REDWOOD CITY, Calif. -- In a decision that appears to set California legal precedent, a judge ruled Tuesday that evidence police gathered using electronic devices to track Scott Peterson after his pregnant wife disappeared can be used in his double-murder trial.

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After nearly two days of testimony from experts on global positioning system technology, Judge Alfred A. Delucchi said he believed the devices were reliable -- despite a defense lawyer's objections they were error prone.

Because GPS technology has yet to be tested in state criminal court, prosecutors had to establish both its reliability and demonstrate that the technology was used correctly.

Delucchi decided Tuesday they had met those legal tests, calling the technology "generally accepted and fundamentally valid." Legal observers believe it is the first time a California judge has admitted GPS evidence in a criminal trial.

Police installed at least three of the devices in vehicles Peterson owned, borrowed or rented after his wife, Laci, disappeared on Dec. 24, 2002.

In January, 2003 the devices tracked him to San Francisco Bay -- where Peterson said he was fishing when his wife disappeared and near where her body and that of their unborn son washed up in April. If convicted of killing the two, Peterson could face the death penalty.

Though the judge decided that prosecutors had to tell Peterson's lawyers where police placed the devices, the GPS ruling was a victory for prosecutors. The judge also had good news for Peterson's defense Tuesday, when he tossed out an interview with a woman whom prosecutors believe witnesses may have mistaken for Laci Peterson.

At least three neighbors have said they saw Laci Peterson walking her dog on Christmas Eve morning -- after prosecutors assert Scott Peterson killed her. Prosecutors have suggested the neighbors saw Kristin Dempewolf, who like Peterson had shoulder-length brown hair and was visibly pregnant. Authorities interviewed her under hypnosis, and while it's not clear what Dempewolf said, defense lawyers wanted to bar that interview.

On Tuesday, Delucchi said he wouldn't allow the interview because authorities didn't follow proper procedures and the hypnotist's credentials were insufficient.

Under a gag order, prosecutors could not comment on that decision.

During much of the day, Peterson's lawyer, Mark Geragos, tried to convince Delucchi that temporary glitches proved the GPS devices that Modesto police secretly attached to vehicles Peterson drove before his April arrest were inherently unreliable.

Geragos seized on tracking errors in several of the devices Modesto police used, including one which he said didn't work for nearly three weeks.

The satellite-based radio navigation system can pinpoint a user's location within feet and is in common use from airplanes to weekend wilderness warriors.

Prosecution expert Hugh Roddis, president of the company which sold police the devices, said that covertly placed GPS technology is a "good investigative tool." He blamed the errors on inaccurate maps, a faulty wireless antenna and a bad microprocessor connection -- but insisted the devices were still fundamentally sound.

Ultimately, the judge agreed.

Also Tuesday, Geragos again urged the judge to sequester the jury and to seat two separate juries -- one for the guilt phase and one for the penalty phase. Delucchi appeared reluctant to grant either request.

In a heated exchange, Geragos said both were necessary because of intense publicity.

"My client is innocent and the guilt phase is the whole ball of wax," Geragos said. "We don't care about the penalty phase because he didn't do it.

"In a case that has been tilted against my client since day one," Geragos said, "I'm not even asking for a level playing field, we're just trying to get into the ballpark to play."

Outside court, Peterson's father reaffirmed his son's innocence.

"It's going to be a blowout," Lee Peterson said, "and my kid's going to walk out of here."

The case resumes Wednesday, when both parties are expected to discuss evidence authorities collected using wire taps.