A new proposal aims to track the movements of Pennsylvania's registered sex offenders with the same technology that helps lost drivers find their way.

State Auditor General Jack Wagner, state Sen. Jane Orie and other officials are calling for Pennsylvania to require five years of global positioning system (GPS) monitoring for certain categories of sex offenders.

Yesterday, 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.

Ms. Orie, R-McCandless, is cosponsoring legislation with state Sen. John C. Rafferty, R-Montgomery, to make GPS monitoring mandatory for offenders who violate Megan's Law, as well as for those who have been convicted of a subsequent sex offense and sexually violent predators whose victims are children.

When worn as ankle bracelets, GPS tracking devices pinpoint a person's location using 32 federal government-operated satellites. Passive monitoring collects data which can be reviewed later, while active monitoring transmits data on an individual's movement in almost real time.

The most up-to-date devices combine GPS with cellular technologies, setting off an alarm and notifying authorities if the wearer enters a programmed restricted zone.

At least 14 Pennsylvania counties -- but not Allegheny --already use GPS technology to track sex offenders in some capacity, as do 33 states, but traditionally, Pennsylvania has relied on radio-frequency monitoring, not GPS.

The state Department of Corrections and the Board of Probation and Parole conducted a GPS pilot program in 2005 and 2006 and found the technology lacking. Because GPS operates by a process called triangulation, tall buildings or narrow valleys can create "dead spots" where transmission fails.

"Philadelphia is not a great place for GPS. Neither is downtown Pittsburgh," said probation board spokesman Leo Dunn, who said the board also encountered problems with false alerts.

"There is not a perfect GPS unit on the market today," said Bob Bierman, vice president of sales and marketing for iSecureTrac, which provides GPS monitoring services. For example, he said if a woman outfitted with an ankle bracelet moves the strap while shaving her legs, it could set off a false tamper alarm.

GPS is a tool, not a cure-all, said Bob McCullough, Chief Adult Probation Officer in Lycoming County, one of the first counties in Pennsylvania to use GPS monitoring.

"That tool doesn't tell you if the individual is drinking or using drugs. It's not telling you if they're having inappropriate contact with someone," he said.

At the same time, GPS monitoring has been a great boon to his office, which monitored 124 individuals in 2007, he said.

While some worry about the Orwellian implications of the technology, Barry Steinhardt, director of the American Civil Liberty Union's Technology & Liberty Program, said as long as it is "a true alternative to imprisonment," the ACLU does not find civil liberty problems with the GPS monitoring of sex offenders.

It is because the practice offers an option besides incarceration that proponents say it saves money. Active monitoring, which Mr. Wagner recommends, costs between $5 and $20 per individual per day. A Bureau of Prisons calculation for fiscal year 2007 determined the cost of imprisonment to be $68.28 per individual per day.

Mr. Wagner said advances since the probation board's pilot have made GPS more reliable.

"Technology is marching forward every day," said Mr. McCullough. While the first units his office deployed were the size of a briefcase, contemporary devices are comparable to a large cell phone.

"If it works in Lycoming County, it will work in Pennsylvania," said Mr. Wagner.

The probation board will conduct another pilot project later this summer.