If you have a cell phone and it is turned on your cell phone company knows where it is within a few hundred yards. If the government wants this data from the provider it generally (emergency exemptions apparently exist) has to get a court order.
However, the FBI has the technology to trigger phones into giving up their serial numbers and their phone numbers. This does not require a court order! Currently they use a van to drive around town and try to find their target. But that doesn’t have to be the way they always do it. All they have to do is put this technology on major travel routes and in travel hubs and they can do a pretty good job of tracking a large proportion of the population. If they put up their own devices on most of cell towers around the country they can track every active cell phone in the country.
I write software for mobile phones for Microsoft and it is rare that a day goes by where we don’t talk about and implement privacy protections for the customer and now I find out the Federal Government is actively working to defeat us.