It’s probably fair to assume that FBI hires the best hackers of the world, and also fair to assume that they probably hire a lot of hackers (who’ve been caught) in return for freedom instead of jail.
Anyways, it’s easy to say that FBI does have these highly sophisticated equipment but have you actually seen one in real life?
If not that’s okay too because the photos posted here and at iFixIt show you exactly that, the inside working of a real FBI GPS tracking device. There’s not much other than a battery that could in theory last 20 years. YES, 20 YEARS!!!