Law enforcement can still be required to obtain a search warrant for access to citizens' mobile phone location data, but police need not uphold the traditional Fourth Amendment standard of "probable cause" in the process of such an investigation, a federal appeals court ruled Tuesday.

While civil liberties groups which argued in favor of stronger privacy protections largely called the ruling favorable, it still leaves room for law enforcement to continue large-scale mobile spying operations.

A friend-of-the-court brief [PDF link] in this case was given to the Third Circuit Federal Appeals Court on behalf of The American Civil Liberties Union, the ACLU of Pennsylvania, the Electronic Frontier Foundation (EFF) and the Center for Democracy and Technology (CDT).

The brief argued mobile phone users should not be subjected to a greater likelihood of government surveillance just by owning the device. Police all over the country have increasingly relied upon largely carte blanche access to mobile phone tracking data via web-based backend systems provided by major cellular carriers and third parties.

"Innocent Americans should not be made to feel the government is following them wherever they go – including in their own home," ACLU attorney Catherine Crump said, in a media advisory. "While there's no question that law enforcement agents should have the tools they need to stop crimes, such tools must be used in a manner that upholds the Constitution and personal privacy."

As Newsweek reported in February:

"How many of the owners of the country's 277 million cell phones even know that companies like AT&T, Verizon, and Sprint can track their devices in real time? Most 'don't have a clue,' says privacy advocate James X. Dempsey. The tracking is possible because either the phones have tiny GPS units inside or each phone call is routed through towers that can be used to pinpoint a phone's location to areas as small as a city block. This capability to trace ever more precise cell-phone locations has been spurred by a Federal Communications Commission rule designed to help police and other emergency officers during 911 calls. But the FBI and other law-enforcement outfits have been obtaining more and more records of cell-phone locations—without notifying the targets or getting judicial warrants establishing 'probable cause,' according to law-enforcement officials, court records, and telecommunication executives."

However, in its decision, the court made a special exception to the Fourth Amendment for mobile phone tracking, saying that requests for location records "does not require the traditional probable cause determination".

On page 25 of their decision [PDF link], the judges compare the case to United States v. Miller, where the Supreme Court decided that because banks were party to transactions, individual citizens' financial records were fair game for warrantless searches.

They held that, in the same logic behind the availability of bank records, because mobile subscribers have shared their personal information with a third party, i.e., the phone service provider, constitutional protections effectively do not apply.

The Government argues that no CSLI can implicate constitutional protections because the subscriber has shared its information with a third party, i.e., the communications provider. For support, the Government cites United States v. Miller, 425 U.S. 435 (1976), in which the Supreme Court found that an

individual’s bank records were not protected by the Constitution because “all of the records [which are required to be kept pursuant to the Bank Secrecy Act,] pertain to transactions to which the bank was itself a party,” id. at 441 (internal quotation and citation omitted), and “[a]ll of the documents obtained, including financial statements and deposit slips, contain only information voluntarily conveyed to the banks and exposed to their employees in the ordinary course of business,” id. at 442.

CNet added:

In this case, the Obama administration has argued that warrantless tracking is permissible because Americans enjoy no "reasonable expectation of privacy" in their--or at least their cell phones'--whereabouts. U.S. Department of Justice lawyers have said that "a customer's Fourth Amendment rights are not violated when the phone company reveals to the government its own records" that show where a mobile device placed and received calls.

Though the court largely agreed with the administration's argument, it also found that location data can be used by defendants to show they were in a private location that is protected by the Fourth Amendment.

The ruling effectively leaves it up to individual judges' discretion as to how mobile tracking data is used and accessed by law enforcement.

"Although the court did not definitively rule on the Fourth Amendment status of cell phone location information, it made clear that under some circumstances the privacy of such data could be constitutionally protected, and that judges have the discretion to require a warrant to avoid potentially unconstitutional seizures of location data," EFF attorney Kevin Bankston explained.