)
• Select the beacon with most number of readings
• Roughly corresponds to strongest radio signal

65
Estimation AlgorithmWindowed MinMode
A
Frequency
B
5
Distance (feet)
5
10
66
Closest Beacon May Not Reflect Correct Space
Room A
Room B
I am at B
67
Correct Beacon Positioning
Room A
Room B
x
x
I am at A

• Position beacons to detect the boundary
• Multiple beacons per space are possible

68
Implementation

• Cricket beacon and listener

RF
RF
Micro- controller
Micro- controller
RS232
US
US

• LocationManager provides an API to applications
• Integrated with intentional naming system for
  resource discovery

69
Static listener performance

• Immunity to interference
• Four beacons within each others range
• Two RF interference sources
• Boundary detection ability
• L1 only two feet away from boundary

Room B
Room A
readings due to interference of RF from I1
and I2 with ultrasound from beacons
I1
I2
Room C
70
Inference Algorithm Error Rates
71
Mobile listener performance
Room A
Room B
Room C
72
Comparisons
System
Attribute
73
Summary

• Cricket provides information about geographic
  spaces to applications
• Location-support, not tracking
• Decentralized operation and administration
• Passive listeners and no explicit beacon
  coordination
• Requires distributed algorithms for beacon
  transmission and listener inference
• Implemented and works!

74
Future work

• Dynamic transmission rate with carrier-sense for
  collision avoidance.
• Dynamic ultrasonic sensitivity.
• Improved location accuracy.
• Integration with other technologies such as Blue
  Tooth.

75
Caveats

• Incompatible with ad-hoc beacon placement
• Does not attempt to interpolate coordinate system
  between beacons, simply presents name of closet
  beacon
• Does not solve problem of fine-granularity
  location..at best within 2-4 foot radius
• Tradeoff between granularity of location and
  interference from neighboring beacons (they had a
  max of 6 beacons in range)
• May mean it works best indoors in confined
  spacesgtLocation-support, not tracking

76
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

77
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

78
Bibliography (contd.)

• PowerPoint The Cricket Location-support
  System, Nissanka B. Priyantha, Anit Chakraborty,
  hari Baiakrishnan, MIT lab for Computer Science
• PowerPoint 6.964 pervasive Computing
  Context-Aware Networking, Stephen J. Garland,
  MIT laboratory for Computer Science