The type of beacon on the plane flown by the adventurer Steve Fossett when he disappeared last week emits false signals 98 percent of the time, the Federal Aviation Administration has concluded.

As a result, after Feb. 1, 2009, the F.A.A. and the National Oceanic and Atmospheric Administration will no longer monitor signals from these devices, the agencies said in a memorandum issued Friday.

The devices, known as emergency locator transmitters, or E.L.T.'s, have been standard in aircraft for decades, but the agencies have concluded that the older models, which operate at a frequency of 121.5 megahertz, are antiquated and unreliable. Furthermore, the analog frequency that the beacons use is often trespassed accidentally by radio signals emitted by other devices, including some stadium scoreboards and video arcade equipment, said Lt. Jeffrey Shoup of NOAA. The authorities have not picked up any signals from Mr. Fossett's plane, which vanished on Sept. 3 in northern Nevada. After February 2009, planes with old transmitters will have to rely on their distress signals being picked up by planes flying overhead, rather than by the satellites that monitor such signals.

The F.A.A. is urging - but not requiring - plane owners to switch to the digital transmitters that emit a signal 20 times stronger at 406 megahertz, to be spotted by satellites. The newer transmitters send identification numbers that the authorities know and can use to contact the owners quickly to determine if there is a real distress.

With the old transmitters, including the one on the single-engine Super Decathlon flown by Mr. Fossett, Coast Guard and Civil Air Patrol officers must visit the site matching the coordinates of each distress signal to discover if a real problem exists.

Lieutenant Shoup said the decision to switch to 406 megahertz was made in 1999 by a Montreal-based consortium of 38 nations that operates the world's search-and-rescue satellites. But Ross Aimer, chief executive of Aviation Experts, a consulting firm in San Clemente, Calif., said that until Friday's memorandum, few aviators knew about the transition. He said he had not known the date when the F.A.A. would stop monitoring the older devices, so it is possible that Mr. Fossett also did not know of it.

Only 10 percent of the 200,000 planes in the United States have 406 megahertz locator transmitters, and most are commercial airliners, because replacing the transmitters can cost more than $4,000, including the device and the retrofitting.

The unreliability of Mr. Fossett's beacon may be one reason the search for the record-setting aviator has failed so far, but other high-tech measures should make up for that. Late last week, the Nevada Wing of the Civil Air Patrol added to its fleet of more than a dozen search planes a special aircraft that is equipped with cameras capable of seeing 15 times more detail than the human eye.

The Archer, an acronym for Airborne Real-time Cueing Hyperspectral Enhanced Reconnaissance, was developed for classified military uses in the 1990s, but the Civil Air Patrol obtained $6 million from Congress in 2002 to buy several and adapt them for search missions.

"It's a better set of eyes than a human set," said Col. Drew Alexa, the patrol's director of advanced technology. "It can see things that are anomalous in the vegetation such as metal or something from an airplane wreckage."

While the Archer has not located Mr. Fossett, it has been instrumental in the discovery of at least eight previously unnoticed plane wrecks in the rugged Sierra Nevada region.

Those wrecks will be examined more closely once the Fossett search ends, officials said.

More Fossett Pieces, all from NYT unless otherwise noted: