On any given day, 4.6 million convicted offenders are out on probation, parole or some other form of community supervision. Nearly 20 percent of them will be re-incarcerated because of a rule violation or new offense, according to the U.S. Department of Justice.In an effort to reduce that recidivism and protect the community, certain districts are using a GPS-based monitoring system developed by Pro Tech Monitoring, which includes a 4 pound GPS receiver (known in the business as "the box") and an anklet containing a radio frequency transmitter that tethers the offender to the box. Central monitoring via the AT&T Wireless network provides 24-hour satellite tracking of those wearing the high-tech jewelry.Unlike other electronic monitoring systems, which?without a GPS component?can only confirm whether a person is in a certain place at a certain time, the box can locate someone at any time and in any place. Law enforcement and corrections agencies pay Pro Tech $10 to $12 a day, and every 10 minutes the GPS device transmits the wearer\u2019s geographic location by means of a wireless call to the central computer, which stores that information. If an offender is in violation of any predetermined rules?say, failing to return home by an established time, going near someone he\u2019s not supposed to or tampering with the anklet?the system sends an immediate alert (either via pager, e-mail or fax) to the supervising agency as well as the offender and the original victim, if applicable. The box can also potentially provide alibi information for anklet wearers suspected of other crimes. "We now know where our highest risk offenders are 24 hours a day," says Richard Nimer, director of the Florida Department of Corrections\u2019 office of programs, transition and post-release services in Tallahassee, who currently has 550 offenders on the box. "With the old radio frequency monitoring system, we could only track someone\u2019s presence or absence from their home telephone. So if someone was supposed to leave his home at 8 a.m. to go to work, he could actually go out and murder someone or commit a heinous sex crime, come home at 6 p.m., and we\u2019d have thought, Gosh darn, this guy\u2019s doing what he\u2019s supposed to do."Some 120 criminal justice agencies in 27 states now use this system to track about 1,200 offenders nationwide. Eventually, more states may let certain prisoners out early because it\u2019s cheaper to track them by GPS than to house and feed them. Chances are, offenders would prefer the little anklet to the big house too.