The reason we prefer a lease model is that we can continually upgrade and improve our product. As an agency receives new units or replacement units, if the units wear out or if the offender has taken off—and we constantly ship the units—we always ship the latest technology unit.
To go back to your question about an offender entering a subway, GPS absolutely will not work in a subway. In fact, we expect the GPS won't work inside most buildings. Sometimes we get lucky, depending upon the structure of the building, and it does work. But with the technology we're using, we have supplemental terrestrial technology whereby we use cell towers. We don't triangulate, but we know the location of cell towers and we know the location of the nearest cell tower, or perhaps the cellphone repeater, which might be down in a subway. It is not as accurate as GPS, but it is supplemental information.
There are customers or other competitors who tout other tracking technologies, but most technologies are not as accurate as GPS.