"A device that records relentlessly your every movement in time and space."
This is how a Supreme Court Justice described GPS in a landmark ruling last August, which will have huge ramifications going forward for how GPS data can be used in legal cases.
Supreme Court to Decide Legality of GPS Tracking Without Warrant
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The U.S. Supreme Court today said it would decide whether law enforcement authorities must get a warrant to use GPS tracking equipment. In its term that starts next October, the high court will review a ruling by the U.S. Court of Appeals in Washington, D.C., that said the federal government violated the Fourth Amendment rights of a former night club owner, Antoine Jones, by attaching a GPS device to his vehicle in a case of suspected drug trafficking without first obtaining a warrant.
Legal Times reported at least two other federal appellate circuits have found that the use of GPS tracking over a long period of time is not a "search" under the Fourth Amendment. Federal prosecutors argued the police did not need a warrant to track Jones' travels because he was moving freely on public roads in the District of Columbia and in Maryland. The authorities initially had a warrant, but it expired. "It is one thing for a passerby to observe or even to follow someone during a single journey as he goes to the market or returns home from work," said Judge Douglas Ginsburg. "It is another thing entirely for that stranger to pick up the scent again the next day and the day after that, week in and week out, dogging his prey until he has identified all the places, people, amusements, and chores that make up that person's hitherto private routine." Here is a link to the original article / criminal case.