REDWOOD CITY, Calif.  —  Scott Peterson's (search) defense lawyer grilled an expert Wednesday about the reliability of satellite technology that police used to track the murder suspect before his arrest.

The hearing will determine whether evidence gathered from global positioning system devices secretly placed on Peterson's vehicles in the weeks after his wife's disappearance can be used in his upcoming trial.

Modesto police used GPS (search) to track Peterson for nearly four months last year -- from Jan. 3 through April 22 -- when they arrested him near San Diego. He was caught carrying $10,000 and his brother's driver's license days after the bodies of his wife, Laci, and unborn son surfaced in San Francisco Bay.

Defense lawyer Mark Geragos (search) wants any GPS-related evidence tossed out and has hired his own expert to question how police used it in the case. On Wednesday, he suggested that the expert called by prosecutors was motivated more by profit than justice.

"I assume you want the judge to rule that this evidence is admissible so you can sell more GPS receivers," Geragos asked in court. "Is that a fair statement?"

"I'd have to say yes to that," answered Peter Van Wyck Loomis, who has worked for Trimble, a Silicon Valley company that produces GPS receivers, since 1988.

Bound by gag orders, neither side has discussed what evidence would be lost or gained from the information gathered by the tracking devices.

The military developed the satellite-based system, which can pinpoint a user's location at any time, in all weather, anywhere in the world. The decades-old technology is now used by everyone from airline pilots to weekend hikers and Sunday drivers.

But GPS technology has yet to be tested in California's criminal courts. As a result, prosecutors first must establish its reliability using properly qualified experts, and then demonstrate the technology was used correctly. Only then could GPS-related evidence be introduced at Peterson's trial.

That process began Wednesday morning when Judge Alfred A. Delucchi agreed with prosecutor Rick Distaso, who said Loomis qualified as an expert.

Loomis acknowledged that at least twice the GPS devices used to trail Peterson provided inaccurate information for several minutes.

Geragos seized on instances when the devices appeared to fail and pressed Loomis to say they were unreliable because of how police hid them on Peterson's vehicles. Loomis rejected that claim.

Prosecutors also filed their opposition Wednesday to Geragos's request to sequester jurors and to have separate juries for the trial and possible penalty phase.

Also Wednesday, Laci Peterson's stepfather, Ron Grantski, spoke to the media, saying the family should be celebrating the first birthday of the Petersons' unborn son, Conner, whose due date was Feb. 10 of last year.

"We shouldn't be going to court," Grantski said. "I should be teasing Laci about Conner keeping her up."