The government has the ability to track cellphones using the portable device pictured above called the Stingray — it was recently revealed in a criminal case in Arizona, but the government doesn't want anyone to know how it works. When the judge in the case asked for more information about the Stingray in order to determine if its use requires a search warrant, the government filed a memo basically arguing both ways: it said Stingray use generally doesn't require a warrant, but concedes that one was required in this specific instance — a huge concession that could cost them the case, just so the Stingray's design and functionality remain a secret.
Although the government's lawyers are willing to tie themselves in knots trying to conceal the Stingray, we do have some information on how it works: experts told the WSJ that it mimics an actual cell tower pinging for a specific device, and the data can be used to triangulate a phone's location. It can be concealed in the back of a van and measure the distance to any type of cell phone from multiple locations — circles drawn from each point will intersect within 100 meters of the phone's location. Our FBI contact told us that tracking a cellphone normally requires a wireless provider's cooperation, which could take weeks to obtain — the Stingray simplifies investigations because cell towers aren't needed. We'll see what happens — if it comes down to keeping the Stingray a secret or allowing law enforcement to track anyone they want without a warrant, we suppose we prefer the first.