Many cities and towns across the United States are turning to technology to help monitor house-arrest prisoners and to keep jails from busting at the seams.

Some are even using global positioning satellites (search) to keep track of offenders.

"It's what we consider a viable alternative to actual incarceration," said Lt. Wayne Garner of Louisiana's LaFourche Parish Sheriff's Office, which has monitored 422 offenders in 2.5 years. "It keeps the jail space open for the more serious offenders."

But the technology has its detractors.

In the legal case of accused double murderer Scott Peterson, defense attorneys tried to convince a judge Wednesday that GPS is inaccurate and unreliable. They claim their client, who is accused of killing his wife and unborn child, was tracked by GPS devices placed by authorities in vehicles he drove after Laci disappeared on Christmas Eve in 2002. Geragos wants all the GPS tracking evidence excluded from the trial.

"The GPS technology has not been generally accepted by the scientific community," Mark Geragos, Peterson's lead attorney, contended in court papers filed in October.

For law enforcement officials, however, GPS is becoming a must-have.

Some say using gadgets to track sex offenders, in particular, is becoming more appealing, particularly since the disappearance of North Dakota college student Dru Sjodin (search).

"It's insurmountable to try to keep track of these people" without using high-tech equipment, said Michael Copley, president and founder of CellTrack (search), an Indiana-based company that combines GPS technology and digital cellular technology in its monitoring system. Copley added that many states can't account for numerous sex offenders who are supposed to register with state sex-offender registries. "That's a huge problem."

GPS is a navigation system made up of a network of satellites placed into orbit that can be used to track felons so long as a signal is available.

Many forms of non-GPS electronic monitoring, which allows for the monitoring and enforcement of curfews and other conditions of community supervision, only let officials know when an offender is home — not where he or she is at other times.

Many states and localities have been using various non-satellite electronic monitoring devices to keep track of lower-risk felons for years. People convicted of domestic abuse, theft or other offenses, for a small fee, can volunteer to wear an ankle bracelet or other device that's hooked up to a computer system, telling parole or probation officials when offenders are home.

Even the federal government wants to put electronic ankle bracelets on illegal immigrants awaiting deportation.

The system reduces jail crowding and the financial burden on taxpayers, monitoring proponents argue. It also allows offenders to keep their jobs, which they would have to give up while doing jail time.

"There's just a tremendous potential in what you can do," said Marilyn Rosenberg, director of the program for the city and county of Denver, which spends about $70 a day to keep someone in jail. Denver's electronic monitoring program has had about a 90 percent success rate over the past 11 years, Rosenberg said.

There are two types of electronic monitoring via GPS — active and passive.

Active is where computer systems pick up a GPS signal telling where the offender is. Using outbound technology built into the system — a pager, short-messaging system, Bluetooth technology or cell phone — a call is sent out to the trackers to give the offender's exact location.

CellTrack's technology, for example, offers two-way voice communication and text-messaging capability with GPS technology built in to the cell phone.

"Not only can you monitor the location of the person continually but you also have the opportunity to communicate immediately with that person should they be in some violation," Copley said. "The offender is continually monitored."

With active monitoring, agencies tracking the offender need to be ready to respond if the felon goes out of the allowed area or if he or she enters an off-limits location.

"That was going to be the panacea that was going to cure everybody's concerns," said Jock Waldo, vice president of business development for BI Inc., which monitors more than 4 million offenders in the United States and has contracts with various agencies to do so.

But if an offender is in a basement or underground, the signal could be lost.

Plus, "the likelihood of an officer being able to immediately respond … quickly and promptly the way they need to, is very limited," Waldo said. But "some day we might get to the point where literally, Big Brother is watching these criminals wherever they go."

Passive GPS, on the other hand, is identical to active tracking except there is no outbound communication capability and no immediate response required.

"The beauty there is, agencies still know what you did but it takes away the responsibility of the immediate follow-up," Waldo said. "What all of these technologies do … is allow an agency to place a great deal of structure into an offender's world."

North Dakota tried using GPS to track high-risk offenders a few years ago but halted the program because it was expensive and limited. But since Sjodin, a 22-year-old co-ed from Minnesota, went missing Nov. 22 and is presumed to have been kidnapped and possibly killed by a known area sex offender, authorities are thinking about breathing new life into the program.

Officials there will retest the technology within the next six months, "given the strong public outcry about being more safe from dangerous offenders," said Warren Emmer, field services director for the North Dakota Department of Corrections (search).

Minnesota Gov. Tim Paulenty and state legislators are also pushing to track dangerous criminals after Sjodin's disappearance.

"I think it's absolutely critical that we have the ability to track these folks," state Rep. John Lesch told the Minneapolis Star Tribune. Lesch is pushing a plan to track sex offenders with GPS.

But Emmer also said these new technologies should not replace good old-fashioned police work.

"Too many states have tried to give people a false sense of security by making them believe electronic monitoring will protect them totally from serious offenders," he said. "That's not the case."

The Associated Press contributed to this report.