Frank Ochoa, imprisoned on an attempted carjacking conviction, thought he had the guards fooled.

The inmate at a minimum-security prison in the California desert slipped his electronic bracelet around a hot cup of coffee and made a run for it.

Ochoa apparently didn't know that by tampering with the tracking device, he had triggered an alarm at the Calipatria prison's security control center. Guards caught Ochoa less than a mile away.

The sophisticated radio monitoring system that helped capture Ochoa two years ago is now being installed at a handful of other U.S. prisons. If widely adopted, it could one day change the way correctional facilities are run.

"It completely revolutionizes a prison because you know where everyone is -- not approximately but exactly where they are," said Larry Cothran, a technology consultant to the National Institute of Justice.

Within 20 feet

Using radio transmitters monitored by a network of receivers, the system tracks prisoners and corrections officers to within 20 feet. Inmates wear tamper- and water-resistant bracelets while officers wear pager-like devices.

It's a high-tech version of the head count, except these head counts are conducted every two seconds versus the old-fashioned method of five to eight times a day.

Any time an inmate tampers with or removes the bracelet -- or strays out of range -- the bracelet trips an alarm. Guards monitoring the prison can not only pinpoint the location but also know who is in the vicinity.

The monitoring device for officers has a red button that allows them to signal for help and an automatic "man-down" alarm if the device ends up in a horizontal position.

If guards aren't careful, the tilt mechanism has been known to trigger alarms if the device gets twisted on their belt or they drop their pants, said Chris Trott, president of the Calipatria guards union.

"Sometimes you go to the bathroom and your alarm goes off," he said.

Location data shows up as dots on a computerized map -- blue for corrections officers and yellow for inmates. A list indicates who each dot represents and all movements are stored in a database, for investigative purposes.

So far, Scottsdale, Ariz.-based Technology Systems International Inc. appears to be the only company selling such a product in the United States.

Called TSI PRISM, the system is based on technology Motorola Inc. developed in the 1980s with an eye to military uses. The company decided instead to license it to TSI, which spent seven years honing it for prisons.

Fewer guessing games

The state of Michigan, which recently installed a $1 million PRISM system at its new maximum-security 200-inmate juvenile prison, thinks the technology will aid investigations of assaults and of sexual contact -- a common violation among inmates, many of whom are sex offenders.

"If someone says someone assaulted him last night, this system can help us figure out if he was telling the truth," said Marlys Schutjer, acting director with Michigan's Bureau of Juvenile Justice. "It should really help a lot in cutting through a lot of guessing games."

Trott said guards using the devices credit them for helping to understand prisoner involvement in fights or other incidents.

"It's almost like having a videotape of the incident, because you can track who was there," Trott quoted guards as saying.

Another system is being installed in a large medium-security prison being built in Logan, Ill.

"If money were not an issue, it would be a welcome addition," said Tim Ochoa, an associate warden at the 300-inmate Calipatria State Prison, which has been testing the system for three years.

Guards at Calipatria initially opposed the system, thinking it would be used to spy on them. But officials agreed not to use monitoring data against them, Tim Ochoa said, and the officers now support its use.

As for prisoners, they don't have a say in the matter.