By the fall, hundreds of convicted criminals could get sprung from Philadelphia's crowded jails and instead be under the "virtual lockdown" of a global positioning system.

But instead of a guardian angel crooning advice into their ears, a voice barking orders from their ankle - like a modern-day version of Maxwell Smart's shoe phone - would scold offenders who ventured someplace forbidden.

That's the vision of Deputy Mayor Everett Gillison, who plans to have a pilot program in place this fall in which more than 200 offenders would wear GPS-tracking anklets embedded with two-way speakers so that guards could immediately detect wayward wearers - and warn them to quit their wrongdoing or risk returning to jail.

The program would alleviate crowding in the city's jammed jails by removing nonviolent inmates, already sentenced for misdemeanors and "light felonies," and hooking them up to the GPS trackers, Gillison said.

Only one-fifth of the 9,300 inmates in city jails are there for violent offenses, a recent study found. About 37 percent of them already have been sentenced, prisons spokesman Robert Eskind said.

GPS monitoring also would save money, Gillison said. Inmates cost the city about $91 a day; Gillison estimates that GPS monitoring would cost $9 to $18 per day, per offender.

"You can't just lock people up and throw away the key; that mentality has pervaded our criminal-justice system for too long," said Gillison, who also is exploring alternatives like day-reporting, which would allow low-risk participants to be employed.

Gillison, Mayor Nutter's public-safety chief, has been meeting with companies offering the GPS technology and plans to put the plan out for bid soon. Not all companies include the speakers - which use cellular technology - with the GPS trackers.

About 800 offenders in Philadelphia already are electronically monitored, Court Administrator Dave Lawrence said. Under electronic monitoring, authorities know when offenders venture beyond defined areas.

The GPS technology allows authorities to know where offenders are at all times.

That capability has some critics questioning whether the technology is too Orwellian. Others worry that offenders might become temporarily impossible to track if they pass through "dead zones" unreachable by the satellite technology.

District Attorney Lynne Abraham said yesterday that she hadn't seen Gillison's plan but that it needs "a lot of discussion."

Such discussion, she said, should include making sure that a fair and adequate assessment is in place to determine who qualifies for GPS tracking and ensuring enforcement so that violators return to jail.

A prison also cannot overrule a court order, Abraham added, so Gillison's plan couldn't allow the release of inmates deemed serious offenders and sentenced to incarceration.

"Public safety is our overarching concern," she said. "Nobody who's smoking a joint goes to prison. These people [jail inmates] are candidates who create certain concerns for public safety."

Such concerns haven't stopped more than 350 jurisdictions nationwide from contracting with Utah-based SecureAlert, said Peter Derrick, a company spokesman.

In perhaps the device's most famous application, authorities last year used one to monitor the post-arrest movements of former astronaut Lisa Nowak, who was accused of driving across country and of donning a disguise in an ill-fated plot in February 2007 to kidnap and kill her perceived rival in a reported love triangle. The case remains unresolved. Derrick said SecureAlert is the only company offering the speaker technology.

It also frees up corrections personnel, because offenders' movements are tracked by Secure-Alert staffers who notify local authorities if violations occur, Derrick said.

One prisoners' advocate applauded the technology as a positive alternative to incarceration.

"There are a number of viable alternatives to locking somebody into a hard cell, and GPS is one," said Bill DiMascio, executive director of the Pennsylvania Prison Society.

"We've been resistant to using some of these alternatives, for fear of being perceived as being soft on crime. But instead, they're a very smart way to deal with this growing and very costly problem [of crowded prisons]."