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Localization is a fundamental technology embedded — often unbeknownst to users — in many devices and services. First, this book reveals how localization systems are driving modern society. The core focus is then on the Global Positioning System (GPS), which is constantly rising in importance due to its global availability and low receiver cost. Still, high energy consumption and susceptibility to fake or reflected signals are challenges for GPS receivers. The limited endurance of battery-powered GPS receivers can be addressed by snapshot receivers which require as little as one millisecond of received satellite signals to determine their location. Using short signals reduces the active time and thus the energy consumption of a GPS receiver drastically. But short signals reduce the received signal energy, which impairs the localization robustness. The collective detection maximum likelihood method performs well with short signals. Furthermore, a tiny and low power snapshot GPS receiver hardware design allows location tracking for months or years using a coin cell. Spoofed GPS satellite signals pose a threat to an increasing number of critical systems relying on GPS. Determining multiple likely localization solutions instead of only one enables correct receiver localization despite malicious signals. Finally, an experimental localization system is presented that leverages status messages transmitted by aircraft. It aims at combining the ubiquitous availability and low receiver cost of GPS with the high signal strength of local localization systems like those based on WiFi or LTE signals. As such, it gives the prospect of robust and possibly indoor localization without the high setup costs of short- range localization systems. This book is suitable for readers with a technical background, but no prior knowledge of localization systems. Readers familiar with GPS can skip the introductory chapters and directly dive into the advanced material.