GPS technology is being used in the US to track sex offenders, violent criminals and even children jigging school.

Electronic surveillance technologies are changing the way authorities in the United States monitor repeat offenders.

In 2002, William Cotter burst into the home of his estranged wife, Dorothy, and shot her in the back with a sawn-off shot gun before taking his own life. Just five days earlier, a court had ordered him to stay away from his wife after decades of drunken violence and she was carrying a panic button linked to the local police station, in Amesbury, Massachusetts.

Fast-forward six years and advocates of the GPS-monitoring systems say it could have saved Dorothy's life. Its detractors fear a widening breach of civil liberties and an illusory sense of protection.

Coast to coast, authorities are expanding electronic monitoring to fight crime - moving beyond its early use in tracking movements of sex offenders to include gang members who have been released on probation, people accused of repeated violence against women and even truant students at schools.

At the heart of the surveillance is a technology best-known for helping people on the road: the global positioning system.

Other countries are watching closely. GPS monitoring is already established in parts of Europe but applied more narrowly, and it's growing fast in Latin America, said Jeff Durski, spokesman for iSECUREtrac Corp, based in Omaha, Nebraska, which manufactures the devices and leases them to police and courts.

Massachusetts, one of the first states to employ it in 2006, now has about 700 people fitted with electronic bracelets that send signals via satellite to computer servers if they go places they shouldn't - so-called "exclusion zones."

Part of the appeal is money. GPS is a cost-effective alternative to prison, said Paul Lucci, deputy commissioner of the Massachusetts Probation Service, pointing to a chart taped to his office wall showing a state-wide surge in use of GPS — mostly to track sex offenders but also for others.

"These people probably should be in jail but the cost of incarceration can be as much as US$30,000 or US$40,000 a year. The GPS costs about US$3,400 a year ... I think it's good on both sides. It is a device to protect the public. Although we can't guarantee anyone's safety, it provides an extra level of supervision. On the other side, for a defence attorney, it is in lieu of incarceration," said Lucci.

The Massachusetts law was inspired in part by Cotter's death and other cases of repeated abuse in a country where authorities say more than 1,000 women are murdered each year by intimate partners. It alerts police whenever an offender enters a restricted zone such as near a woman's home or office.

"It's more than just slapping a GPS on a guy. You have to really have an intelligent coordinated approach to it and then it really can save lives," said Diane Rosenfeld, a professor at Harvard Law School who helped draft the Massachusetts law.

US authorities see GPS tracking as an alternative to overflowing prisons in a country with the world's highest incarceration rate.

The number of people in US prisons has risen eight-fold since 1970 to 2.2 million people — nearly a quarter of the world's total, according to the Sentencing Project, a research and advocacy group.

There are other concerns. Los Angeles Superior Court Judge Michael Linfield warned a Harvard Law School panel in February that GPS may offer only a "high tech illusion of safety" that fails to do more to protect women than traditional restraining orders, according to the law school's newspaper, The Record.

"We don't ever say to anyone that this will save your life," said Barry Bryant, deputy director of the Governor's Crime Commission in North Carolina.

"It doesn't really guarantee much because the truth is it's real time. If someone has entered a zone where they shouldn't be, can you get there before they do something violent? I don't know. But it's an added measure of safety."

In Australia, RFID chips have been used to track prisoners while they are incarcerated.

The Alexander Maconochie centre in the ACT is designed to function as an "open plan" prison, housing young offenders, male and female prisoners of varying degrees of security all within the same facility, without substantial physical security to separate each section or razor wire barriers.

"Without this technology a prison without razor wire would be far more difficult to manage," ACT Corrective Services IT manager Andreas Wullen told ZDNet.com.au earlier this year.

According to Wullen, prisoners are fitted with an RFID bracelet upon entering the facility, enabling guards to track their location every two seconds if necessary.

Marcus Browne contributed to this report.