The defendants have to wear an anklet and keep with them at all times a tracking device that hooks onto a belt, hangs in a car window or is mounted at home. Deputies log the defendants' schedules into a computer system and map out where they may and may not travel.If the defendants go where they are not allowed, stray too far from the tracking device or tamper with the anklet or any of the equipment, deputies are notified via text messages sent to their cell phones. Deputies can call the defendant and check in. Or they can use computers in their patrol cars or at the office to see where the defendant is and send nearby deputies there.
_____________________
A GPS device can be carried or worn by a sex offender to accurately and consistently track the offender’s location. If an offender enters a restricted zone or comes within a predefined radius of a restricted zone, such as a school or a play ground, an alert will be generated and sent to the person monitoring the offender. Also, a digital message or call can be sent to police to notify them of the violation. A GPS device carried or worn by the offender is known as an "active tagging system" because it must be with the offender at all times and it constantly sends out a signal. With GPS devices, authorities are able to locate a sex offender at all times and are notified if the offender enters a "hot zoned" area, which are areas such as a former victim’s house, school, or place of work. In addition to this, the GPS devices are often equipped with beepers that will omit a beeping noise when the offender enters a hot zoned area. This beeper serves the purpose of alarming potential victims that something is wrong and acts as a reminder to offenders that they should leave the area.
http://thinkgeo.com/Default.aspx?tabid=389
http://cbs2chicago.com/local/local_story_360165526.html