The government is scrapping a $20 million prototype of its highly touted "virtual fence" on the Arizona-Mexico border because the system is failing to adequately alert border patrol agents to illegal crossings, officials said. The move comes just two months after Homeland Security Secretary Michael Chertoff announced his approval of the fence built by The Boeing Co. The fence consists of nine electronic surveillance towers along a 28-mile section of border southwest of Tucson. Boeing is to replace the so-called Project 28 prototype with a series of towers equipped with communications systems, new cameras and new radar capability, officials said.

Less than a week after Chertoff accepted Project 28 on Feb. 22, the Government Accountability Office told Congress it "did not fully meet user needs and the project's design will not be used as the basis for future" developments.

A glaring shortcoming of the project was the time lag between the electronic detection of movement along the border and the transmission of a camera image to agents patrolling the area, the GAO reported.

Although the fence continues to operate, it hasn't come close to meeting the Border Patrol's goals, said Kelly Good, deputy director of the Secure Border Initiative program office in Washington.

"Probably not to the level that Border Patrol agents on the ground thought that they were going to get. So it didn't meet their expectations."

The Border Patrol had little input in designing the prototype but will have more say in the final version, officials said.

Agents began using the virtual fence last December, and the towers have resulted in more than 3,000 apprehensions since, said Greg Giddens, executive director of the SBI program office in Washington.

But that's just a fraction of the several hundred illegal immigrants believed to cross the border daily near southwest of Tucson.

The virtual fence is part of a national plan to use physical barriers and high-tech detection capabilities to secure the Mexican border - and eventually the Canadian boundary.

Boeing was awarded an $860 million contract to provide the technology, physical fences and vehicle barriers.

"Boeing has delivered a system that the Border Patrol currently is operating 24 hours a day," Boeing spokeswoman Deborah Bosick said. She declined further comment.

Project 28 was not intended to be the final, state-of-the-art system for catching illegal immigrants, Giddens said. "I think some people understood that and some didn't. We didn't communicate that well."

Written By ARTHUR H. ROTSTEIN

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