JUST SIX YEARS after Philadelphia started using Motorola's expensive - and much-maligned - 800 megahertz digital radio system, city officials are considering a pricey upgrade.

Frank Punzo, deputy commissioner of the Department of Public Property, said recently that he's hoping to soon see a final proposal from Motorola on a newer system, which could come with a $13 million price tag.

Punzo said that the maintenance agreement on the current system - which was hailed as being state-of-the-art just a few years ago - will expire in 2010.

"It's coming up on its sunset," Punzo said, noting that 2010 is "the last year they [Motorola] want to support that version."

The upgrade proposal - which Punzo hopes to present to City Council before the end of the year - will be more of a retooling than a complete overhaul.

"We would retain the basic infrastructure we have now," Punzo said, "and make some adjustments and additions."

The upgraded system - which would take about 18 months to be installed - would add 10 communication channels to the current 35. Some new channels could be used by the School District of Philadelphia's police force, Punzo said.

The upgrade could enable the city to add new broadcast towers in signal trouble spots.

It also would be compatible with digital mobile and fixed repeaters, the devices that amplify radio signals for firefighters and cops who might respond to emergencies that are underground or in other signal trouble spots. (The "old" $62 million system isn't compatible with the current crop of digital repeaters.)

"With the next system, firefighters could take a repeater with them to the scene and even into buildings," Punzo said.

Motorola initially submitted plans for a $54 million upgrade, but Punzo said that the city doesn't "have that kind of money. They came back with a more reasonable number that was about a quarter of that."

If Motorola hadn't reduced its asking price, he said, the city would have considered soliciting bids for a new system from other companies - a path that Punzo thinks would have ultimately led to a dead end.

"If we didn't have $54 million to spend on Motorola, then we wouldn't have $54 million to spend on . . . anyone else," he said. *