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A secret aircraft that debuted in Vietnam and usually protects U.S. fighter jets is getting a new type of task over Iraq – trying to stop the scourge of roadside bombs by jamming ground signals from mobile phones and garage door openers.

The EA-6B Prowler is thought to be one of the most effective U.S. weapons against the bombs, the biggest killer of military personnel in Iraq. But no one can be sure: Even supporters say its effectiveness is hard to measure.

The aircraft debuted at the tail end of the Vietnam War and was used in Kosovo and the 1991 Gulf War, escorting U.S. attack jets while jamming military radios, hostile radars and air-defence batteries aimed at them.

These days, the Prowler focuses its jammers on smaller signals: those emanating mobile phones and garage door openers that are used to trigger roadside bombs in Iraq, U.S. Navy Captain David Woods said.

Often, it is hard to prove that a roadside bomb failed to explode because of a Prowler's jamming its signals, Capt. Woods said. Still, he is confident that the plane is making a difference against the bombs, which the military calls improvised explosive devices (IEDs).

“When it's flying, we have greater success and fewer IEDs going off,” he said. “It's kind of an insurance policy.”

Capt. Woods, the commander of Carrier Air Wing Eleven and one of the Navy's most experienced Prowler pilots, said few people understand the EA-6B's mission, which is to control the electromagnetic spectrum so that allies can use it but not enemies.

The Prowler and its electronic warfare system is so valuable it has never been exported – even to close allies. Details about the training of crew members are secret.

The Prowler is a homely plane, hung with torpedo-shaped pods and covered in bumps packed with an array of computers, transmitters, antennae and receivers that can analyze and block ground transmissions. A sinister-looking prong protruding from its nose is a refuelling nozzle.

The EA-6B's bulbous nose cradles a crew of four: a pilot and three electronic countermeasures officers who operate the jamming gear.

Outside experts say the Prowler remains the world's most effective electronic-warfare aircraft, but the aging U.S. fleet of about 120 aircraft is overworked.

Still, the Pentagon considers the Prowler critical enough to ensure that no U.S. aircraft carrier heads to battle without four or five. There are two squadrons now at sea in Mideast waters: one aboard the carrier USS John C. Stennis, the other aboard the Nimitz.

Another two land-based Prowler squadrons are in Iraq, Capt. Woods said.

In 1999, Serb air defences shot down a U.S. Air Force F-117 stealth fighter probably because it strayed too far from the jamming beam of its Prowler escort, said Loren Thompson, a military analyst with the Washington-based Lexington Institute.

On the Nimitz, which operated inside the Persian Gulf in recent days, Prowlers could be seen screaming off the deck along with packs of F/A-18 Hornet fighters, heading to Iraqi airspace.

Over Iraq, the fighters fly patterns above U.S. ground operations, waiting to be called to drop 500-pound bombs, if needed.

The Prowlers fly between 20,000 and 30,000 feet, Capt. Woods said, steering waves of electromagnetic signals over areas where insurgent bombs may be waiting for U.S. convoys.

According to outside experts, receivers inside the Prowler's tail collect radio signals from the ground that are analyzed by an on-board computer. As threats are identified, the plane's crew floods the area with electromagnetic energy that blocks the signal.

The plane's computer is loaded with a “threat library” of hostile signals that are used to match those on the ground. The jammers can block transmissions across wide range of frequencies, everything from TV and radio signals to mobile phones and the Internet.

But its jamming gear has no effect on bombs that are hard-wired to their triggers, Capt. Woods said.

The Pentagon is spending $9-billion to replace the Prowler with 90 Boeing F/A-18 fighters outfitted with electronic warfare gear. The first two, known as the EA-18G Growler, are already being tested.

The first Growlers are supposed to begin service by 2009 and replace the carrier-based Prowler squadrons by 2013. The job is expected to eventually be taken over by unmanned planes.

Capt. Woods is among a rare breed with more than 1,000 aircraft carrier landings – nearly all with the Prowler. He said he will miss the plane he has been flying since 1984.

“It's like an old girlfriend or your first car,” he said, pointing to a photo of the plane on the wall of his quarters on the Nimitz. “There are things about it you just can't replace.”