In United States v. Skinner, a Sixth Circuit panel held that repeatedly pinging a cell phone in order to obtain its GPS coordinates (or something like that) was not a Fourth Amendment search and thus does not necessitate evidence suppression. No. 09-6497 (6th Cir., Aug. 14, 2012). In a concurring opinion, one judge argued that obtaining the data was a search, but the good faith rule saves the evidence from suppression.
Before trial, the defendant sought to suppress the search, arguing that the "use of GPS location information emitted from his cell phone was a warrantless search that violated the Fourth Amendment." The evidence was not suppressed, and he was found guilty on multiple counts.
On appeal, the Sixth Circuit concluded that no reasonable expectation of privacy existed "in the data given off by [the] voluntarily procured pay-as-you-go cell phone." The court continued:
If a tool used to transport contraband gives off a signal that can be tracked for location, certainly the police can track the signal. The law cannot be that a criminal is entitled to rely on the expected untrackability of his tools.... It follows that Skinner had no expectation of privacyThe court considered this situation simply an advancement of the Supreme Court's 1983 decision in Knotts as it is law enforcement adapting to technological change. They also distinguished the case from Jones because no physical intrusion occurred in Skinner. "Skinner himself obtained the cell phone for the purpose of communication, and that phone included the GPS technology used to track the phone’s whereabouts." Further, the court noted that Justice Alito's concurrence also does not apply. Alito suggested that "relatively short-term monitoring" of movements may not violate the Fourth Amendment, and Skinner was only tracked for three days (opposed to 28 days in Jones).
in the context of this case, just as the driver of a getaway car has no expectation of privacy in the particular combination of colors of the car’s paint.
In a concurring opinion, Judge Donald argued that Skinner did have a reasonable expectation of privacy in the GPS data, making the act by law enforcement here a search. However, she also noted that the Leon good faith exception would prevent the need for suppression "because the officers had probable cause to effect the search in this case and because the purposes of the exclusionary rule would not be served by
If my analysis of the technology doesn't make sense, it's because the facts in the opinion leave a lot to be desired. This might be about cell site location data. The fact that Skinner was tracked to his home would make you think it was GPS data due to the accuracy, and the court called it GPS throughout. The court also referenced agents pinging the phone. No one - including the court, apparently - is really sure what was happening here. Check out Professor Kerr's discussion of this issue here (and the reader comments).
UPDATE: Professor Kerr dug into the case a little more. Be sure to read what he came up with on Volokh Conspiracy.