Satellite navigation devices used to track Scott Peterson's (search) car after his wife's disappearance sometimes developed glitches, once indicating he was driving 30,000 mph, an expert in the technology acknowledged Monday at Peterson's murder trial.

Prosecutors allege Peterson acted like a guilty man and lied to friends and relatives about his whereabouts during the search for his wife, Laci. Defense lawyers maintain he was trying to avoid media attention.

People involved in making and selling the global positioning satellite devices police used to monitor Peterson's travels defended the reliability of the devices but acknowledged they have limitations.

"It's not exactly perfect," said Hugh Roddis, chief technology officer for Nova Scotia-based Orion Electronics, which sold the GPS devices to Modesto police.

Peter Van Wyck Loomis (search), whose company, Trimble Navigation, made the devices, testified "there were some rare cases where it was off by 15 to 20 yards."

In an effort to head off defense attacks on the devices, prosecutor Rick Distaso noted one instance when a tracking device showed Peterson's vehicle was moving at more than 30,000 mph.

Loomis said that was about a "100-second" glitch.

Prosecutors are trying to prove that Peterson killed his 8-months-pregnant wife on or around Dec. 24, 2002, then dumped her weighted body in San Francisco Bay.

The bodies of Laci Peterson (search) and her fetus washed up on shore in April 2003 not far from a marina where Scott Peterson says he launched his boat that Christmas Eve morning for a solo fishing trip.

Police have testified they followed Peterson to the Berkeley Marina three times as authorities scoured the bay for evidence. Defense lawyers claim their client was checking up on search efforts, hoping police would find clues, as well as looking for two witnesses who may have been at the marina that Christmas Eve morning.

Witnesses testified Monday that the GPS (search) devices tracked Peterson to the marina at least three more times in January 2003 after police ended their physical surveillance of Peterson on Jan. 11, when they figured he had become aware he was being followed.

During pretrial hearings, defense lawyers fought to keep out the GPS testimony, claiming it was error-prone.

In what legal experts believed set California precedent, Judge Alfred A. Delucchi in February ruled the testimony would be allowed at trial because he was convinced it met legal requirements.

The technology uses signals from a network of satellites to pinpoint the location of GPS devices. It is commonly used by people from airline pilots to wildlife management officials and weekend hikers.

In cross-examining Roddis, defense attorney Mark Geragos noted that some dates and times on a printout shown to jurors were inconsistent with Peterson's actual movements.

"It's not intended to be terribly accurate," Roddis said, adding that the printouts of stored data are strictly for investigators to get a rough timeline.

Roddis said that with GPS technology, a wind gust or door slam can make a vehicle appear to be in motion when it is standing still. Conversely, at times the devices detect no movement from a vehicle traveling on a smooth highway, he said.

In cross-examining a Modesto detective who was in charge of collecting some of the GPS data, Geragos said that, according to the tracking units, Peterson also visited two other spots miles from the marina after learning of media reports that police were considering searching for Laci there.

The detective said he was unaware of such reports.

Later Monday, state Department of Justice criminalist Pin Kyo testified that she found no blood on two mops and a bucket seized from the Petersons' home. Prosecutors have implied Peterson used the mops to clean the murder scene, but have presented no evidence to support that theory.

Kyo also testified no blood was found on several other items, including a blue tarp and Peterson's boat cover.

Kyo later said specks of blood were found on the Petersons' comforter cover, but didn't elaborate. A source close to the case later told The Associated Press that a DNA test indicated the blood came from Scott Peterson.