Location Systems An Introduction to the Technology Behind Location Awareness iii Synthesis Lectures on Mobile and Pervasive Computing Editor Mahadev Satyanarayanan, Carnegie Mellon University Mobile computing and pervasive computing represent major evolutionary steps in distributed sys- tems, a line of research and development that dates back to the mid-1970s. Although many basic principles of distributed system design continue to apply, four key constraints of mobility have forced the development of specialized techniques. These include unpredictable variation in network quality, lowered trust and robustness of mobile elements, limitations on local resources imposed by weight and size constraints, and concern for battery power consumption. Beyond mobile computing lies pervasive (or ubiquitous) computing, whose essence is the creation of environments saturated with computing and communication yet gracefully integrated with human users. A rich collection of topics lies at the intersections of mobile and pervasive computing with many other areas of com- puter science. RFID Explained Roy Want 2006 Controlling Energy Demand in Mobile Computing Systems Carla Schlatter Ellis 2007 Application Design for Wearable Computing Dan Siewiorek, Asim Smailagic, and Thad Starner 2008 Location Systems: An Introduction to the Technology Behind Location Awareness Anthony LaMarca and Eyal de Lara 2008 Copyright © 2008 by Morgan & Claypool All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form or by any means—electronic, mechanical, photocopy, recording, or any other except for brief quotations in printed reviews, without the prior permission of the publisher. Location Systems: An Introduction to the Technology Behind Location Awareness Anthony LaMarca and Eyal de Lara www.morganclaypool.com ISBN: 9781598295818 paperback ISBN: 9781598295825 ebook DOI: 10.2200/S00115ED1V01Y200804MPC004 A Publication in the Morgan & Claypool Publishers series SYNTHESIS LECTURES ON MOBILE AND PERVASIVE COMPUTING #4 Lecture #4 Series Editor: Mahadev Satyanarayanan, Carnegie Mellon University Series ISSN ISSN: ISSN1933-9011 print ISSN: 1933-902X electronic Location Systems An Introduction to the Technology Behind Location Awareness Anthony LaMarca Intel Corporation Eyal de Lara University of Toronto SYNTHESIS LECTURES ON MOBILE AND PERVASIVE COMPUTING #4 vi ABSTRACT Advances in electronic location technology and the coming of age of mobile computing have opened the door for location-aware applications to permeate all aspects of everyday life. Location is at the core of a large number of high-value applications ranging from the life-and-death context of emer- gency response to serendipitous social meet-ups. For example, the market for GPS products and services alone is expected to grow to US$200 billion by 2015. Unfortunately, there is no single location technology that is good for every situation and exhibits high accuracy, low cost, and universal coverage. In fact, high accuracy and good coverage seldom coexist, and when they do, it comes at an extreme cost. Instead, the modern localization landscape is a kaleidoscope of location systems based on a multitude of different technologies in- cluding satellite, mobile telephony, 802.11, ultrasound, and infrared among others. This lecture introduces researchers and developers to the most popular technologies and sys- tems for location estimation and the challenges and opportunities that accompany their use. For each technology, we discuss the history of its development, the various systems that are based on it, and their trade-offs and their effects on cost and performance. We also describe technology-independent algorithms that are commonly used to smooth streams of location estimates and improve the ac- curacy of object tracking. Finally, we provide an overview of the wide variety of application domains where location plays a key role, and discuss opportunities and new technologies on the horizon. KEYWOrds localization, location systems, location tracking, context awareness, navigation, location sensing, tracking, Global Positioning System, GPS, infrared location, ultrasonic location, 802.11 location, cellular location, Bayesian filters, RFID, RSSI, triangulation vii Contents 1. Introduction .......................................................................................................1 1.1 Lecture Overview ................................................................................................ 3 2. The Global Positioning System ............................................................................5 2.1 GPS Origins ........................................................................................................ 6 2.2 System Architecture ............................................................................................ 7 2.2.1 Earth-Orbiting Satellites ......................................................................... 7 2.2.2 Ground Stations ...................................................................................... 8 2.2.3 GPS Receivers ......................................................................................... 8 2.3 Basic GPS-Positioning Algorithm ...................................................................... 9 2.3.1 Satellite Range Estimation .................................................................... 10 2.3.2 Satellite Coordinate Estimation ............................................................ 11 2.4 GPS Errors and Biases ...................................................................................... 12 2.4.1 Ranging Errors ...................................................................................... 12 2.4.2 Satellite Geometry Errors ...................................................................... 13 2.5 Differential GPS ............................................................................................... 15 2.5.1 Real-Time Differential GPS ................................................................. 15 2.5.2 Real-Time Kinematic ............................................................................ 15 2.6 Future GPS Enhancements ............................................................................... 16 2.7 Other Global Navigation Satellite Systems ....................................................... 16 2.8 Summary ........................................................................................................... 18 3. Infrared and Ultrasonic Systems ........................................................................ 19 3.1 Room-Level Localization via Proximity ............................................................ 19 3.2 Subroom Accuracy With Ultrasonic Time of Flight ......................................... 23 3.3 Absolute Location With Time of Flight and Angle of Arrival ......................... 25 3.4 Comparison of Approaches ............................................................................... 28 4. Location Estimation With 802.11 ...................................................................... 33 4.1 Signal Strength Fingerprinting ......................................................................... 34 4.2 Signal Strength Modeling ................................................................................. 39 viii LOcatiON SYstems 4.2.1 Constructing the AP database ............................................................... 41 4.3 Privacy Considerations ...................................................................................... 43 4.4 Improvements and Variants ............................................................................... 43 5. Cellular-Based Systems ..................................................................................... 49 5.1 Cell ID-Based ................................................................................................... 49 5.2 Radio Modeling Approaches ............................................................................. 50 5.3 Assisted GPS ..................................................................................................... 52 5.4 Signal Strength Fingerprinting ......................................................................... 54 5.5 Standardization Efforts and Discussion ............................................................ 57 6. Other Approaches ............................................................................................. 59 6.1 Instrumented Surfaces ....................................................................................... 59 6.2 Vision ................................................................................................................ 61 6.2.1 Mobile Cameras .................................................................................... 61 6.2.2 Fixed Cameras ....................................................................................... 61 6.2.3 Visual Tags ............................................................................................ 64 6.2.4 Practical Considerations ........................................................................ 65 6.3 Laser Range Finders .......................................................................................... 65 6.4 Audible Sound ................................................................................................... 66 6.5 Internet Protocol Measurement ........................................................................