PERHAPS no element in American deterrence against nuclear attack is more vital than the ability of the Navy to communicate with its missile-carrying submarines, considered by many to be the least vulnerable leg of the military triad, the other two being land-based missiles and bombers.
Yet, since those vessels first went to sea, never has there been an entirely adequate technology for such communication. Last week's Defense Department announcement that President Reagan has approved ELF, an advanced communications system designed to improve the ability of the United States to control its missile-bearing submarine fleet, is only the latest development in a debate that has simmered for years.
The problem with the systems currently in use is this: For a submarine to receive the order to launch missiles from the President, it has to trail a buoy or buoyant antenna near the surface. Under such circumstances, either the antenna, its wake or the submarine itself may be detectable. Once detected, it is vulnerable to pre-emptive attack.
Such communications, at very low frequencies, are also believed susceptible to disruption by jamming or by the long-range electromagnetic pulse from a nuclear explosion.
For more than two decades communications specialists have sought to resolve this problem. Several schemes have emerged. One would use message-carrying laser beams at blue-green wavelengths that can penetrate to considerable depths in the sea. The beams would either be generated aboard earth satellites or reflected by them from landbased generators. But it is estimated that such a system could not become operational much before the end of this century.Continue reading the main story
An approach at an even earlier stage of development would modulate naturally flowing electric currents in the upper atmosphere, or ionosphere, to generate globally observable radio signals at very low or extremely low frequencies. Dr. Anthony J. Ferraro of Pennsylvania State University, who has been testing the concept, says it was possible early this year to modify such a current over Norway in a manner observable at his campus in University Park.
The controversial approach that was given the go-ahead last week by Mr. Reagan would transmit extremely low-frequency, or ELF, signals from interconnected antenna farms in Wisconsin and northern Michigan. It would exploit a special property of radio waves: the lower their frequency, the deeper they can penetrate into the sea.
At present the Navy uses very low-frequency, or VLF, transmissions from shore stations, relayed from aircraft, to maintain contact with its far-flung submarine fleet. In this system, known as Tacamo, for ''Take Command And Move Out,'' a modified C-130 Hercules aircraft is constantly over the Atlantic Ocean using a ''random walk'' flight plan to avoid interception and, according to published reports, trailing an antenna six miles long.
The submarine, to receive messages while cruising at slow speeds in its assigned patrol area, tows a buoy that, like a small submarine, can keep itself at the depth to which VLF waves penetrate, some 40 feet below the surface. This inhibits the submarine from high speeds or quick turns and prevents it from diving very deep.
On high-speed runs, heading to its patrol area, for example, the submarine trails an antenna within a buoyant casing that insulates most of its 200-foot length. A 1,700-foot tow line enables the submarine to remain at considerable depth while towing the antenna near the surface.
Even though the buoys are submerged, they or their wakes could be detected by patrol aircraft using radar, infrared sensors or acoustic devices, those who favor the ELF system argue.
ELF tests have shown that submarines can trail an antenna at considerable speed and at depths of hundreds of feet yet pick up such transmissions in most of the distant operational areas envisioned for the new Trident submarines. ELF Has Its Disadvantages
One disadvantage of such transmissions is the stringent limit on how much information they can carry. They cannot handle long messages in any reasonable length of time. According to published reports, a three-letter code devised by the Navy can be used to indicate any one of 17,000 catalogued messages. One, for example, could command a submarine to deploy an antenna near the surface for additional information.
The history of ELF proposals has been long and controversial. In 1958, before the first Polaris-armed submarine had gone to sea, scientists met at the Lawrence Radiation Laboratory in Livermore, Calif., to explore the communications problem. Among them was Nicholas Christofilos, a physicist from Greece who early that year had made a secret, and sensational, prediction to the American military establishment:
If an atomic bomb were fired a few hundred miles above the earth but within the closed lines of its magnetic field, he said, highenergy particles from the explosion would become trapped in that magnetism, enclosing the earth in a shell of radiation that could disrupt missile defenses. The secret detonation of three atomic bombs by the United States over the South Atlantic bore out his prediction.
Mr. Christofilos's solution was ELF, whose radio emissions would resonate within a vast global cavity bounded above by the ionosphere and below by the earth's surface.
The ionosphere is the electrified region above 45 miles where sunlight dislodges electrons from the atoms of air. Shorter radio waves, such as those used by police cars and small craft, pass through it into space and rarely travel beyond the horizon. Longer wavelengths are bent back toward the earth by the electron-rich layers, making possible beyond-the-horizon communications. 2,500-Mile Wavelength
Whereas wavelengths of transmissions from police cars and the like are measured in tens of meters (dozens of feet), those favored for ELF communications, oscillating at about 75 hertz, or cycles per second (close to the 60-hertz rate of alternating-current supplies), are 2,500 miles long.
Normally, to generate such waves would require an antenna many hundreds of miles in length. But Mr. Christofilos realized that a comparable effect could be achieved by using a large volume of the earth's interior as part of the antenna.
Feeder lines from a central transmitter on the surface would run for a number of miles in opposite directions, then be grounded to electrodes in the earth. If the earth were a poor conductor, flow lines of the current between the electrodes would spread into a wide range of paths, forming a more efficient wave generator than if the current were concentrated in a few closely packed paths.
Bedrock ideal for such transmissions is the Canadian Shield, or Laurentian Plateau, a vast region of very ancient rock that extends south into Wisconsin and northern Michigan and lies beneath only a thin layer of topsoil.
A 1962 test of the concept described by Roland S. Starkey Jr. in the journal Military Electronics/Countermeasures was conducted in Wyoming. A 43-mile-high voltage line between Cheyenne and Laramie was disconnected from service during the evening and grounded at both ends. Its 60-hertz emissions could then be detected more than 900 miles away in California. An antenna of similar length was then installed between Lookout Shoals, N.C., and Algoma, Va. Tests with this system apparently flickered lightbulbs in the area and caused spurious ringing of telephones. The Sanguine Project
These tests in the late 1960's led to the Sanguine Project, which, in its original form, would have involved 41 percent of Wisconsin. A ''farm'' of antennas totaling 6,000 miles in length were to be buried five feet deep and driven by about 100 buried transmitters. Various elements of the antenna array could be activated to control the direction of transmission.
The cost was to exceed a billion dollars, but it was believed such a system, because of its redundance, could survive a direct hit by a nuclear weapon.
Opposition came from many directions, and in 1973 Melvin R. Laird, then Secretary of Defense and himself a former Wisconsin Congressman, killed the project. Within two years, however, it had been revived on a smaller scale as Project Seafarer.
A northern Michigan site was favored, with the transmitter inside K.I. Sawyer Air Force Base. The network of buried antennas 3.7 miles apart would have totaled 2,400 miles. The scope of the plan was somewhat reduced. It received a generally favorable health effects assessment by the National Academy of Sciences, but was vetoed by the Governor and terminated by President Carter in 1979.
This led to the ''Austere ELF'' project which presumably will serve as model for the plan initiated by the President last week. It would link an existing ELF test site at Clam Lake in the Chequamegon National Forest area of Wisconsin with an array to be built 165 miles to the northwest at Sawyer Air Force Base. At Clam Lake there are two 14-mile antennas mounted on poles along east-west and northsouth lines. At the Michigan site there were to be three antennas, each 30 or 40 miles long, most of them along existing roads. Less Safe From Attack
This will be far less safe against nuclear attack than the grandiose Sanguine scheme, but the latter would have depended on power from large commercial plants that themselves would have been vulnerable. The power requirements of the new plan are more modest. One of its two units could function when the other was inoperative for repairs or because of war damage, and some backup power supply might be feasible.
It is believed that an ELF system, in contrast to one using higher frequencies, is relatively safe from jamming or disruption by the electromagnetic pulse from a nuclear explosion. Messages can also be received with it beneath the ice cover of the Arctic Ocean.Continue reading the main story