The electronic 'ankle bracelet'

In the aftermath of several terror attacks in Germany, politicians and journalists are debating whether such violent acts could be prevented by the use of electronic ankle bracelets for radicalized individuals known to authorities, referred to in German as "Gefährder" ("endangerers").

The discussion is getting pretty heated, but it's important to keep some basic truths in mind about what such a system can and cannot do: "Offender tracking systems" are useful to support offenders who actually want to reintegrate into society.

They can also serve to spare an accused suspect investigative jail time or to enable sentenced prisoners on work release programs to go to work during the day. Some prisoners may also serve their sentence in house arrest wearing an ankle bracelet.

A precondition for the successful use of "offender tracking" is that the offender in question actively supports the program and has no reason or incentive to flee. It must be clear to the participant that the ankle bracelet - or another less intrusive monitoring method - is the lesser evil compared to spending time in prison.

Ankle bracelets cannot prevent determined violent offenders from committing a crime. Criminals who are planning to flee and go into hiding will be able to do so. The bracelet can easily be removed with a strong pair of scissors. And if someone plans a suicide attack, an offender tracking system isn't going to stop him, anyway.

A judge decides about the monitoring level

Offender tracking was first introduced in the US. Some states in Germany agreed to jointly implement the system five years ago. A central computer collects data from the monitoring devices. So far, the program only includes sentenced offenders on work-release programs or on probation, and accused suspects who are awaiting trial.

But not all of these people are actually wearing an ankle bracelet monitoring their movements. There are different kinds of monitoring devices, including less intrusive ones. Which of them is right for the respective offender depends on the gravity of the crime, the willingness of the person to leave the criminal past behind and  the offender's overall behavior.

A warning signal to the one who wears it - and to potential victims

The actual ankle bracelet is mostly for culprits who are strictly banned from leaving or entering certain areas. This would apply, for example, to someone on house arrest, but also to a potentially violent ex-husband who is banned from approaching the home or workplace of his former wife or his children's school.

The bracelet itself is an encapsuled, waterproof device only slightly larger than a sports watch. In the house of the monitored offender, authorities will install a receiver, which detects signals from the bracelet within a certain range.

The range of the device can then be extended to include part or all of the city or region where he lives. The GPS navigation system will determine where the person is, and transfer the information about their movements to the central server.

The movements have to comply with a specific individual time plan or other requirements set by the court. If an offender is allowed to leave the prison during daytime and go to work, such systems can ensure that they take the most direct route to work and back to jail in the evening.

If the offender oversteps his boundaries, he will receive a warning: The anklet will start to vibrate. It's also possible that a potential victim receives the alarm as well: The wife or ,for example, a known stalking victim can receive a warning message on her phone.

An alcohol tester to check if the sentenced person drinks

Ankle bracelets aren't the only way to go - there are several other devices that can be used to monitor a person. In case the offender committed his crimes while intoxicated, a court may rule that his house arrest is linked to the condition of staying sober. Authorities would then use a remote alcohol monitoring system. The standard device in Germany is made by the company 3M. It comes equipped with a camera and face recognition software.

A timeplan will stipulate that the person has to stay home during certain hours of day and night. In random intervals, the system will ask the offender to blow into the device. The attached camera will then capture his image and check if the biometric data match, so that no one else can take the test for the offender while is off drinking with his buddies.

A less intense form of offender monitoring needs nothing more than a fixed-line telephone. The main aspect of the monitoring system is the offender's proven intention to comply with the rules. The person must basically have his "ankle bracelet" in his head.

In this case the computer system will call the offender's home or work. The server works with voice recognition software that can identify the person on the other end of the line - kind of like an acoustic fingerprint. If the offender isn't there at the time of the call or someone else answers the phone, the system will detect the fraud and send a message to police or to the parole officer.

This can also be used in the case of, for example, a potentially violent soccer hooligan, who was sentenced by court to stay home during games of his club. He cannot bank on being left off the hook after receiving one call: The system can call at any time, also repeatedly. A recording of the voice won't help either - the machine asks questions that are just as random as the times of the call.

Support of therapy and reintegration

The system can't just monitor house arrest. There might be times where the offender is not supposed to be home - like during work hours or when he has an appointment with a therapist. The system could call then and if the offender answers the phone, an alert will be sent to the central monitoring computer.

There is one golden rule: If the person under observation attempts toto cheat or tamper with any of the systems, he will lose his credibility and all privileges connected to it. And that usually means going back to jail.