Satellite tracking explained
Using Global Positioning Systems to effectively monitor an offender
Satellite tracking is used to control and monitor the location of an individual to ensure that they are complying with the restrictions placed on them. It effectively limits an offender’s freedom whilst not employing a full time custodial sanction.
Subjects wear an electronic ankle tag, which communicates using radio frequency to a tracking device usually worn around the waist. A G4S Control Centre tracks the electronic monitor and records the location of the offender to within a few metres using satellite GPS technology backed up by mobile telephone technology. Should a subject make any unauthorised movements, we will immediately notify the relevant agencies.
We can track offenders using this technology either as part of licence conditions following release from prison; or to monitor compliance with an exclusion order.
The offender wears a tracker device on belt, with wireless connection to ankle tag. An alarm is triggered if the tag is removed from the ankle. A trigger also occurs if the unit is moved away from the tag.
Subjects are also given a Home Extension Unit which is located normally at the subject’s home, and is the size of a small radio. This unit allows the subject to move freely around at home, whilst charging the tracking unit battery.
Through the GPS, the satellites transmit signals to the tracking unit which contains GPS receivers. Each GPS satellite transmits data that indicates its location and the current time. When the receiver estimates the distance to at least four GPS satellites, the offender’s location can be calculated to within just a few metres.
Location data transferred from belt device to telephone network.
Data sent to control centre, which can be specifically alerted if offender enters exclusion zone or breaks other conditions.
Offender's position is shown as location trails on computer screen map.
Types of Satellite Tracking
There are two types of tracking: retrospective and exclusion monitoring. In both types of tracking, the device worn by the subject determines his/her location in real time and retains the information. The only difference is how and when the information is transmitted to the G4S Control Centre.
- Retrospective monitoring: The equipment monitors and stores details of the subject’s movements in real time. The information is relayed to the Control Centre when the device communicates its data by calling in at a pre-scheduled time, using the GSM network. Information is transmitted retrospectively, and it therefore shows where the subject has been since the last transmission.
- Exclusion monitoring: The monitoring equipment works in the same way, except that when the subject breaches the tracking conditions or requirements, for example by entering an exclusion zone – the tracking device switches to active mode and transmits a real-time alert to the Control Centre. The information is shown as location trails on a computer screen map detailing which street and in what direction the subject is moving.
Key Benefits
- Assists rehabilitation of offenders as satellite tracking ensures that the offender’s family life and work can continue with minimal disruption.
- Acts as a deterrent to repeat offending.
- Provides extra intelligence to public protection agencies about a subject’s movements, allowing them to intervene swiftly if restrictions are contravened.
- Ensures effective surveillance of an offender without restricting movement.
- Ensures as far as possible that restrictions placed on a subject are adhered to with definitive proof of compliance.
- Provides a high level of protection to the public, by ensuring that a subject does not enter proscribed areas such as a victim’s neighbourhood.