<P> China has proposed using this technology to track commuting patterns of Beijing city residents . Aggregate presence of mobile phone users could be tracked in a privacy - preserving fashion . </P> <P> In Europe most countries have a constitutional guarantee on the secrecy of correspondence, and location data obtained from mobile phone networks is usually given the same protection as the communication itself . </P> <P> In the United States, there is no explicit constitutional guarantee on the privacy of telecommunications, but the use of location data is limited by statutory, administrative, and case law . Law enforcement can obtain permission to position phones in emergencies where people, including criminals, are missing . In some instances, law enforcement may even access a mobile phone's internal microphone to eavesdrop on conversations while the phone is switched off . </P> <P> A secret interpretation of The Patriot Act, confirmed to exist, has been linked to secret widespread location tracking . </P>

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