Over the past few years, data brokers and federal military, intelligence, and law enforcement agencies have formed a vast, secretive partnership to surveil the movements of millions of people. Many of the mobile apps on our cell phones track our movements with great precision and frequency. Data brokers harvest our location data from the app developers, and then sell it to these agencies. Once in government hands, the data is used by the military to spy on people overseas, by ICE to monitor people in and around the U.S., and by criminal investigators like the FBI and Secret Service.
Link: How the Federal Government Buys Our Cell Phone Location Data