A dozen years ago, when the first edition of this book appeared, stories like the one above were plausible. Now, given the right equipment* and a map of the area, you could be led blindfolded to any spot in the great out-of-doors and determine exactly where you were.† This happy capability is due to some ingenious electronics and a dozen billion dollars‡ spent by the U.S. government. I refer to NAVSTAR (NAVigation System with Time And Ranging; informally the “Navigation Star”)—a constellation of from 24 to 32 satellites orbiting the Earth, broadcasting data that allows users on or near the Earth to determine their spatial positions. The more general term in the United States for such an entity is “Global Positioning System” or “GPS.” The Russians have such a navigation system as well, which they call GLONASS (GLObal Navigation Satellite System). (One might reflect that, for some purposes, the cold war lasted just long enough.) A more general, recent acronym for such systems is GNSS, standing for Global Navigation Satellite Systems. In the Western world, GPS usually implies NAVSTAR, so I will use the two designations interchangeably in this text.