State Auditor General Jack Wagner says a new proposal to track the movements of Pennsylvania's registered sex offenders using global positioning system technology is necessary to protect the state's most vulnerable citizens, but critics of the measure say it would be the first step down a slippery slope of Orwellian proportions.

"Lawmakers need to be cautious about this," said Marc Rotenberg, executive director of the Electronic Privacy Information Center in Washington, D.C.

"Sometimes these proposals are looked at piecemeal, and people don't look at the broader issue of the ways we're being watched," said Andy Hoover, a legislative assistant for the American Civil Liberties Union of Pennsylvania.

Last week, state Sens. Jane Orie and John Rafferty announced they would cosponsor a bill mandating five years of GPS monitoring for certain categories of sex offenders.

When worn as ankle bracelets, GPS tracking devices pinpoint a person's location using 32 satellites operated by the federal government. Passive monitoring collects data that can be reviewed later, while active monitoring transmits data on an individual's movement as it happens.

Barry Steinhardt, director of the national ACLU's Technology & Liberty Program, said the organization doesn't have a fundamental problem with GPS monitoring of sex offenders as long as it is "a true alternative to imprisonment." But he said broader applications are "very different."

When such technologies are proposed for "groups that have little public support," said Mr. Rotenberg, over time their use grows. "There are millions of people tied into the U.S. criminal justice system that could soon be wearing various types of tracking devices if we're not careful," he said.

From parole and probation officers to worried parents, the current markets for GPS monitoring reveal the technology's expansive breadth.

At least 14 Pennsylvania counties -- but not Allegheny -- already use GPS technology to track sex offenders in some capacity, as do 33 states. California, which along with Florida leads the nation in using GPS monitoring to fight crime, is currently monitoring 5,138 sex offenders, 2,208 with active GPS.

In some states, GPS tracking is replacing paper restraining orders for victims of domestic violence: If the offender enters a programmed "restricted area," the device simultaneously notifies the police and the person who sought the order of protection. Many states monitor parolees and individuals on probation who are deemed to be "high risk."

In other areas, GPS aids pretrial supervision. The same devices are used to curb gang violence in California and juvenile truancy in Dallas.

Nationwide, parents can purchase units to track their children's cars, and several companies market "wander prevention" systems for elderly relatives with Alzheimer's.

As to the future, Jay McKeage, vice president of corporate development for Digital Angel, a company that markets GPS devices, said there is a "tremendous amount of interest" in an implantable microchip with GPS capabilities.

"Lots of parents call and say, 'Do you have this for my kid? I'm estranged from my husband, and I'm afraid he's going to kidnap my child,' " said Mr. McKeage. Because of current battery technology limitations, a GPS microchip is futuristic at best. But "eventually? Sure," he said.

For those who worry about the implications of GPS monitoring, the question, said Mr. Hoover, is "where and when does it stop?"

Mr. Hoover emphasized that using GPS to monitor sex offenders "would be going through a lot of trouble" for a group with a low rate of recidivism. He cited a 2003 Department of Justice study in which 5.3 percent of sex offenders were arrested for another sex crime within three years of their release. He also said since most victims of sex crimes know the perpetrator, GPS monitoring would be minimally effective in reducing sex crimes.

Mr. Wagner, though, said the study stating the low rate of recidivism among sex offenders is misleading.

"The average molester of girls committed 50 crimes prior to being caught," he said. "The average molester of boys has committed 100 crimes of sexual molestation prior to being caught."

The new proposal was initially put forward after Mr. Wagner released a report disclosing that Pennsylvania had lost track of 923 of the state's 9,800 sex offenders. Megan's Law requires convicted sex offenders to register their names and addresses on a publicly accessible Web site, but if offenders fail to notify authorities when they move, they can drop off the map.

The proposed GPS bill would require monitoring for sex offenders who violate Megan's Law, as well as those who have been convicted of a subsequent sex offense and sexually violent predators whose victims are children.

"We have studied this issue in great depth, and this is not a reactionary response," Mr. Wagner said in defense of his recommendations. "For the ACLU to protect sexual offenders when we have modern technology that can protect our children is simply out of line."

In response to civil rights concerns, Steven Logan, CEO of Satellite Tracking of People LLC, said GPS monitoring is a double-edged sword.

Mr. Logan's company extracts crime data from law enforcement databases and checks it against the movement of individuals wearing GPS bracelets. Hits are e-mailed to the local police.

While GPS data can prove an offender was near the scene of a crime, the same system provides a flawless alibi if a suspect's data shows he was somewhere else when a crime was committed.

"It's natural for that concern to come up," he said, of Orwellian accusations, "but I think, if thought about in this manner, it's intended to benefit those offenders who are complying."

"The ACLU doesn't understand what is in the best interest of those who have been convicted and have served time," said Mr. Wagner. "This is a deterrent to recommitting sexual crimes."

Mike Sarfert, a spokesman for Sen. Orie, said the proposal to monitor sex offenders targets specific individuals who "have diminished rights due to their own actions."

While he understands fear of "the overexpansive use of Big Brother tracking," watchdogs like the ACLU are never far behind, he said.