That’s called Stingray; there are many FR articles on it.
https://www.aclu.org/map/stingray-tracking-devices-whos-got-them
“Stingray Tracking Devices: Who’s Got Them?”
“The map below tracks what we know, based on press reports and publicly available documents, about the use of stingray tracking devices by state and local police departments. Following the map is a list of the federal agencies known to have the technology. The ACLU has identified 71 agencies in 24 states and the District of Columbia that own stingrays, but because many agencies continue to shroud their purchase and use of stingrays in secrecy, this map dramatically underrepresents the actual use of stingrays...