15-446 Networked Systems Practicum Lecture 8 – Localization 1

15-446 Networked Systems Practicum

Lecture 8 – Localization

1

Outline

• General Techniques

• GPS

• WiFi

• Audio

• Trajectory

2

GPS Basics

• GPS stands for Global Positioning System which measures 3-D locations on Earth surface using satellites

• GPS operates using radio signals sent from satellites orbiting the earth

• Created and Maintained by the US Dept. of Defense

• System as a whole consists of three segments• Satellites (space segment)• Receivers (user segment)• Ground stations (control segment)

GPS History

• Development began in 1973• First satellite became

operational in 1978• Declared completely functional

in 1995• A total of 52 satellites have

been launched in 4 phases• 30 satellites are currently

functional• Managed by the U.S.

Department of Defense• Originally developed for

submarines• Now part of modern “smart

bombs” and highly accurate missiles

Satellites

• At least 4 satellites are above the horizon anytime anywhere

• GPS satellites are also known as “NAVSTAR satellites”

• The satellites transmit time according to very accurate atomic clocks onboard each one

• The precise positions of satellites are known to the GPS receivers from a GPS almanac

Map from P. Dana, The Geographer's Craft Project, Dept. of Geography, U. Texas-Austin.

Satellites

• The satellites are in motion around the earth

• Like the sun and moon satellites rise and set as they cross the sky

• Locations on earth are determined from available satellites (i.e., those above the horizon) at the time the GPS data are collected


Receivers

• Ground-based devices read and interpret the radio signals from several of the NAVSTAR satellites at once

• Geographic position is determined using the time it takes signals from the satellites to reach the GPS receiver

• Calculations result in varying degrees of accuracy that depend on:• Quality of the receiver • User operation of the receiver (e.g., skill of user and

receiver settings)• Atmospheric conditions• Local conditions (i.e., objects that block or reflect the

signals)• Current status of system

Ground Stations

• Control stations• Master station at Falcon (Schriever) AFB, Colorado• 4 additional monitoring stations distributed around the world

• Responsibilities• Monitor satellite orbits & clocks• Broadcast orbital data and clock corrections to satellites


How GPS Works: Overview

• Satellites have accurate atomic clocks onboard and all GPS satellites transmit the same time signal at the same time• Think “synchronize your watches”

• The satellite signals contain information that includes• Satellite number• Time of transmission

How GPS Works: Overview

• Receivers use an almanac that includes• The position of all satellites every second• This is updated monthly from control stations

• The satellite signal is received, compared with the receiver’s internal clock, and used to calculate the distance from that satellite

• Trilateration (similar to triangulation) is used to determine location from multiple satellite signals

How GPS Works

• Distances between satellites and receivers is determined by the time is takes the signal to travel from satellite to receiver• Radio signals travel at speed of light (186,000 miles/second)• All satellites send the identical time, which is also generated by the

receivers• Signal travel time = offset between the satellite signal and the

receiver signal

• Distance from each satellite to receiver = signal travel time * 186,000 miles/second

1sec

Receiver signal

Satellite signal

How GPS Works: Trilateration

• Start by determining distance between a GPS satellite and your position


• Adding more distance measurements to satellites narrows down your possible positions



• The 4th satellite in trilateration is to resolve any signal timing error • Unlike GPS satellites, GPS receivers do not

contain an atomic clock• To make sure the internal clock in the receiver

is set correctly we use the signal from the 4th satellite

GPS Error Sources

• Satellite errors• Satellite position error (i.e., satellite not exactly where it’s supposed to be)• Atomic clocks, though very accurate, are not perfect

• Atmospheric• Electro-magnetic waves travel at light speed only in a vacuum• Atmospheric molecules, particularly those in the ionosphere, change the

signal speed

• Multi-path distortion• The signal may "bounce" off structures before reaching the GPS receiver –

the reflected signal arrives a little later

• Receiver error: • Due to the receiver clock or internal noise

• Selective Availability• No longer an issue

Sources of Error

• Satellite Clock & Satellite Position• Atomic clock errors• +/- 2 meters of error

• Satellite is not in precise orbit• +/- 2.5 meters of error

Sources of Error

• Atmospheric Delays/Bending• +/- 5 meters or error

Sources of Error

• Multi Path Interference (signal bouncing off of buildings, trees, etc.)• +/- 1 meter of error

Sources of Error

• Receiver Timing/Rounding Errors • +/- 1 meter of error (depends on the quality of the GPS receiver)

Quadruple Redundant Atomic ClocksAccurate to Nanoseconds

$800,000 in clocks on each satellite2:02:01.23456789012

Powered by 4 AA Batteries~$2.99

2:02:01.2345

GPS - Selective Availability

• A former significant source of error• Error intentionally introduced into the satellite

signal by the U.S. Dept. of Defense for national security reasons

• Selective Availability turned off early May 2, 2000

Good Satellite Geometry

Good Satellite Geometry

Poor Satellite Geometry

Poor Satellite Geometry

DGPS Site

x+30, y+60

x+5, y-3

True coordinates = x+0, y+0

Correction = x-5, y+3

DGPS correction = x+(30-5) and y+(60+3)

True coordinates = x+25, y+63

x-5, y+3

Real Time Differential GPS

DGPS ReceiverReceiver

National Differential Global Positioning System

Yellow areas show overlap between NDGPS stations. Green areas are little to no coverage. Topography may also limit some areas of coverage depicted here.

NDGPS Ground Stations

National Differential Global Positioning System

Yellow areas show overlap between NDGPS stations. Green areas are little to no coverage. Topography may also limit some areas of coverage depicted here.

NDGPS Ground Stations

Geostationary WAAS satellites

GPS Constellation

WAAS Control Station (West Coast)

Local Area System (LAAS)

WAAS Control Station (East Coast)

Wide Area Augmentation System

+ -3

meters

+-15 meters

With Selective Availability set to zero, and under ideal conditions, a GPS receiver without WAAS can achieve fifteen meter accuracy most of the time.*

Under ideal conditions a WAAS equipped GPS receiver can achieve three meter accuracy 95% of the time.*

* Precision depends on good satellite geometry, open sky view, and no user induced errors.

How good is WAAS?

Outline


• GPS

• WiFi

• Audio

• Trajectory

31

Test Environment

• 3 Base Stations

• 10500 sq ft

• Lucent WaveLAN cards.

• 200m/50m/25m range for open/semi-open/closed areas.

Map of Testbed

Empirical Data Collection

• Mobile host 4 UDP packets per second with 6-byte payload.

• Each base station records the signal strength with timestamp (t, bs, ss)

• User indicates current location on mobile application• Store orientation since it causes variation in detected

signal.• Mobile node records (t,x,y,d)• Data collection phase repeated for 70 distinct

locations for 4-directions.

Information Collected

• Use signal information• Off-Line Phase

• Construct/validate models for signal propagation

• Real-Time Phase• Infer location of user

• Information Passed• Signal strength (SS)• Signal-to-noise ratio (SNR)• SS is a stronger function of location; therefore

authors do not use SNR

Analysis

• Convert physical space to signal space (ss1,ss2,ss3)

• Nearest Neighbor in Signal Space (NNSS) using Euclidean distance.

222 )3'3()2'2()1'1( DistanceEuclidian ssssssssssss

Comparison

• Empirical Method is more accurate than other tracking methods.

K-nearest neighbors

• Average k neighbors (in signal space)

• Result: Small k has some benefit and large k is not accurate.

• K-neighbors in signal space are not near in physical space.

An illustration of how averaging multiple nearest (N1, N2, N3) can lead to a guess (G) that is closer to the user’s true location (T) than any of the neighbors is individually.

Other Analysis Methods

• Accuracy did not decrease with number or data points.

• Accuracy decreased with decreased samples.

• Ignoring radio orientation decreases accuracy

• Tracking Mobile User as sequence of location determination problems. • Use 10 sample window. Results are only slightly worse.

Outline


• GPS

• WiFi

• Audio

• Trajectory

39

Determining Distance

• A beacon transmits an RF and an ultrasonic signal simultaneously• RF carries location data, ultrasound is a

narrow pulse• Velocity of ultra sound << velocity of RF

RF data(location name)

Beacon

Listener

Ultrasound(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

Uncoordinated Beacons

• Multiple beacon transmissions are uncoordinated

• Different beacon transmissions can interfere• Causing inaccurate distance measurements

at the listener

Beacon A Beacon B

timeRF B RF A US B US A

Incorrect distance

t

S/b

r/v (max)

S - size of space stringb - RF bit rater - ultrasound rangev - velocity of ultrasound

Bounding Stray Signal Interference

(RF transmission time) (Max. RF US separation at the listener)

S r

b v

Bounding Stray Signal Interference

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

– The reading is discarded

tRF A US A

RF B US B

Closest Beacon May Not Reflect Correct Space

I am atB

Room A Room B

Correct Beacon Positioning

Room A Room B

x x

I am atA

Position beacons to detect the boundary

Multiple beacons per space are possible

Outline


• GPS

• WiFi

• Audio

• Trajectory

46

Accelerometer Pedometer

47

Error-prone Trajectories

48

Signatures

49

UnLoc Design

50

Other Techniques

• Ambient noise

• Angle of arrival

• Camera views

• Etc…

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Documents

15-446 Networked Systems Practicum Lecture 8 – Localization 1