1 / 37

A Survey of Localization Methods

A Survey of Localization Methods. Presented to CS694 November 19, 1999 Jeremy Elson. 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?

heinz
Download Presentation

A Survey of Localization Methods

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  1. A SurveyofLocalization Methods Presented to CS694 November 19, 1999 Jeremy Elson

  2. 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

  3. 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.

  4. 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?

  5. 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

  6. 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

  7. triangulation Land Landmarks Works great -- as long as there are reference points! Lines of Sight Unique Target Sea

  8. compass triangulation cutting-edge 12th century technology Land Landmarks Lines of Sight North Unique Target Sea

  9. 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 Encyclopedia Britannica

  10. 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

  11. vor for localization 2 VORs plus a map will uniquely determine 2-D position

  12. 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

  13. 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

  14. 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

  15. 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)

  16. 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 http://www.trimble.com/gps/howgps/gpsfram2.htm

  17. 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

  18. effects of clock bias true distance biased distance

  19. 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!

  20. solving for clock bias true distance biased distance

  21. sources of gps errorper 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

  22. 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)

  23. 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

  24. 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

  25. 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

  26. 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/

  27. 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

  28. 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

  29. 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

  30. 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

  31. 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

  32. 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

  33. 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

  34. 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

  35. an initial test Signal Strength Indicator Distance in Meters Nirupama Bulusu and Puneet Goel

  36. 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 :)

  37. that’s all, folks! And, remember: wherever you go, there you are.

More Related