C H A P T E R 1
Fifty years of developments in electronic location technology, coupled with the advent of mobile
computing, fueled by smaller, less expensive, and more capable devices, have opened the door for
location-aware applications to permeate all aspects of everyday life. Location is at the core of a
number of high-value applications including emergency response, navigation, asset tracking, ground
surveying, and many others. The market for GPS products and services alone is expected to grow to
US$200 billion by 2015 [141]. Location is also commonly used to infer other contexts. For example,
symbolic location is often a good proxy for activity (e.g., grocery store is indicative of shopping).
Similarly, social roles and interactions can be learned from patterns of colocation [39], and physical
activities and modes of transportation can often be inferred from the changes in coordinate-based
location [129, 157].
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. The modern localization landscape is
thus populated by a kaleidoscope of location systems based on a multitude of different technologies
including satellite, mobile telephony, 802.11, ultrasound, and infrared among others.
The aim of this lecture is to inform researchers and developers about the most popular tech-
nologies and systems for location estimation, and the challenges and opportunities that accompany
their use. Throughout this lecture, we will illustrate the trade-offs made by various systems and
describe the effect on cost and performance. For each technology, we introduce the history of its
development, the various systems that are based on it, and the performance characteristics of those
systems. We characterize each of the technologies we discuss across the following dimensions:
Accuracy: This is the most often-cited metric of location systems and refers to the correct-
ness of a systems location estimates. For coordinate-based location systems, accuracy is
expressed as an error distribution, typically in centimeters or meters. It is most common
for location accuracy to be expressed as a median error” which indicates that 50% of the
location estimates are at least that accurate. Systems that exhibit normally distributed error
often have their error expressed in meters at 1 (or 2) sigma, meaning that 66% (or 95%) of

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