http://www.washingtonpost.com/world/national-security/secrecy-around-police-surveillance-equipment-proves-a-cases-undoing/2015/02/22/ce72308a-b7ac-11e4-aa05-1ce812b3fdd2_story.html
Stingray pretends to be a cellphone tower, and then uses jamming of certain frequencies to push your phone to use the Stingray device rather than a local cell phone tower.
http://www.washingtonpost.com/world/national-security/how-a-stingray-works/2015/02/23/b4cfcce2-bb70-11e4-8668-4e7ba8439ca6_graphic.html
Letter of Senator Bill Nelson to Feds asking WTF?
http://www.commerce.senate.gov/public/index.cfm?p=PressReleases&ContentRecord_id=9c857ed7-dd75-49db-9101-5aa429f91689