PHILADELPHIA — Passengers and other people opposed to a Federal Communications Commission plan to allow the use of cell phones during airline flights now have some fresh data on their side.

A study by engineers at Carnegie Mellon University, published this month in the technology magazine IEEE Spectrum, found that cell phones, laptops and other personal electronic devices can cause greater interference with an airplane's critical electronics than was previously believed.

"These devices can disrupt normal operations of key cockpit instruments, especially Global Positioning System receivers, which are increasingly vital for safe landings," Bill Strauss, a co-author of the study, said in the article. Strauss, who recently earned a doctorate from Carnegie Mellon, is now an aircraft electromagnetic compatibility expert at the Naval Air Warfare Center at Patuxent River, Md.

The researchers concluded something else surprising by extrapolating data from tests they conducted in late 2003 on 37 flights in the eastern United States: One to four cell-phone calls are typically being made aboard every airline flight in the country, despite the fact that the calls are illegal and that flight crews tell passengers not to do it.

Strauss said the Carnegie Mellon study and other data collected by NASA "suggest to us that there is a clear and present danger: Cell phones can render GPS instruments useless for landings."

GPS navigation instruments are used by most general-aviation aircraft, such as corporate jets, but not by most commercial airliners. Still, use of GPS by airlines is expected to "increase significantly over the next several years," Strauss said.

The FCC is considering a proposal that would allow passengers to use cell phones when an aircraft is in the air, in addition to just before takeoff and after landing. Its main concern in banning calls in the air has been potential interference with cell-phone use on the ground.

Opposition to the plan is widespread, based on comments to the agency, surveys and testimony at a U.S. House aviation subcommittee hearing on the issue in Washington last summer.