Modern espionage is not primarily a matter of men in trench coats and meetings in bazaars. Electronic spying - with dish antennas, satellite cameras, and computer analysis - is more widespread than most Americans probably realize.
In recent months, a series of unprecedented revelations has hinted at the extent of Western high-tech snooping.
President Reagan, defending the invasion of Grenada, made public a sharp satellite or plane photo of a Cuban-built Grenadian runway. At the United Nations, the United States played for delegates a tape of the Soviet radio transmissions that doomed the downed Korean Air Lines Flight 7.
And now, stories from Britain indicate that Western antennas last week intercepted a Libyan radio message that ordered Libyan officials in London to shoot anti-Qaddafi demonstrators.
Espionage today ''depends on both human and electronic intelligence. One thing you learn very early in the intelligence game is you don't want to depend on a single source of information,'' said retired Vice-Adm. Bobby Inman, former director of the National Security Agency (NSA), in a recent interview.
The NSA is the chief US agency involved in electronic eavesdropping. It is six times the size of the Central Intelligence Agency, according to one congressional estimate. Its headquarters, halfway between Baltimore and Washington, is a huge building with so many radar domes that it looks like an airport terminal.
NSA listening posts are all over the world. According to James Bamford's book ''The Puzzle Palace,'' they range from a Virginia antenna farm that likely eavesdrops on Washington embassies to Pine Gap, an Australian station that receives signals from Soviet missiles in flight.
NSA satellites can tell whether a plane on a Siberian runway is a bomber or a prototype space shuttle, according to one intelligence source. Records of the American Telephone & Telegraph Company show an incredible number of ''receive only'' phone lines flowing into NSA headquarters - many of which may carry intercepted radio messages.
But Western SIGINT (government talk for ''signals intelligence'') is far from a purely American effort. NSA stations are probably run in conjunction with other nations - and many US allies have their own SIGINT efforts.
The KAL 7 tape could have been recorded by Japanese listening posts. Britain's spy agency, MI5, is well known; its SIGINT agency, Government Communications Headquarters, is less famous.
It is probable that the British themselves intercepted the Libyan radio message, one knowledgeable source muses.
The capture of the Libyan communication, however, points out the limits of electronic eavesdropping. Catching messages is not too hard - but turning them into useful intelligence is. The Libyan message, according to published reports, was not decoded and translated in time to stop the April 17 shooting outside the Libyan embassy in London.
Ciphers are today so tough that state-of-the-art code breaking requires powerful supercomputers and highly trained analysts. (The NSA, for instance, is a major employer of PhDs in mathematical statistics.) In fact, it is widely thought that large countries such as the US and the USSR can, if they want, transmit in codes that are currently unbreakable. Only the top ciphers of smaller nations - such as Libya - can be readily unraveled.
Until about 10 years ago, third-world nations relied on World War II surplus code machines, says David Kahn, author of ''The Codebreakers,'' the definitive history of cryptology.
These machines produced codes readily crackable by larger countries. Today, however, ''probably the machines used by Libya and other third-world countries are manufactured by commercial firms - in particular, Crypto AG,'' a Swiss company, says Mr. Kahn.
Even though they are bought off the rack, these machines ''aren't lousy systems,'' he says.
For Libya and other small countries, the danger lies not so much in using bad machines as in having clumsy clerks. Third-world code operators are less well trained than their Western counterparts, and more prone to the transmission errors that ''are really where your code breaks come from,'' says Kahn.