Transcript New Directions in Network Intrusion Detection

A Survey
of
Localization Methods
Presented to CS694
November 19, 1999
Jeremy Elson
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what’s the problem?
• WHERE AM I?
• 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
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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.
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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?
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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
computing
– “Closest printer” really means the one in this
room, not on the other side of a wall
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topological
localization
• ParcTab and Active Badges
– Infrared transmission picked up by recivers in all
rooms
– 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
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triangulation
Land
Landmarks
Works great -as long as there
are reference points!
Lines of Sight
Unique Target
Sea
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compass triangulation
cutting-edge 12th century technology
Land
Landmarks
Lines of Sight
North
Unique Target
Sea
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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
harder
•
Note: Points to non-flatness of earth
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Encyclopedia Britannica
VOR: modern
triangulation
• 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
http://www.interlog.com/~bitewise/aviation/navplan/radionav.htm
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vor for localization
2 VORs plus a map will uniquely determine 2-D position
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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
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distance-to-reference
systems
• 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
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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:
small overlapping region
Reference points close together:
large overlapping region
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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)
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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
http://tycho.usno.navy.mil/gpsinfo.html
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http://www.trimble.com/gps/howgps/gpsfram2.htm
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
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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!
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solving for clock bias
true distance
biased distance
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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)
http://www.trimble.com/gps/howgps/gpsfram2.htm
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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)
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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
http://www.pinpointco.com/_private/whitepaper/rfid.html
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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
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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 this25
active bats
• Research project at ORL-cum-AT&T
• Similar goals as Pinpoint: indoor LPS
100mm x 60mm x 20mm
http://www.uk.research.att.com/bat/
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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
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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
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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
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as well as position of objects
active bat accuracy
95% within 14cm for raw measurments
95% within 8cm when averaged over 10 samples
ftp://ftp.uk.research.att.com/pub/docs/att/tr.97.10.pdf
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active lew-bats
• Goal: distance between two robots
• One robot simultaneously:
– Sends a message over the network to the target
robot
– Emits an audio chirp from the sound card
• Target robot:
– Waits for network message
– Listens for chirp, calculates time of flight
• Evaluation in progress
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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
SCADDS/SCOWR
– Sound: A half baked idea of mine
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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
http://www.ascension-tech.com/products/flockofbirds
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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
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Signal Strength Indicator
an initial test
Distance in Meters
Nirupama Bulusu and Puneet Goel
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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 :)
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that’s all, folks!
And, remember: wherever you go, there you are.
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