Hundreds of convicted criminals could be sprung from crowded city jails by fall and be placed in a "virtual lockdown" by being fitted with GPS tracking devices.

A voice barking orders from their ankles 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 for about 200 offenders.

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

The devices would be embedded with two-way speakers so that guards could immediately detect wayward convicts and warn them to quit their wandering or risk returning to jail.

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

About 800 offenders in Philadelphia already are electronically monitored, Court Administrator Dave Lawrence said. Under electronic monitoring, authorities only know when offenders venture beyond defined areas. The GPS technology allows authorities to know where someone is at all times.

That capability has some 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 GPS satellites.

District Attorney Lynne Abraham voiced concern that public safety might be compromised.

"Nobody who's smoking a joint goes to prison," she said. Those serving sentences in city jails are people "who create certain concerns for public safety," she said.

Such concerns haven't stopped hundreds of jurisdictions nationwide from contracting with companies that provide GPS-tracking services for inmates and those awaiting trial.