Positioning systems

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Development of Complex Curricula for Molecular Bionics and Infobionics Programs within a consortial* framework**

Consortium leader

PETER PAZMANY CATHOLIC UNIVERSITY

Consortium members

SEMMELWEIS UNIVERSITY, DIALOG CAMPUS PUBLISHER

The Project has been realised with the support of the European Union and has been co-financed by the European Social Fund ***

**Molekuláris bionika és Infobionika Szakok tananyagának komplex fejlesztése konzorciumi keretben

***A projekt az Európai Unió támogatásával, az Európai Szociális Alap társfinanszírozásával valósul meg.

PETER PAZMANY CATHOLIC UNIVERSITY

SEMMELWEIS UNIVERSITY

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Peter Pazmany Catholic University Faculty of Information Technology

ELECTRICAL MEASUREMENTS

Positioning systems

www.itk.ppke.hu

(Elektronikai alapmérések)

Helymeghatározó rendszerek

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Electrical measurements: Positioning systems

Lecture 4 review

• About the decibel

• Description signal in transform domain (Fourier and Laplace transformation)

• The bandwidth of signal

• Analog-to-Digital Conversation

• Tha noise

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Outline

• Principle of positioning

• Satellite based positioning system (GPS)

• Other satellite based positioning systems

• Feature of GPS

• The coordinate systems

• The time systems

• Applciations of GPS

• The NMEA Protocol

• Real Time Positioning Systems

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Abbreviations

• GNSS – Global Navigation Satellite Systems

• GPS – Global Positioning System (USA)

• NAVSTAR-GPS – NAVigation System with Timing And Ranging GPS

• SPS – Standard Positioning System (in commercial use)

• PPS – Precise Positioning System (in military use)

• A-GPS – Assisted GPS – GPS supperted by terestrial mobile cellular wireless communication systems.

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Basic concepts of positioning

• Passive one-way measurement: points A and B have independent but shyncronized clock. We know the time when we send a message. The receiver uses this message it receives to determine the transit time of message and computes the distance to transmitter.

A B B’

¡ cΔt ¡ cδt

Where: Δt – transit time (from A to B) c – speed of light

δt – clock error

(transmitter) (receiver)

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Positioning based on time measurements

• Active measurements:

A B

¡ ¡

Δt’=2 Δt AB= ½cΔt

(transmitter) (receiver)

There is not clock failure effect!

Why not use? The number of users is limited!

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Principle of positioning

¡

p₁

p₂ p₃

¡ x_Ay_A

x₂y₂ x₃y₃

2 3 2

3 3

2 2 2

2 2

2 1 2

1 1

) (

y y

x x

p

y y

x x

p

y y

x x

p

A A

− +

−

=

− +

−

=

− +

−

=

3 equations, 2 variables,

pseudo distant AB= cΔt

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Principle of positioning taking into account clock failure

¡

p₁

p₂ p₃

¡ x_Ay_A

x₁y₁

x₂y₂ x₃y₃

t c y

y x

x p

t c y

y x

x p

t c y

y x

x p

A A

δ δ δ +

− +

−

=

+

− +

−

=

+

− +

−

=

2 3 2

3 3

2 2 2

2 2

2 1 2

1 1

) (

3 equations,

3 unknown variables!

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Satellite based positioning system: GPS

• The GPS is a space-based GNSS that provides location and time information in all weather, anywhere on or near the Earth, where there is an unobstructed line of sight to four or more GPS satellites (as reference points).

• Details of coordinate systems: cartesian, geographic, topocentric, inertial.

• Details of time systems: universal time, coordinated universal time, GPS time.

• Signals are encoded using code division multiple access (CDMA) allowing messages from individual satellites to be distinguished from each other based on unique encodings for each satellite (that the receiver must be aware of).

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Satellite based positioning system: GPS (cont’)

• GPS uses satellites as reference points to calculate accurate positions.

• Consists of 24 GPS satellites in medium Earth orbit (The region of space between 2000km and 35,786 km)

• Each satellite orbits the earth every 12 hours (2 complete rotations every day).

• Constellation design: at least 4 satellites in view from any location at any time to allow navigation (solution for 3 position + 1 station clock unknowns)

± ±

±

ª

p₁ p₂ p₃ p₄

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Trilateration

2 2 2

1 1 1 1

2 2 2

2 2 2 2

2 2 2

3 3 3 3

2 2 2

4 4 4 4

( ) ( ) ( )

A A A

p x x y y z z c t

δ δ δ

δ

= − + − + − +

Pseudoranges

Clock errors

Remark: Trilateration is used to determine the position based on three satellite's pseudoranges. Using more than four is an over-determined system of equations with no unique solution, which must be solved by least-squares or a similar technique: ^{Δ =}^ˆ

(

^T

)

⁻¹ ^T ⁻¹^Δ^ˆ

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Available satellite based positioning systems

• PS was created and realized by the U.S. Department of Defense (USDOD) and was originally run with 24 satellites. It became fully operational in 1994.

• While originally a military project, GPS is considered a dual- use technology, meaning it has significant military and civilian applications.

• Characterized by continuous development and modernization.

• There are alternative and complement GNSS systems:

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GPS system

• The navigational signals transmitted by GPS satellites encode a variety of information including satellite positions, the state of the internal clocks, and the health of the network.

• The location is expressed in a specific coordinate system (World Geodetic System, WGS84)

• The GPS consists of 3 main segments:

– Space Segment: the constellation of satellites

– Control Segment: operation and monitoring of the GPS System – User Segment: all GPS receivers and processing software's

– We might add a 4th segment. Ground Segment: permanent civilian networks of reference sites, associated analyses and archives (e.g. IGS)

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• 24+ satellites

• Satellites equally distributed in 6 orbiting planes around the Earth

• 55 degree inclination

• 20200 km above Earth

• GPS satellites repeat their ground tracks after: 1 sidereal day = 23 h 56 min = 2 orbital periods. The same geometry is reached 4 minutes earlier every day.

• 5 hours view in horizon

• Constellation design: at least 4 satellites in view from any location at any time to allow navigation (solution for 3 position + 1 station clock unknowns)

• Each GPS satellites then transmit signals to the GPS receivers . These signals indicates satellite’s location and the current time.

• Each GPS satellite has special clocks to provide very accurate time reference (atomic clocks).

GPS space segment

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GPS satellites

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GPS dedicated monitor stations (Control segment)

Hawaii

Ascension Diego Garcia

Kwajalein

Colorado Springs

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• All GPS receivers on land, on sea, in the air and in space.

• GPS receivers are generally composed of an antenna, tuned to the frequencies transmitted by the GPS satellites.

• Knowing the distance from at least 4 GPS satellites, the GPS receiver can calculate their position in ground or in air (for aircraft).

• GPS also can tell you

– What direction you are heading – How fast you are going

– Your altitude

– A map to help you arrive at a destination – How far you have traveled

GPS user segment

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GPS history

• 1978-1992

– the first experimental Block-I GPS satellite was launched (1978).

– the first modern Block-II satellite was launched (1989).

• 1993

– GPS achieved initial operational capability (IOC), indicating a full constellation (24 satellites) was available and providing the SPS.

– 3 channel receivers, 10 min. setup time, and the receivers were 5 times expensive than today.

• 2000

– 12 channel receivers, 15-20 sec. setup time

– improving the precision of civilian GPS from 300 meters to 20 meters

• 2005

– first modernized GPS satellite was launched and began transmitting a second civilian signal (L2C) for enhanced user performance.

• 2010

– GPS Next Generation Operational Control System (OCX) to improve accuracy and availability of GPS navigation signals, and serve as a critical part of GPS modernization.

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The european brother

• Galileo is a global navigation satellite system (GNSS) currently being built by the European Union (EU) and European Space Agency (ESA). The €3.4 billion project is an alternative and a complement to the U.S. NAVSTAR Global Positioning System (GPS) and the Russian GLONASS. On 30 November 2007 the 27 EU transportation ministers involved reached an agreement that it should be operational by 2013, but later press releases suggest it was delayed to 2014.

• The political aim is to provide an independent positioning system upon which European nations can rely even in times of war or political disagreement, since the USA could disable use of their national system by others (through encryption).

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Galileo and GPS

• One of the reasons given for developing Galileo as an independent system was that GPS is widely used worldwide for civilian applications, which until 2000 had universal Selective Availability (SA) enabled (and still bears the possibility of being reenabled). This could intentionally render the locations given via GPS inaccurate. Galileo's proponents argued that civil infrastructure, including aeroplane navigation and landing, should not rely solely upon GPS.

• As old satellites are replaced in the GPS modernization program, SA will cease to exist. The modernization programme also contains standardized features that allow GPS III and Galileo systems to inter-operate, allowing a new receiver to utilise both systems to improve precision. By combining GPS and Galileo, it can create an even more precise GNSS system.

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GPS signals

The frequency of atomic clocks on satellite: 10,23 MHz Carrier frequency:

1: f1 154 10,23 1575,42

L = ⋅ MHz = MHz

2 : f2 120 10,23 1227,60

L = ⋅ MHz = MHz

1 2

19,05 24,45

cm cm λ

λ

=

The modulation:

( )

1

( ) ( ) ( ) ( )

1 1

( ) ( ) ( )

1

1 cos sin

L t = a P t W t D t f t + a C t D t f t

Amplitude

P-code

W-code

Data code, 50 bit/s C/A-code

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GPS signals

• Bits encoded on carrier by phase modulation:

– C/A-code (Clear Access / Coarse Acquisition): 1.023 MHz (λ =300 m ), 10.23/10

– P-code (Protected / Precise): 10.23 MHz (λ = 30 m ) at fundamental frequency

– Navigation Message: (system time, “Broadcast” orbits, satellite clock corrections, almanacs, ionospheric information, etc.), 50 bps on both L1 and L2

• The C/A code is a 1,023 bit deterministic sequence called pseudorandom noise (PRN). Each satellite transmits a unique PRN code, which does not correlate well with any other satellite's PRN code.

• The P-code is also a PRN but longer than C/A. The extreme length of the P-code increases its correlation gain.

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GPS codes

P-code (Protected):

code length: 2,36*10¹⁴ bit accuracy: 0,3m

C/A-code (Clear Acquisition):

Code length: 1023 bit accuracy: 3m

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BPSK modulation technique

1 -1 1 1 -1

Carrier wave

PRN code (modulation)

Modulated wave cycle

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Error sources in GPS

Sources Type C/A-code P-code

Satellite Clock error 3.0 3.0

Orbit error 1.0 1.0

other 0.5 0.5

Control stations Satellite track error 4.2 4.2

other 0.9 0.9

Propagation ^Ionosphere ^{5.0 – 10.0} ^2.3

Troposphere 2.0 2.0

Multipath 1.2 1.2

Receiver ^{Mérési zaj} ^7.5 ^1.5

Remark: this applies the pseudorange.

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Receiver clock errors

• Receivers use inexpensive quartz crystal source. The reason is to keep the receiver costs to an affordable level.

• The receiver clock error is larger than the satellite clock errors.

An error of 1 micro second (0.000001 seconds) causes a range error of about 300 metres.

• If the receiver clock is in error, the error will affect all the measurements to all satellites. The receiver clock error is identical for all satellites observed simultaneously.

• To determine the 3D position, three unbiased satellites measurements are required. To account for the receiver clock error, an additional satellite is observed.

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Coordinate systems

• Cartesian coordinate system:

– It rotates together with the Earth – geocentric

– The coordinate origin is the Earth's center of mass – the x-, y-, and z-axes in a right-handed system.

– The SI unit is meter.

– Z-axis pointing to the reference pole – X-axis is on the meridian plane

A A

A

X

r Y

Z

⎡ ⎤

⎢ ⎥

= ⎢ ⎥

⎢ ⎥

⎣ ⎦ G

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Coordinate systems (cont’)

• Spherical coordinate system:

– The spherical coordinates of a point A are then defined as follows:

• the radius or radial distance is the Euclidean distance from the origin O to A.

• the inclination (or polar angle) is the angle between the zenith direction and the line segment OA.

• the azimuth (or azimuthal angle) is the signed angle measured from the azimuth reference direction to the orthogonal projection of the line segment OA on the reference plane.

(

^{, ,}

)

A = ϕ λ h

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Coordinate systems (cont’)

• Latitude and Longitude are spherical coordinates on the surface of the earth.

• Latitude is measured North or South of the Equator.

• Longitude is measured East or West of Greenwich.

• GPS uses Latitudes and Longitudes to reference locations.

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Coordinate transformation

( ) ( )

( )

(

²

) ⁽ ⁾

cos cos cos cos

cos sin cos sin

1 sin sin

X N h R h

Y N h R h

Z e N h R h

ϕ λ ϕ λ

ϕ ϕ

= + ≅ +

= − + ≅ +

It is a conversion from one system to another :

where N, R, e characterizes the elipsoid

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Coordinate systems: WGS84

• World Geodetic System (WGS) 1984:

– The coordinate origin is meant to be located at the Earth's center of mass.

– the meridian of zero longitude is the IERS Reference Meridian.

– Z tengelye egybeesik a Föld forgástengelyének 1900-1905. évi középhelyzetével

– XY síkja a forgástengelyre merőlegesen a tömegközépponton átmenő sík – the X-, Y-, and Z-axes in a right-handed system.

• Current geodetic realizations of the geocentric reference system family International Terrestrial Reference System (ITRS) maintained by the IERS are geocentric, and internally consistent, at the few-cm level, while still being metre-level consistent with WGS 84.

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Universal Time(s)

• Greenwich Apparent Sidereal Time: is the hour angle of the vernal equinox at the prime meridian at Greenwich, England.

• Universal Time (UT1): conceptually it is mean solar time at 0° longitude (precise measurements of the Sun are difficult).

• Greenwich Mean Time (GMT): a term originally referring to mean solar time at the Royal Observatory in Greenwich, London.

• Greenwich Mean Sidereal Time: a time-keeping system astronomers use to keep track of the direction to point their telescopes to view a given star in the night sky

• Universal Time Coordinated (UTC): an atomic timescale that approximates UT1. It is the international standard on which civil time is based.

• International Atomic Time (TAI): s a high-precision atomic coordinate time standard based on the notional passage of proper time on Earth's geoid.

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Universal Time(s)

( )

1 12^h

UT = GMST −α T −

( )

T a bTu cTu² dTu³

α = + + +

Due to avarage Right ascension and precession

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Universal Time(s)

• 1980. január 6: GPS time was set to match Coordinated Universal Time (UTC) in 1980, but has since diverged

• GPST = 604800 WN + TOW

– WN (Week Number): is transmitted as a ten-bit field in the C/A and P(Y) navigation messages

– TOW (Time Of Week) or seconds-into-week number

• The difference is that GPS time is not corrected to match the rotation of the Earth, so it does not contain leap seconds or other corrections that are periodically added to UTC.

• The difference between GPS time and UTC, which as of 2011 is 15 seconds because of the leap second added to UTC December 31, 2008.

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Applications

• Military.

• Search and rescue.

• Disaster relief.

• Surveying.

• Marine, aeronautical and terrestrial navigation.

• Remote controlled vehicle and robot guidance.

• Satellite positioning and tracking.

• Shipping.

• Geographic Information Systems (GIS).

• Recreation.

• Location of inventory or permanent plots.

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NMEA protocol

• National Marine Education Association

• It is a combined electrical and data specification for communication between marine electronic devices such as echo sounder, sonars, anemometer, gyrocompass, autopilot, GPS receivers and many other types of instruments.

It has been defined by, and is controlled by, the U.S.-based National Marine Electronics Association.

• The NMEA standard uses a simple ASCII, serial communications protocol that defines how data is transmitted in a "sentence" from one "talker" to multiple "listeners" at a time. Through the use of intermediate expanders, a talker can have a unidirectional conversation with a nearly unlimited number of listeners, and using multiplexers, multiple sensors can talk to a single computer port.

• Most GPS manufacturers include special messages in addition to the standard NMEA set in their products for maintenance and diagnostics purposes. These extended messages are not standardized at all and are normally different from vendor to vendor.

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NMEA grammer

• Each message's starting character is a dollar sign.

• The next five characters identify the talker (two characters) and the type of message (three characters).

• All data fields that follow are comma-delimited.

• The first character that immediately follows the last data field character is an asterisk, but it is only included if a checksum is supplied.

• The asterisk is immediately followed by a two-digit checksum representing a hexadecimal number. The checksum is the exclusive OR of all characters between the $ and *. According to the official specification, the checksum is optional for most data sentences, but is compulsory for RMA, RMB, and RMC

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NMEA example

Sentence Description

$GPGGA Global positioning system fixed data

$GPGLL Geographic position - latitude / longitude

$GPGSA GNSS DOP and active satellites

$GPGSV GNSS satellites in view

$GPRMC Recommended minimum specific GNSS data

$GPVTG Course over ground and ground speed

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$GPGGA Sentence (Fix data)

Field Example Comments

Sentence ID $GPGGA

UTC Time 092204.999 hhmmss.sss

Latitude 4250.5589 ddmm.mmmm

N/S Indicator S N = North, S = South

Longitude 14718.5084 dddmm.mmmm

E/W Indicator E E = East, W = West

Position Fix 1 0 = Invalid, 1 = Valid SPS, 2 = Valid DGPS, 3 = Valid PPS Satellites Used 04 Satellites being used (0-12)

HDOP 24.4 Horizontal dilution of precision

Altitude 19.7 Altitude in meters according to WGS-84 ellipsoid

Altitude Units M M = Meters

Geoid Seperation Geoid seperation in meters according to WGS-84 ellipsoid

Seperation Units M = Meters

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VisualGPS

It is a NMEA 0183 GPS compliant software

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VisualGPS plots

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Real Time Location Systems

• The wireless devices are becoming more and more integrated into our daily lives.

• Wireless devices are becoming more context aware: a system is context aware if it uses contexts to provide relevant information and services (time, location, temperature, speed, orientation, biometrics, audio/video recordings, etc.) to the user, where relevancy depends on the user’s tasks.

• Between these variables that define a context, location is probably the most important inputs that define a specific situation.

• Localization serves as an enabling technology (Real Time Location Systems) that makes numerous context-aware services and applications possible (Location Based Services).

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Taxonomy of location systems

• Signaling scheme

– Infrared signal (inexpensive, low power; it is susceptible against sunlight; it cannot penetrate through obstructions )

– Optical signal (LoS, low power, it is affected by sunlight, it provides high accuracies in the short ranges (10m))

– Ultrahang jelek (high accuracies in the short range, inexpensive in LoS conditions, power hungry)

– Radio frequency (most commonly used, it can penetrate through obstacles and can propagate to long distances.)

• UWB, CDMA, OFDM, etc.

• Cellular systems, WLAN, WPAN, RFID, WSN

• Location estimation unit

– handset-based (self-positioning, eg.: GPS) – network-based (remote-positioning, eg.: WSN)

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Why is localization important?

• Very fundamental component for many other services

– GPS does not work everywhere

– Smart Systems – devices need to know where they are – Geographic routing & coverage problems

– People and asset tracking

– Need spatial reference when monitoring spatial phenomena

• In many applications we are interested in identifying the exact location:

– Where something has happened ? – Where is an Object ?

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Taxonomy of location systems (cont’)

• Localization type:

– Active Localization: system sends signals to localize target.

– Cooperative Localization: the target cooperates with the system.

– Passive Localization: system deduces location from observation of signals that are “already present”.

– Blind Localization: system deduces location of target without a priori knowledge of its characteristics.

• Centralized versus distributed

• Software-based versus hardware-based

• Relative coordinate versus absolute coordinate

• Based on performance

accuracy vs. precision, calibration, cost, energy consumption, Electrical measurements: Positioning systems

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• Position-related parameters:

– received signal strength (RSS)

P(d)=P₀−10nlog10(d/d₀) – angle of arrival (AOA)

r_i(t)=αs(t − τ_i) + n_i(t)

τ_i ≈ d/c +(l_i sin ψ)/c, ahol l_i = l(N_a+ 1)/2 − i) – time-of-arrival (TOA)

correlation based, synchronization is needed – time difference of arrival (TDOA)

Taxonomy of location systems (cont’)

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Localization algorithm

• Cell ID localization (the nearest reference node)

• Geometrical methods

– Triangulation (at least three nodes)

– Trilateration (in 2D at least three node, in 3Dat least four nodes) – Multilateration

• Statistical methods

• Fingerprint based or pattern-matching

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Computation models

• Each approach may be appropriate for a different application

• Centralized approaches require routing and leader election

• Fully distributed approach does not have this requirement

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Accuracy requirements

Applications Range Accuracy

Core apps. Sports tracking (NASCAR, horse races, soccer) 150m 10-30cm

Cargo tracking at large depots 300m 300cm

Children in large amusement parks 300m 300cm

Animal tracking 300m 150cm

Military

Military training facilities 300m 30cm

Military search and rescue: lost pilot, man overboard, coast guard rescue operations

300m 300cm

Civil Tracking guards and prisoners 300m 30cm

Aircraft landing systems 300m 30cm

Tracking firefighters and emergency responders 300m 30cm

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Measurements technologies

• Available Technologies: Bluetooth, Cellular, Satellite, Television, Wi-Fi, ZigBee, Ultra Wide Band, RFID, Infrared, Ultrasound, Laser

• Ultrasonic ToA

– Common frequencies 25 – 40KHz, range few meters (or tens of meters), avg.

case accuracy ~ 2-5 cm, lobe-shaped beam angle in most of the cases Wide- band ultrasonic transducers also available, mostly in prototype phases

• Acoustic ToA

– Range – tens of meters, accuracy =10cm

• RF ToA

– Ubinet UWB claims = ~ 6 inches

• Acoustic AoA

– Average accuracy = ~ 5 degrees (e.g acoustic beamformer, MIT Cricket)

• RSSI based localization

– WSN: Accuracy = 2-3 m, Range = ~ 10m – 802.11: Accuracy = ~3m

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Challenges in WSN

• Physical layer imposes measurement challenges

– Multipath, shadowing, sensor imperfections, changes in propagation properties (RSSI based localization)

• Extensive computation aspects

– Many formulations of localization problems, how do we solve this optimization problem? We have to solve the problem on a memory constrained processor.

– How do we solve the problem in a distributed manner?

• Networking and coordination issues

– We are using it for routing [→ see Chapter 9], it means we have routing support to solve the problem!

• System Integration issues

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Available localization systems

Technology Location method Accuracy Remarks

GPS ToA satellite based 1-5m Expensive, Not works indoors Ekahau

(WLAN)

RSS-based pattern

matching 1m No extra cost over existing wireless LAN structure, extensive utilities Microsoft

RADAR

RSS-based pattern

matching 3-4m Scalability problems, no extra cost over existing wireless LAN structure

LOKI

(WLAN) Closest AP cell size Installed as a free software. Used for locating the closest restaurant

Ubisense TDOA and AOA 30cm Maximum tag-sensor distances greater than 50m

Indoor GPS AOA 1mm

Laser positioning system for indoors.

Transmission range expandable from 2 to 300 m.

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Available localization systems: WSN

Technology Location method Accuracy Remarks

Active Badges

Infra-red-based proximity of wearable badges to predeployed sensors

Room-size

Installation costs, cheap tags and sensors, sunlight and fluorescent interference,

Active Bats Ultrasound ToA 9cm Ceiling sensor installation costs Cricket RSS and ultrasound-

based localization 1m $10 beacons and receivers, installation costs

SpotON RSS-based ad-hoc localization

Depends on cluster size

$30 per tag, inaccuracy of RSS metric

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Tracking in WSN

• Tracking mobile targets involves finding out the location of mobile targets based on wireless sensor nodes with known positions (tracing the path).

• Given the locations of the nodes and accurate range information to the target, it is straightforward to determine the target's position.

• Traditional tracking applications tend to be split into two separate phases:

– Localization phase: the network is localized using a specialized algorithm.

– Tracking phase: after localization completes, target positions are estimated based on the discovered sensor positions.

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GSM based positioning systems

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Summary

• Location is probably the most important inputs of context aware systems.

• Common characteristic of numerous location system: each of them is wireless system.

• Network-based services that integrate a derived estimate of a mobile device’s location or position with other information so as to provide added value to the user.

• Most location-based services will include two major actions: (1) Obtaining the location of a user, and (2) Utilizing this information to provide a service.

• The accuracy and precision requirements of location-based applications are highly dependent on the application characteristics.

• There are numerous localization technologies currently available which have different ranges, accuracy levels, costs, and complexities.

• Next lecture: Theoretical approach to networks and systems