* U.S. to build additional hi-tech fencing on Mexico border

* Government says problems with prototype "got fixed"

By Tim Gaynor

TUCSON, Ariz., May 12 (Reuters) - The U.S. government has begun building more "virtual" fencing along the border with Mexico at a cost of $100 million after overcoming some of the technological glitches of an ill-starred prototype, officials said on Tuesday.

The U.S. Customs and Border Protection agency said work had begun on the first of two new stretches of the fencing covering a total of 53 miles (85 kms) along the Arizona border, combining tower-mounted radars, cameras and other sensors to spot for smugglers crossing from Mexico.

The program follows a much-criticized prototype built by Boeing Co (BA.N: Quote, Profile, Research) that went live along a 28-mile (45-km) stretch of the border in 2007. That drew fire from Congress for a slew of problems including radar and communications faults.

"The prototype didn't work, but the system now does work ... It actually got fixed," Mark Borkowski, executive director of the agency's Secure Border Initiative, told a news conference in Tucson.

Work began this month on a new stretch of fencing covering 23-miles (37-km) of the border, near Sasabe, southwest of Tucson. It comprises 17 sensor and communication towers and 200 unattended ground sensors and is expected to become operational this year.

Additional work to build 12 communication and sensor towers along a second, 30-mile (48-km) stretch of border near the desert town of Ajo, will begin in coming months. That is expected to be completed next year and turned over to the Border Patrol, Borkowski said.