The GPS systems use an embedded GPS capability that the person wears, and that GPS capability produces a system log of where that person has been based on a connection with the GPS satellite system. That information is either downloaded in real time using something like cellphone technology through a monitoring site, or it could be downloaded at the end of the day to show where the person had been during the day. Those are the basic core principles of a GPS-type system.
The radio frequency systems tend to be based upon a system technology where you only want to know whether the person is within a certain distance from a particular base station. If the person's wearing a small bracelet and they go more than 100 metres, or 100 feet—whatever the range is—away from the station, an alarm is set off.
For the biometric base systems, which people don't tend to talk about, you're looking at a situation where you simply want to know whether the person came home that night. You could have a biometric scan of some sort—a retinal scan, a thumbprint, or something like that—to give you the confidence that the person, at one point during the day, actually was at that site.
Again, all these things depend upon the operational requirement.