Here's a nightmare scenario for you. In order to better be able to track people, the government would require them to carry around a radio transmitter which broadcasts their location at all times. This information would be precise to 300 yards or better (much better with GPS-enabled transmitters), and would be accessible to any law-enforcement officer at all without a warrant.

Reality, of course, is different from this nightmare scenario. In reality, the devices are called "cell phones", and rather than force you to carry them, they get you to pay for the privilege.