111.11.2009

Positioning MARK ESSIEN

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Table of ContentsTable 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.

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 BasedPositioning

Systems

Such as:

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How do we do get locations?Location Techniques

TrackingExample: Infrared

Example: RFID

PositioningExample: GPS

Example: WIPSDevice

discovers its own position

Sensor Network tracks

the device

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Basic techniques to discover locations

Cell of Origin

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Basic techniques to discover locations

Time of Arrival

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Basic techniques to discover locations

Angle of Arrival

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Basic techniques to discover locations

Measuring Strength

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Basic techniques to discover locations

Processing Video Data

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Satellite Positioning2

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

Disadvantages•Expensive to launch satellite•Cannot position indoors

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Basic Mechanism of Satellite Navigation

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.

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Basic Mechanism of Satellite Navigation

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)

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What about the distance r from the satellite?

r = c * tr = Distance from Satellitec = Speed of Lightt = Time it took signal to reach device

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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

•A system of 21 to 30 satellites in orbit around the earth and providing positioningInformation.• Conceived in 1970, satellites launched in 1984, 12 working satellites by 1990•Full operational capability in July, 1995•Made up of 3 segments

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Space Segment / The GPS Satellite

• 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.

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User Segment / The GPS receiver

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Control Segment /GPS Master Control Station

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

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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 insatellite orbit, disturbances of theatmosphere/ionosphere and multipath errors

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Improving GPS accuracy with DGPS

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

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Correction Stations in Australia

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.

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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

•Abandoned in 2007 by industry, taken over 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 Positioning3

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Network Based Positioning

Using existing networks for positioning

The GSM network Wireless LAN networks

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• 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

Overview

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Sony Ericsson developed the MPS (MobilePositioning 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

Improving Accuracy

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• 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

WLAN: Overview

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Skyhook

•Used in iPhone 2G to discover the location of users even withouta GPS radio

•Reasonable accurate

•Company drives around in metroareas and maps available wirelessLAN devices and signal strengthsto the GPS location

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Coverage (Skyhook)

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Indoor Positioning4

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Indoor Positioning

Indoor positioning does not work with satellitesbecause 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:

InfraredUltrasound VideoRadio

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1. Infrared Beacons

Infrared devices are cheap and easy to get, and so are often used for indoorpositioning. Two sample system are the Active Badge System and the WirelessIndoor 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

•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

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2. Radio BeaconsUsing radio, time of arrival as well as signal strength methodscan be used to compute positions in buildings (in a mannersimilar to satellites). Positioning in 3 dimensions becomespossible 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

Ultrasound devices use ultrasound transmitters to transmitthe position of a user. Sensorsare placed in the building, andthe transmitter sends ultrasound signals on request.

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The Active Bat System

•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

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4. Video Based Systems

Indoor positioning canalso be achieved bycapturing andprocessing video data torecognize and positionobjects. This method iscomputing intensive.

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Using Visual Tags

• 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

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Positioning

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

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Conclusions & Perspectives5

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ConclusionIn conclusion, positioning is an importanttechnology that is of critical importance inlocation based systems. We have gone throughthe most important methods in thispresentation, and imparted an understanding ofhow modern positioning works.

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Future of positioning

There are still a lot of problems left in the positioningarea. 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

markessien@gmail.com

Thank you

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References• Data Collection

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

• Positioning Systems Jörg RothChapter 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.htmAccessed 11.11.2009