Positioning MAR MARK K ESS ESSIEN IEN Seminar Location-based - - PowerPoint PPT Presentation

11.11.2009

Positioning

MAR MARK K ESS ESSIEN IEN

Seminar Location-based Services (S19567) Instructor: Prof. Dr. Agnès Voisard

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Table of Contents Table of Contents

• Introduction

A quick overview of positioning systems

• Satellite Based Positioning

Positioning with GPS

• Network Based Positioning

GSM and Wireless LAN based positioning techniques

• Indoor Positioning

Infrared, Radio, Ultrasound, and Video based techniques

• Conclusion and Future Perspectives

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INTRODUCTION TO POSITIONING

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What are positioning systems?

These are systems that allow us detect the location of a person or object.

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There are lots of different such systems in different

• areas. Here we will talk about the systems we

need for Location Based Services.

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Indoor Positioning Systems Satellite Positioning Systems Network Based Positioning Systems

Such as:

How do we do get locations?

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Location Techniques Tracking Example: Infrared Example: RFID Positioning Example: GPS Example: WIPS

Device discovers its

• wn position

Sensor Network tracks the device

Basic techniques to discover locations

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Cell of Origin

Basic techniques to discover locations

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Time of Arrival

Basic techniques to discover locations

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Angle of Arrival

Basic techniques to discover locations

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Measuring Strength

Basic techniques to discover locations

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Processing Video Data

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Satellite Positioning

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Advantages

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• Can be used anywhere on earth
• Not disturbed by enviromental conditions
• Precise

Disadvantages

• Expensive to launch satellite
• Cannot position indoors

Basic Mechanism of Satellite Navigation

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The user knows the distance of the satellite to him, as well as the position of the satellite So he can calculate a radius is somewhere on. But he does not know where on the radius he is.

Basic Mechanism of Satellite Navigation

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By looking at the intersection of the coverage radius of at least 3 satellites, he can discover his exact position

How does the device know the position of the satellite?

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Satellites are on fixed, known orbits. Additionally, the position of all satellites is updated by sending a so-called almanac with currrent position information to the device.

(By the way, if you don’t use your GPS device for a long time, it needs to download this almanac, which is why it takes much longer to start)

What about the distance r from the satellite?

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r = c * t

r = Distance from Satellite c = Speed of Light t = Time it took signal to reach device

Problems

• The speed of light is very high. An error of 1 μs

leads to a 300m inaccuracy

• Satellites have atomic clocks, and so their times

are very accurate, but the devices do not. The clocks are hence not synchronised.

• To correct the unknown factor, a fourth satellite

signal is drawn in, and equations that factor in the time offset built

• These non-linear equations can be calculated

using Kalman filters or Taylor series

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Global Positioning System

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• A system of 21 to 30 satellites in orbit around the earth and providing positioning

Information.

• Conceived in 1970, satellites launched in 1984, 12 working satellites by 1990
• Full operational capability in July, 1995
• Made up of 3 segments

Space Segment / The GPS Satellite

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• Weighs between 1.5 and 2 tons
• Energy supplied by solar cells
• Central computer is a 16 Mhz CPU
• Expected lifetime of 7.5 years
• 12 hours for an orbit
• 60 days to launch after failure
• Programmed in ADA
• About 25.000 lines of code. In comparison,

Microsoft Office has 30 million lines of code.

User Segment / The GPS receiver

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Control Segment /GPS Master Control Station

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Schriever Air Force Base, near Colorado Springs, U.S.A

GPS Properties

• SPS (Standard Positioning Service): Available

for civilian users – Less Accurate (100m horizontal)

• PPS (Precise Positioning Service): 22m
• horizontal. For military use
• Data channel with 50bps

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GPS Accuracy

Accuracy affected by clock errors, fluctuation in satellite orbit, disturbances of the atmosphere/ionosphere and multipath errors

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Improving GPS accuracy with DGPS

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DGPS involves a system of base stations with fixed, known positions that broadcast correctional information to the devices directly.

Correction Stations in Australia

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AMSA's Differential Global Positioning System provides a network of radio beacons that improve the accuracy and integrity of the Global Positioning System (GPS) around selected areas of Australia's coast.

Improving GPS accuracy with WAAS

• Base stations calculate correction data, then

transmit it to geostationary satellites, who then pass it on to the devices

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Selective Availability

• Artificial Distortion of GPS signal by U.S

government

• Switched off in 2000
• New Satellites being launched 2009 do not

have this capability any more

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Other Satellite Based Navigation Systems

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• Abandoned in 2007 by industry, taken
• ver by EU
• Slated for operation start by 2013
• Bases in Germany and Italy
• Partners include China, Israel, Ukraine,

Morroco, South Korea

• GLONASS – Russian alternative to GPS
• Launched in 1996
• Financial problems, only 10 satellites

by 2000

• Partnership with India, 12 satellites

added by 2008

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Network Based Positioning

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Network Based Positioning

Using existing networks for positioning

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The GSM network Wireless LAN networks

• Already covers a wide area
• User knows his position already, based off the

cell he is in

• However, cell accuracy from 1km to 35km, so

very inaccurate

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Overview

Sony Ericsson developed the MPS (Mobile Positioning System) that improves the accuracy. It does so using multiple methods.

– Detect Cell – Detect the segment antennae user is, allowing an angle of antennae to user – Use Timing Advance to determine distance. Accuracy is circa 555m – Signal runtimes to 4 base stations

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Improving Accuracy

• Position can be detected by measuring signal

strength of all wireless LAN access points

• Requires a training phase where the locations

are mapped to signal strengths

• Realized as prototype by Microsoft, as well as

with the Nibble system

• Outdoor variant in use in the iPhone 2G, using

a service by the company SkyHook

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WLAN: Overview

Skyhook

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• Used in iPhone 2G to discover

the location of users even without a GPS radio

• Reasonable accurate
• Company drives around in metro

areas and maps available wireless LAN devices and signal strengths to the GPS location

Coverage (Skyhook)

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Indoor Positioning

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Indoor Positioning

Indoor positioning does not work with satellites because the radio signals do not penetrate the

• walls. So other systems are required. The indoor positioning

devices can be split up into categories based on the technology in use:

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Infrared Ultrasound Video Radio

• 1. Infrared Beacons

Infrared devices are cheap and easy to get, and so are often used for indoor

• positioning. Two sample system are the Active Badge System and the Wireless

Indoor Positioning System (WIPS).

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The Active Badge System

• Every user carries a transmitter in the

shape of a card

• Infrared signal with pulse length 0.1s is

sent every 15 seconds

• Signal identifies user with unique code
• Low cost with a long battery life

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Active Badge: Technology

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• Signals do not penetrate walls, but rather reflect of walls, so can receive signals

even without line of sight

• Signals flood room, so positioning is accurate to room level
• Long battery life because the pulse duration very short compared to idle time
• Signals do not collide because of low differences in pulse duration

• 2. Radio Beacons

Using radio, time of arrival as well as signal strength methods can be used to compute positions in buildings (in a manner similar to satellites). Positioning in 3 dimensions becomes possible if transmitters are on multiple floors.

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The SpotON System

• Uses the strength of the signals at the spot from where

the measurement occurs

• The signal strength is assumed to decrease with the

square of the distance. This is not always the case, however, as there may be obstacles.

• Accuracy of 3m can be achieved with this

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RFID

• Small systems with processor,

memory, antennae, but without power supply

• Use the energy from the radio waves
• Distance of 1 meter
• Often used to track objects – can be

used to find out if an object has passed a certain point

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• 3. Ultrasound

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Ultrasound devices use ultra sound transmitters to transmit the position of a user. Sensors are placed in the building, and the transmitter sends ultra sound signals on request.

The Active Bat System

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• The server transmits a send request by radio to the so-called “Bat”. A

specific Bat is always chosen to avoid collisions

• The bat replies with an ultra-sonic impulse
• Receivers on the ceiling, which are spread out at 1.2m intervals, receive

the signal

• The receivers pass the signals on to a location server via a wired network
• Position is calculated similar to satellite positioning – non-linear system of

equations using signal runtimes. Runtimes of ultrasound are much lower than for radio signals, leading to simpler calculations

• 4. Video Based Systems

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Indoor positioning can also be achieved by capturing and processing video data to recognize and position

• bjects. This method is

computing intensive.

Using Visual Tags

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• The evaluation of the video data

can be simplified with the use of visual tags

• The tags can store information,

for example by the position of squares to each other

• The size of the tag can be used as

a reference for the distance of the user to the camera

Positioning

If two cameras detect the same tag, they can pinpoint the users location using triangulation

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Conclusions & Perspectives

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Conclusion

In conclusion, positioning is an important technology that is of critical importance in location based systems. We have gone through the most important methods in this presentation, and imparted an understanding of how modern positioning works.

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Future of positioning

There are still a lot of problems left in the positioning

• area. In particular:
• The speed of position detection can be improved
• Power consumption issues should be solved
• Accuracy should get better, independent of the

location

• Ideal positioning systems should combine the

different methods available

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markessien@gmail.com

Thank you

References

• Data Collection

Jörg Roth In Jochen Schiller, Agnès Voisard (eds), Location-Based Services, Morgan Kaufmann Publishers, May 2004

• Positioning Systems

Jörg Roth Chapter 3 of the thesis "A Decentralized Location Service Providing Semantic Locations“ (habilitation), 2005

• Fact Sheet: Differential Global Positioning System (DGPS)

Australian Maritime Safety Authority Canberra, ACT Australia August 2007

• Website: How it Works – Skyhook Wireless: http://www.skyhookwireless.com/howitworks/

Accessed 11.11.2009

• Website: Glonass Official page: http://www.glonass-ianc.rsa.ru.

Accessed 11.11.2009

• Website: Galileo: http://ec.europa.eu/transport/galileo/index_en.htm

Accessed 11.11.2009

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