They don't need to plant a tracking device on you.

Your cellphone is a tracking device. You planted it on yourself.

The government has routinely used records of cellphone calls and caller locations to show where a suspect was at a particular time, with access to those records obtainable under a lower legal standard. (Wireless operators keep cellphone location records for varying lengths of time, from several months to years.)...

Prosecutors, while acknowledging that they have to get a court order before obtaining real-time cell-site data, argue that the relevant standard is found in a 1994 amendment to the 1986 Stored Communications Act, a law that governs some aspects of cellphone surveillance.

The standard calls for the government to show "specific and articulable facts" that demonstrate that the records sought are "relevant and material to an ongoing investigation" - a standard lower than the probable-cause hurdle.

The magistrate judges, however, ruled that surveillance by cellphone - because it acts like an electronic tracking device that can follow people into homes and other personal spaces - must meet the same high legal standard required to obtain a search warrant to enter private places.

"Permitting surreptitious conversion of a cellphone into a tracking device without probable cause raises serious Fourth Amendment concerns, especially when the phone is monitored in the home or other places where privacy is reasonably expected," wrote Stephen W. Smith, a magistrate in Federal District Court in the Southern District of Texas, in his ruling.

"The distinction between cell site data and information gathered by a tracking device has practically vanished," wrote Judge Smith. He added that when a phone is monitored, the process is usually "unknown to the phone users, who may not even be on the phone."

Very interesting and important.