In an excellent report explaining the technology behind GPS systems and its implications this report examines cell phone location data and its uses and risks.
The report noted, "The number of mobile phone subscriptions in the United States grew from about 3.5 million in 1989 to approximately 291 million by the end of 2009, according to the most recent Federal Communications Commission (FCC) data.1 The Centers for Disease Control and Prevention reported that one-third of U.S. households had mobile phones but no landline phones as of December 2011.2 Increasingly, Americans’ mobile phones are smartphones, which use advanced operating systems to provide computing functions, including Internet access and a variety of applications, in addition to basic voice service. According to The Nielsen Company, as of June 2012, smartphones accounted for just over half of all mobile phones in the United States, up from less than one-quarter in early 2010."
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