A stingray is a mobile surveillance device that presents itself as a cell tower, appearing to be trusted infrastructure so that targeted smartphones connect to it automatically.
The device, also known as an IMSI (International Mobile Subscriber Identity) catcher or a cell site simulator, is used in government and law enforcement operations. Stingrays may be placed in fixed locations or can be mobile in a vehicle or a drone. In addition to detecting the location of targeted users, the devices can also intercept calls and texts from targeted phones and others within the target area. According to privacy groups, stingray devices don't target precisely but can collect data from thousands of phones when used in an urban area.
In September 2015, the United States Department of Justice announced a policy requiring that law enforcement representatives obtain warrants before tracking mobile phones. The DOJ’s "Enhanced Policy for Use of Cell-Site Simulators" also stipulates requirements for handling the data collected by the devices, including an auditing program to ensure that data is deleted properly. Furthermore, according to the policy, the devices cannot legally collect other types of communication data, such as emails, texts, contact lists and images.
See a tutorial on cell site simulators: