How does EM work?

Tagging devices in the criminal justice system usually use radio-frequency (RF) or GPS technology.

RF devices monitor whether the person stays in a particular area. You need two pieces of equipment for this: a home unit and a tag. The home unit, which stays indoors at all times, communicates with the tag, which is people tend to wear around the ankle. The home unit checks continuously whether the person is around. It sends an alert to the monitoring centre when the person leaves the house and when they return. The criminal justice system uses this technology for, for instance, monitoring curfews at home. If a person on a tag leaves home during the curfew hours, they risk having to go to prison.

GPS systems in the criminal justice system use the same technology that your smartphone uses when you open a map to find your way around the city. The tag around the ankle calculates the location based on the data received from multiple satellites around the globe. It reports to the monitoring centre about the location of a person, multiple times within a minute. This way the entire route of someone on a tag can be tracked. The authorities are able to set up zones where you are allowed to go and where you don’t, on a virtual map. The monitoring centre gets an alert when their device receives a sign from a prohibited zone.

Tags are designed to be difficult, but not impossible, to remove. Breaches happen when people break their curfews by leaving the nominated place, enter an exclusion zone or leave an inclusion zone. Tampering or removing tags, tampering with the home units, not charging GPS devices and threats and/or violence against staff all result in breaches.

England and Wales use both technologies.

If you have any comments or suggestions, please contact Eszter Parkanyi (e.parkanyi[at]leeds.ac.uk).

Last updated: 12/12/2018