Slight tangent - there was a small kerfuffle recently when a guy found an GPS tracking device in his car. He posted pics asking what it was, then FBI agents then appeared and demanded the return of their property. People are upset because his car was bugged and his movements followed without any kind of warrant. Courts are split as to whether you even need a warrant to bug a car, because roads are public therefore you can't claim an expectation of privacy against government search, but the bug is more intrusive than just following a car.
Well, bugs like that will eventually be a thing of the past. They simply won't be needed. The new system will look like this:
High above a city, there will be an eye in the sky. Initially this might be an aeroplane, but that's a dumb way to do it. Eventually the task will be carried out by solar "stratellite" - a small unmanned electric blimp, solar powered, maintaining a GPS-fixed location and altitude. With no need to come down for fuel or food, the device could eventually become a permanent fixture in the sky.
The eye in the sky is like a digital camera, but its resolution is measured in gigapixels (current designs are just one or two GP), and it views at a lower frequency of light than the visible spectrum, thus it can more easily see through clouds and other weather systems. It takes a photo every couple of seconds.
So, quick summary:
1) A realtime google-earth view of the city (in monochrome, not colour), but otherwise about the same level of detail or better.
2) It is video, rather than a photo (low framerate)
3) The video is stored. You can thus rewind the entire activity of anyone and everyone in the entire city, fast forward, whatever. Thus it becomes a time machine.
4) This surveillance (of everyone, all the time) is already legal.
Intuitive-but-dumb objections include the idea that if you exit a building amongst a crowd of other people, there is no-way that you could be personally identified - you're just a few pixels, a little blob just like everyone else, so what's the point? But obviously, that's just failing to think it through - the Time Machine neatly solves the problem. Pick a random person of hundreds of anonymous people pouring out of a shopping mall on a Saturday afternoon. Want their ID? Fast-forward to where they go to their car. Rewind until that car arrives in that parking spot, follow that car backwards through time until it exits the driveway of the owner's home. Look up the address. Not only is the shopper identified, but it was a trivial task to do so. (Thus, computer automation will eventually do most or all of such identification).
Similar intuitive but not-thought-out objections are that people would get shuffled and lost as groups moved in and out of sight, or that clouds would block the view, or that storing so much data would be expensive, or that (hand-waving) because it just sounds too... science-fictiony!
(It's not science-fictionary, it's a very basic and obvious use for fairly mundane real-world technology.)
Once it is in place there are no need for GPS tracking devices because everyone has already been tracked, and when you want to investigate someone (say, to get the details of a random pretty girl you passed on your way to work, find out where and with whom she goes for fun, go to those places too, and just randomly happen to "meet" her, and just happen to share all her interests), you don't need to start following them, because you've already being following them for years, you can just rewind their life.
I expect the first of these devices will be deployed by the US military over cities in warzones or under tension, probably within 10 years, but since there is no legal reason to not set them up for domestic spying, and no shortage of police and agencies that will be crying out to set these up domestically, I'd expect to see them used domestically within 20 years.
And the best bit, of course, is that the omnipotence the time machines gives to its operators is illusionary - but many won't really be competent enough to act accordingly. Much like the prosecution of Iraq by going public with satellite images of Iraq's WMD, when there were none, if you meet a friend of a friend to drop off the cellphone they left at your place at the party the other night, that's going to make great drug-deal-going-down footage. Even as it helps solve some crimes, the time machine will exacerbate investigation incompetence and laziness, and vaporize privacy.
Even today, privacy is a myth, but it's a myth people still believe in, and something people will still fight for. When a generation grows up having never known that the world has ever been different, with no expectation of or experience with privacy, I think the concept will largely die out. Modern living will mean things like having electricity and internet and running water and other people being able to know what you do.
A lot of kids in America these days have their hobbies assigned to them according to what will help them get into a good college, so they're already living a partially fake life in order to have their lives look "right" on paper. A huge number of American's structure their activities so that their lives will look "right" to the surveillance apparatus of the almighty Credit Score. The internet is another one. As surveillance becomes more and more ubiquitous, a larger and larger chunk of the things we do will be the maintenance of a fake flaw-free life, done for show.
People having to put significant time and energy into this kind of non-productive keeping-up-appearances bullshit is the opposite of a vibrant and efficient society. It is the path to a burdened, wasteful, unhappy society. As these qualities accelerate within a society, it seems that either something has to give (like how in the 60's a rebellion started against the social constraints of the 50's), or else I guess the whole society slowly falls into decline.
I think the USA has plenty of other, more pressing reasons to slide into decline, so I'm not betting on a surprise comeback, but knowing how and why things are going to happen can at least allow me to be better situated for their effects.