[TIME, April 25, 1969]

The pilots of Sopwith Tabloids, French Nieuports and German Taubes opened the age of aerial combat by taking potshots at one another with rifles in the skies of World War I in Europe. But the first military function of aircraft in that war was gathering intelligence. Tiny, unarmed biplanes scurried behind enemy lines to spy out troop dispositions and act as airborne forward artillery observers. Warfare has grown immensely more complex in the half-century since then, but gathering intelligence nonetheless remains one of the airplane's most significant and fascinating functions.

Present-day spy planes, with their elaborate electronic gadgetry, come in two main varieties. The more glamorous type is the fast, sleek jets that darts through another country's airspace to photograph anything of military interest, from missile installations to arms depots. Best known is the subsonic U-2, which precipitated a major cold-war crisis when the Soviet Union shot down one piloted by Francis Gary Powers in 1960. Its replacement is the SR-71 , the 2,000-m.p.h. Blackbird, which is probably the world's fastest airplane in sustained flight

The less spectacular type of spy plane is the slower patrol aircraft that measures radar capabilities and eavesdrops obliquely on enemy radio communications from a distance. The plodding, prop-driven EC-121 shot down by by North Korean MIGs last week is a military version of the Super Constellation airliner. The EC-121 is an ungainly bird, its basically graceful lines awkwardly broken by wartlike plastic radar domes above and below the fuselage. Four piston engines give it a cruising speed of only 300 m.p.h., but it has immense range. It can fly 6,500 miles, staying aloft for more than 20 hours-which enables it to monitor communications longer and more intensively than could a speedier jet.

The EC-121's working altitude of 25,000 ft. gives its snooping gear a much wider reach than that of a surface ship like Pueblo. Because many of the signals to be monitored travel in straight lines rather than bending with the earth's curvature, an airborne collector sees a much more distant horizon and can keep signals within range far longer. One EC-121 radar can sweep a 40,000-sq. mi. area. The plane carries six tons of electronic gear and a crew of 31, large enough to allow technicians and translators to spell each other frequently at tasks that demand intense concentration.

The two main sorts of data collected by aircraft of this type are 'Comint,' for communications intelligence, and 'Elint,' for electromagnetic intelligence. 'Comint' primarily means verbal radio messages while 'Elint' covers such non-verbal signals as radar, automatic landing aids and computer traffic. Since the early 1950s, EC-121s have flown the Atlantic and Pacific regularly as radar picket aircraft.

In Viet Nam and in North Korea, the planes have been used to eavesdrop on the enemy. They also plot the types and sites of radar installations and other electronic gear. They ply the Mediterranean, the Caribbean environs of Cuba and the entire East Asian coast from Viet Nam northward.

The EC-121 first pinpoints a radar site and then, by analyzing the signal picked up, determines just what that particular radar is used for. The experts can tell whether the radar under observation is meant to warn of possible threats from an enemy, whether it is intended to guide defensive surface-to-air missiles, or whether it is designed to control a network of offensive nuclear weapons. The aircraft's antennas, tuned to a wide range of radio frequencies used in military communications, can overhear conversations between major command posts 200 miles away and thus plot troop movements and combat readiness. Analysis of EC-121 data can reveal how much traffic is moving in and out of a military airfield.

One ingenious way to test a potential enemy's alertness is known as 'exercising.' That means feeding a fake signal back to the adversary's tracking radar at precisely timed intervals to simulate an intrusion in his airspace. The defender is lured into sending his interceptors aloft and activates all his secret radar equipment to bag this fictitious intruder. Meanwhile, from a distance, the spy plane can carefully monitor everything that is done by the enemy in order to meet the electronically manufactured threat. There is no indication, however, that the downed EC-121 was 'exercising.'

All this is part of U.S. intelligence gathering designed to prepare against any kind of military attack-for instance, a North Korean strike at South Korea. It also helps to keep the U.S. from getting caught in the kind of nuclear-blackmail situation that would have resulted had photo reconnaissance not turned up the Soviet IRBM installations in Cuba in 1962. Sophisticated electronic satellites have made some of the monitoring flights redundant, but the lumbering EC-121 is still more versatile and reliable, if more vulnerable to attack than a satellite orbiting in space.

The Soviets, naturally, have electronic spies of their own. Their trawler fleet makes up their most visible snooping force, showing up regularly in the South China Sea off Viet Nam and seaward from Cape Kennedy during U.S. space shots. The Soviets launch military reconnaissance satellites as regularly as does the U.S. TU-95 Bear turboprop converted bombers have been working near Alaska since the early 1960s. Most recently they have been keeping tab on the U.S. Sixth Fleet in the Mediterranean-sometimes flying with Russian markings, sometimes with Egyptian. A shorter-range TU-16 Badger, until a year ago made frequent flights down the Pacific coast of Japan to spy on Japanese radar installations; it earned the nickname 'Tokyo Express.' But since the sort of military information that is secret in Communist countries is often openly available in the West, the Soviet Union generally has an easier espionage chore than the U.S.