New Directions in Network Intrusion Detection by sofiaie


									      A Survey
Localization Methods

    Presented to CS694
    November 19, 1999
       Jeremy Elson
    what’s the problem?
• But what does this mean, really?
• Frame of reference is important
  – Local/Relative: Where am I vs. where I was?
  – Global/Absolute: Where am I relative to the world?
• Location can be specified in two ways
  – Geometric: Distances and angles
  – Topological: Connections among landmarks
localization: relative
• If you know your speed and direction, you
  can calculate where you are relative to
  where you were (integrate).
• Speed and direction might, themselves, be
  absolute (compass, speedometer), or
  integrated (gyroscope, accelerometer)
• Relative measurements are usually more
  accurate in the short term -- but suffer from
  accumulated error in the long term
• Most robotics work seems to focus on this.
  This talk will focus on absolute localization.
localization: absolute
 – Proximity-To-Reference
   • Landmarks/Beacons: ParcTab, Active Badges

 – Angle-To-Reference
   • Visual: manual triangulation from physical points
   • Radio: VOR

 – Distance-From-Reference
   • Time of Flight
      – RF: GPS, PinPoint
      – Acoustic: Active Bat, Lew
   • Signal Fading
      – EM: Bird/3Space Tracker
      – RF: SCADDS/SCOWR, Niru
      – Acoustic: Jer?                                   4
      topological maps
• Really the most “natural”: how did you get
  to class today?
  – You have a map of known landmarks and the
    connections among them
  – You even convert metric maps to topological!
• Probably the most useful for location-aware
  – “Closest printer” really means the one in this
    room, not on the other side of a wall

• ParcTab and Active Badges
  – Infrared transmission picked up by recivers in all
  – Works precisely because infrared propagation
    matches topological boundaries of environment
• Reverse is also possible: landmarks
  – SCADDS localization beacons
• Problems: difficult to control granularity;
  apps may need geometric map

                               Works great --
                               as long as there
                               are reference points!
              Lines of Sight

                               Unique Target
   compass triangulation
            cutting-edge 12th century technology



                Lines of Sight

                                                   Unique Target
     celestial navigation
• Same idea, except in
  1D, and reference
  point is a star
• Angle of between
  north star and horizon
  determines latitude
• Works only because
  north star is close to
  axis of Earth’s rotation
• Longitude is much
•   Note: Points to non-flatness of earth                             9
                                            Encyclopedia Britannica
              VOR: modern
• VOR is an aircraft navigation system still
  widely prevalent today
• Same concept as visual landmarks, except
  that radio beacons emit directional signals
• Aircraft can determine (within 1o) their
  bearing to a VOR station
• 1 VOR fix will tell you bearing-to-target; 2
  tells you absolute position   10
vor for localization
2 VORs plus a map will uniquely determine 2-D position

            VOR’s magic
• VOR stations transmit two signals:
  – An omnidirectional reference signal, with a 30
    Hz amplitude modulation
  – A highly directional continuous signal that
    sweeps through 360 degrees at 30Hz
• Result: aircraft sees two sine waves:
  reference modulated by transmitter, azimuth
  signal modulated by directionality
• Receiver computes phase shift between
  them to get bearing
• Measure distance from
  ref point to target
• For n dimensions, n
  measurements give you
  2 sol’ns; n+1 is unique
• Domain knowledge can
  often be used instead of
  n+1’th measurement

  accuracy constraints
• Accuracy depends on:
  – Precision of the distance measurements (represented
    below as thickness of the circles)
  – Geometric configuration of the reference points
 Reference points far apart:   Reference points close together:
  small overlapping region        large overlapping region

    measuring distance
• Measure time-of-flight
  – Biggest problem: time synchronization
     • Time sync and localization are often intertwined
  – If only Einstein was wrong, and information
    could travel instantaneously...
  – GPS, PinPoint, Active Bat all deal with the
    time problem in different ways
• Measure signal strength
  – Used less often because relationship between
    strength and distance is harder to model (also
    not linear)                                           15
        gps: global
     positioning system
• 24 satellites launched by U.S. DOD,
  originally for weapons systems targeting
• Gives time & position anywhere in the world,
  although often only outdoors
• Typical Position Accuracy:
  – Civilian: Horiz 50m, Vert 78m, 3D 93m, 200ns
  – Diff: Horiz 1.3m, Vert 2.0m, 3D 2.8m, 350ns
• Military accuracy might be usable in 2000
         the basic idea
• Satellites constantly transmit beacons along
  with the time-of-beacon and position (in
  predictable, corrected, and observed orbits)
• Receivers listen for (phase-shifted) signals
  and compute distance based on propagation
  delay (assume magically synced clocks for now)
• 3 satellites gives you 2 points (in 3d); throw
  out the one in deep space
• Compute position relative to satellites; use
  satellite position to get Earth coordinates 17
      effects of clock bias

true distance

biased distance               18
solving for clock bias
• Critical point: satellites are perfectly
  synchronized (using expensive atomic
  clocks synchronized before launch)
• If all signals are received simultaneously,
  they are all off by a constant bias
• This means that by adding an additional
  satellite, we can solve for clock bias.
  (Would not work if off by a constant factor)
• This gives us both position and time!
     solving for clock bias

true distance

biased distance           20
    sources of gps error
                 per satellite

•   Satellite clock drift (1.5 m) (1usec = 300m)
•   Orbit estimation errors (2.5 m)
•   Atmospheric and relativistic effects (5.5 m)
•   Receiver noise (0.3 m)
•   Multipath interference (0.6 m)
•   Intentional randomization to reduce civilian
    grade accuracy (30m)   21
       differential gps
• A way of getting more accurate GPS data
• Receivers at known positions find the
  difference between computed & true position
• Computed error correction factor transmitted
  to other GPS receivers in the area
• Corrects for all errors that the receiver has in
  common with the reference (atmospheric,
  relativistic, orbital, sat clock, randomization)

          pinpoint 3d-id
•   Local positioning system by Pinpoint Co.
•   Meant to track large numbers tags indoors
•   Tags should be cheap and all have IDs
•   Infrastructure knows where tags are; tags
    don’t know anything
    – Compare to GPS: Infrastructure knows nothing,
      tags know where they are
• ~1-3 m accuracy
     the clock problem
• Their solution:
  – Interrogator transmits a test signal
  – Tag simply changes the signal’s frequency and
    transponds it back to the interrogator (with tag
    ID modulated in)
  – Interrogator receives transponded signal
• Subtracting out fixed system delays yields
  time of flight
• They avoid the clock sync problem by
  making the transmitter and receiver the
  same device                                          24
implementation details
• Area to be monitored is divided up into
  “cells” - each with antennas & controller
• Coarse-grained location first (which cell?),
  then fine-grained location within the cell
  – Query driven: “Tag 5 raise your hand!”, or
  – Tag driven: all tags periodically beacon (impl.)
• Tag beaconing frequency might depend on
  inertial system
• Collision reduction through various
  techniques, including reducing beacon time
  – They note non-linear increase in perf due to this
           active bats
• Research project at ORL-cum-AT&T
• Similar goals as Pinpoint: indoor LPS

            100mm x 60mm x 20mm
           bats at work
• Tags have unique IDs, radio receivers and
  ultrasound transducers
• Interrogator consists of a radio transmitter
  and “microphones” (ultrasound detectors)
• Interrogator sends radio message: “Tag 5,
  signal now!”
• Tag 5 receives the radio message and sends
  an ultrasonic pulse
• Microphones pick up the sound; time of
  flight calculated                            27
     the clock problem
• Use two modalities: RF for control (very
  fast), sound for measurement (slow)
• We can simulate instantaneous info flow
  because it is almost instant relative to what
  we’re measuring
  – Speed of sound: 344 m/s
  – Speed of light: 300M m/s (30m = 0.1 usec)
  – 0.1 usec * 344m/s = 0.000 034 4 m
• Like Pinpoint, subtract out fixed delays
  (empirically derived) to get flight time        28
implementation details
• Multiple peaks may be detected (echoes -
  audio version of multipath interference)
• Two heuristics for eliminating echos:
  – Difference in distance between two
    measurements can’t be larger than the distance
    between the two microphones.
     • If so, larger one must be a reflection
  – Do statistical tests to identify outliers; repeat
    until variance is low or only 3 points remain
• Nice extension: use 3 tags to detect 3d pose
  as well as position of objects              29
active bat accuracy

   95% within 14cm for raw measurments
95% within 8cm when averaged over 10 samples                 30
        active lew-bats
• Goal: distance between two robots
• One robot simultaneously:
  – Sends a message over the network to the target
  – Emits an audio chirp from the sound card
• Target robot:
  – Waits for network message
  – Listens for chirp, calculates time of flight
• Evaluation in progress
 distance measurement:
  using signal fading
• Another class of localization systems uses
  reduction in the strength of a field to
  measure distance
  – Magnetic Field: Ascension “Flock of Birds”,
    Polaris 3space tracker
  – RF: No (??) commercial products; work here on
  – Sound: A half baked idea of mine

flock of birds
• Measures 3D position
  and orientation
• Consists of largish transmitter & small
  receiver connected to the same controller
• Receiver picks up orthogonal magnetic fields
  from transmitter (details unknown)
• Specs claim 0.02”/0.1o precision over 10’ area
  – Not really that good; and metal screws it up
  – Magnetic field falls off as r4 (?)
• Mostly head tracking apps & similar                         33
 radio signal strength
• Work going on here (SCADDS, SCOWR:
  Nirupama Bulusu, Puneet Goel)
• Can radio signal strength be used as a
  reliable distance measurement?
• Very difficult to model indoor radio prop.
• Current test implementation
  – Radiometrix RPC radio transmitter
  – RxM receiver module with RSSI output pin

                            an initial test
Signal Strength Indicator

                                   Distance in Meters           35
                              Nirupama Bulusu and Puneet Goel
                       sound off
• Half baked idea: can we measure falloff in
  audio volume as a distance estimate?

• ...I told you it was half baked, that’s all I have to say about that :)

that’s all, folks!

And, remember: wherever you go, there you are.


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