GPS ankle bracelets are devices that allow the corrections system to monitor offender behavior without requiring full-time incarceration. In many cases, they represent an intermediate step between probation and prison, and can allow someone convicted of a crime some semblance of a normal life while restricting their activities. There are a number of different types of GPS tracker available, each suited toward a particular kind of offender.
Global Positioning System Technology
All GPS anklets share the same technology. Each contains a GPS receiver and some sort of communication device, usually a cellular modem. The GPS receiver detects signals from the network of GPS satellites in orbit, providing a pinpoint location every minute or so. The cellular connection then transmits that data to a central monitoring hub, giving the corrections department a real-time picture of the wearer's movements.
Geofencing Exclusions
In most cases, GPS anklets use geofencing technology to define restricted areas. For example, the system may allow the wearer to go wherever he wants within a certain radius of his home. The tracking system will still follow his movements within this radius, but if he ventures out of it, the system will send out an alert. Other locations like work or school may be set as acceptable destinations, and the limits may vary from day to day or hour to hour. For instance, a tracked subject may have permission to visit a family member on a specific day, or drive to pick up his children from school at a scheduled time.
Active Monitoring
In some cases, GPS anklets may require active monitoring and exclusion. For instance, a domestic violence offender may have a restriction that bans him from traveling within a certain radius of his victim, and alarms may sound if he approaches the victim's home or workplace. In some cases, a victim may carry a GPS tracker of their own, and if the two devices come within a prescribed distance from one another, alarms will sound. Another active exclusion involves sex offenders, who can be barred from approaching schools or other areas where they may find victims.
Additional Security
In some cases, GPS anklets may include additional communication or sensor devices. For instance, someone convicted on a drug- or alcohol-related charge may receive an anklet that periodically monitors the sweat from his skin to determine whether he remains sober. In other cases, the GPS tracker may contain a two-way communication system, to allow correctional officers to warn the wearer if he approaches the limit of his safe zone or ventures into a restricted area. Many also include a monitoring system that will sound an alarm if the offender removes his anklet, in order to prevent escape attempts.