Like millions of motorists, Eric Hanson used a GPS unit in his Chevrolet TrailBlazer to find his way around. He probably didn't expect that prosecutors would eventually use it too - to help convict him of killing four family members.

Prosecutors in suburban Chicago analyzed data from the Garmin GPS device to pinpoint where Hanson had been on the morning after his parents were fatally shot and his sister and brother-in-law bludgeoned to death in 2005. He was convicted of the killings earlier this year and sentenced to death.

Hanson's trial was among recent criminal cases around the country in which authorities used GPS navigation devices to help establish a defendant's whereabouts. Experts say such evidence will almost certainly become more common in court as GPS systems become more affordable and show up in more vehicles.

"There's no real doubt," said Alan Brill, a Minnesota-based computer forensics expert who has worked with the FBI and Secret Service. "This follows every other technology that turns out to have information of forensic value. I think what we're seeing is evolutionary."

Using technology to track a person's location is nothing new. For years, police have been able to trace cell phone signals and use other dashboard devices such as automatic toll-collection systems to confirm a driver's whereabouts.

But the growing popularity of GPS systems - in cars, cell phones and other handheld devices - gives authorities another powerful tool to track suspects.

Among recent cases:

- In September, a man in Butte, Mont., pleaded guilty to rape shortly after a judge ruled that evidence from the GPS unit in his car could be used against him at trial. Prosecutors planned to use it to show that Brian D. Adolf "prowled" through town looking for a victim.

- In New Brighton, Pa., a trucker's GPS system led police to charge him with setting his own home on fire. GPS records showed his rig was parked about 100 yards from his house at the time of the fire.

- In the case of a missing Chicago-area woman named Stacy Peterson, investigators sought GPS records from the SUV owned by her husband, former police officer Drew Peterson. She still hasn't been found, and no one has been charged.

Developed for the military, GPS navigation systems started showing up in cars in the 1990s. Prices have dropped sharply in the past few years, and many units are now available for less than $150.

The Consumer Electronics Association estimates 20 percent of American households own a portable GPS system and 9 percent have vehicles equipped with in-dash systems.

A GPS unit receives signals from satellites to determine its position on the ground.