According to the Associated Press, police are able to track your cell phone with a device called Stingray. The mechanism is a suitcase-sized tool manufactured by the Harris Corporation that mimics a cell phone tower.
There is not much known about the device, but back in 2011 the city of Miami inadvertently published the price and description of a unit in some other budget items they posted online. The info was removed, but not before it was downloaded by the folks at PublicIntelligence.net [PDF].
That price list describes the Stingray as a "4-channel multi-transmit interrogation and direction-finding transportable unit."
The price for the base model Stingray II was $148,000. There are several software packages that are also available that go with the unit. Those cost around $22,000 each. There is one software package that is restricted to only Federal "customers" that costs $50,000. Other accessories are listed, including cables for around $200, a computer mouse for $50, a 20-foot antenna mast for $2,990, and a choice of regular and "ruggedized" laptops to run it all with.
It is not known if the City of Miami purchased the Stingray system, but if they did, they can now track cell phone users.
In one court case, a Federal agency did acknowledge that the Stingray can affect the cell phone functions of innocent civilians, who are not under investigation, in the area in which it is being used.
Not much detail is known. What can be divined is that the unit tricks a cell phone into thinking that it is a cell tower. The cell phone then transmits all the usual info to the unit, allowing police to track the phone. This is simpler than triangulation of tower signal. Anyone who has been stuck with a phone that does not have GPS, but tries to approximate it with cell tower triangulation (ahem - Blackberry Curve), can tell you that triangulation only gets you so far.
But what if this Stingray can also catch data, not just voice? Location services info from phones to apps is common. If the Stingray can intercept what your phone is sending to, say, Facebook (which uses that info for "check-ins"), then police can follow you within 7.8 meters of where you stand (according to GPS.gov).
Much other information about the Stingray unit is kept confidential. Federal and local law enforcement is required by Harris Corp. to sign non-disclosure agreements about its capabilities. Police records that mention it are often redacted when they are made public. Lots of people question the wisdom - and legality - of a corporate NDA superseding American rights and transparency laws.
Image via Harris Corp.