In the farmlands a few miles from America's bloodiest battlefield, the GWEN tower of Gettysburg rises above a quiet cow pasture -- a slender link in the Reagan administration's preparations to fight a nuclear war.

The gray Gettysburg tower and scores of identical structures across the continent are intended to transmit messages to missile silos, bomber bases and submarine ports in the event of a nuclear attack. But GWEN -- the Ground Wave Emergency Network -- is designed to do more: to survive a nuclear attack, aid U.S. leaders during a prolonged nuclear war and marshal whatever forces remain for the next conflict.

Or in the jargon of a Joint Chiefs of Staff document, GWEN would "support operations in the trans- and post-attack period, including reconstitution."

The projected cost of the GWEN system is $800 million, according to Air Force officials, a relatively small investment by Defense Department standards. But GWEN is part of a complex, $40 billion plan, drawn by the administration and approved by Congress, for a military infrastructure that can survive a protracted nuclear war.

Most senior officials stopped talking publicly about nuclear war-fighting early in the administration, after Defense Secretary Caspar W. Weinberger triggered a storm of criticism by calling for a U.S. strategy to "prevail" in a nuclear conflict. Critics quickly insisted that nuclear weapons are so terrible that no one can win a war in which they are used, and President Reagan began saying that such a war "can never be won and must never be fought."

The change in rhetoric did not reflect a change in programs. Behind the scenes, often in secret, the military has spent nearly $20 billion for a command, control, communications and intelligence network -- what the Pentagon calls C3I (and pronounces "cee-cubed-eye") -- much of which is intended to survive a protracted nuclear war into the "post-attack phase."

"In other words," said critic and nuclear weapons expert William M. Arkin, "it will get the United States ready for World War IV."

The C3I network, in turn, fits into a larger administration program that includes civil defense plans to protect local officials in a nuclear war and weapons -- such as the "Stealth" bomber and SRAM II short-range nuclear missile -- designed for a flexible, war-fighting strategy. Scientists at the Defense Nuclear Agency, meanwhile, are studying how to "train our troops to better understand the impact of enemy nuclear firepower and thereby better prepare them to cope with operations on the nuclear battlefield," according to congressional testimony.

According to interviews, testimony and unclassified documents, many pieces of a purportedly "enduring and surviving" C3I network are being constructed:

The Milstar satellite constellation, which will cost $10 billion to $20 billion and will be, according to Pentagon C3I czar Donald C. Latham, the first communications satellite able to support a "multiple-exchange campaign," in which adversaries would fire nuclear weapons in salvos.

A satellite-based Nuclear Detection System that could report which targets have been destroyed and which need to be hit again, capable of "long-term operation in an enhanced nuclear environment," according to Air Force testimony to Congress.

A fleet of "covertly deployed" 18-wheeler trucks from which generals could run the war in the days and weeks after airborne command posts have been destroyed or have been unable to land -- designed "to operate beyond the initial stages of a nuclear conflict," as Weinberger has said.

Other systems being developed or studied include rapidly inflatable balloons that could be sent aloft trailing radio antennas if GWEN towers were destroyed; communications satellites that could be launched from submarines after existing ones no longer functioned; command posts built deep underground, aboard small merchant ships, or on railroad cars, and a new generation of airplane command posts that could land on highways after all U.S. airstrips are gone, refuel from hidden caches and take off.

"We've looked at everything you can imagine to see how you would survive the C3I function and missions in strategic nuclear war," Latham said in an interview.

Latham and Franklin C. Miller, the Pentagon's director of strategic policy, said in interviews that U.S. preparations aim to deter the Soviet Union from launching a nuclear attack and do not reflect any U.S. belief that a nuclear war could be won. As long as the United States is prepared to fight any kind of war, they said, the Soviet Union will see no advantage in starting one.

In addition, Latham said that what he termed Soviet preparations for a protracted nuclear war -- including deep underground bunkers and the ability to reload missile silos -- force the United States to follow suit.

"It's the other guy who appears to be gearing up," Latham said, "so that if you ever did go to nuclear war by some madness or miscalculation, that it would not simply be a spasm situation that would be over in a matter of minutes . . . . It's not likely to end in the first hour or the first day. And so you need to be prepared."

Critics charge that preparing for anything beyond assured retaliation is foolish, because of the utter devastation that a nuclear attack would cause. And they say such preparations are not only wasteful but dangerous if they convince leaders that the nation could ride out a nuclear war.

Paul C. Warnke, a senior Pentagon official in the Kennedy-Johnson years and the chief arms control negotiator under President Jimmy Carter, said in an interview: "To the extent that you prepare to fight a limited and protracted nuclear war, you may find yourself closer to using nuclear weapons . . . . You can't conduct a limited or protracted nuclear war, and I know of no combat general or admiral who believes you can."

Jerry DePew, a Gettysburg carpenter who led an unsuccessful move to block the GWEN tower here, said: "They can waste all the money on this stuff they want, as long as they promise not to use it," he said. "But I don't trust the promise." McNamara's Declaration

During the early days of the administration, when officials inadvertently energized a nuclear freeze movement with talk of fighting and surviving a nuclear war, they protested that they had been unjustly singled out.

It was Defense Secretary Robert S. McNamara, after all, who declared in 1962 that "we will use nuclear weapons to prevail, if this becomes necessary." It was Defense Secretary James R. Schlesinger who called in 1974 for a strategy of "limited options" in nuclear war. And it was Carter who emphasized the need for survivable command and control in a 1980 presidential directive.

"The requirement for C3 that can endure in a protracted conflict is nothing new," Miller said recently.

What is different, he said, is this administration's determination to back doctrine with hardware.

"For the first time in maybe 20 years, we've really put our money where our mouth is in indicating that we're serious about command and control capabilities," Miller said.

Miller denied during the interview that the administration is seeking the ability to fight a "protracted" nuclear war, although he used the word at times. He said that most of the administration's C3I improvements are aimed at giving the government sufficient warning and time to permit a considered judgment about retaliation, perhaps defusing a crisis that would otherwise explode out of control, capabilities that are applauded by liberals and conservatives alike.

Miller also noted that the military in the early 1980s had been roundly criticized for failing to invest sufficiently in early warning systems and reliable communications equipment capable of enduring an attack.

In fact, during a public hearing in Gettysburg before several dozen GWEN opponents, Miller dismissed as "patent nonsense" a critic's assertion that GWEN is intended to survive a long war.

"Nobody has tried to plan for a protracted nuclear war," he said at last fall's hearing. "I don't know what protracted nuclear war is."

But in testimony to Congress and at other times when they are not in the presence of their critics, officials speak differently. GWEN would continue to operate "even after a nuclear laydown," Latham told the Senate Armed Services Committee; furthermore, an entire generation of systems is being developed to provide "high confidence that our strategic C3 systems will survive and endure through trans- and post-attack phases of a Soviet nuclear strike," the Air Force said in a fiscal 1985 report.

During the first minutes of a nuclear attack, according to military plans, U.S. forces would be controlled from fixed command posts, including centers built inside mountains in Maryland and Colorado. Strategists assume, though, that the Soviets could blow those up, along with Washington, and they count on generals, and perhaps the president or vice president, to carry on the war from airborne command posts -- Boeing 747 and 707 jets crammed with computers and radios.

With the help of refueling planes, the command posts might stay aloft for three days before their engine lubricating oil ran out, forcing the jets to land. Previous administrations assumed those three days would suffice, but Anthony D. Salvucci, assistant deputy commander for strategic systems at the Air Force Electronic Systems Division, said in a recent interview that the military is looking for more endurance.

"We are demanding more on-station time for the aircraft, but at the same time we are recognizing that you probably have to go a step beyond that," Salvucci said. "In the next generation, they're talking about a system that's capable of not only surviving because of its mobility, but surviving for longer periods of time by being able to land if it had to, and get whatever assets or resources it needed to take off again."

The next generation of command post jets, he said, will carry more spare parts, including fuel nozzles that could operate at commercial airports, and might land on highways where parts and fuel had been cached.

"We can be faced with the fact that, 'Jeez, everything looks great except the airfield we're supposed to land at isn't available, now what do I do?' " Salvucci said. "And the guy says, 'Well, there's a highway nearby that isn't too bad, we might be able to make a landing there, and we could haul off across the grass to pump the fuel out of the tanks . . . . We're looking to operate an airborne command post in a not-very-friendly atmosphere and a not-very-friendly ground environment."

Gen. Larry D. Welch, recently installed Air Force chief of staff, said in a March article in Signal magazine that such a new system is needed "for effective wartime force management."

In the meantime, the Pentagon is purchasing 40-foot trucks that resemble ordinary vans rumbling down the highway but carry sophisticated communications gear for fighting a nuclear war. In his 1983 annual report, Weinberger said the Defense Department was "concerned" about the airborne command posts' "ability to operate beyond the initial stages of a nuclear conflict" and was developing mobile systems to "supplement or take over" as needed.

Discussion of that program was subsequently dropped from his annual reports, and a spokesman for the Army -- purchasing agent for the vans under the code name "Island Sun" -- said last week that the subject is classified.

But companies that produce the vans advertise openly at industry conventions. Goodyear Aerospace distributes color brochures promoting its 18-wheel Survivable/Mobile Command Control System ("designed to be utilized in covert operations") and the smaller Survivable Enduring Shelter ("to meet the most stringent technical requirements for survival during a nuclear event").

A Goodyear executive who asked not to be identified said his company's equipment would last longer than any troops exposed to nuclear attack.

"But new troops could be moved in," he said. "Our equipment will last longer than theirs the Soviets' , so in the long run we have an advantage."

The Strategic Air Command in Omaha, according to congressional testimony, has bought a mobile command post called HERT (Headquarters Emergency Relocation Team). The Aerospace Defense Command in Colorado Springs last year requested $46 million for RAPIER (Rapid Emergency Relocation Team).

Together, Latham told Congress in 1983, the airborne and highway-driven command posts "provide a strategic C3 system capable of spanning the full spectrum of modern warfare, from crises operation through execution of an initial nuclear exchange and conduct of a prolonged nuclear war to conflict termination."

Still, Latham said recently, the Pentagon continues to weigh other options for survivable command posts: at sea, on trains, deep underground. Deep bunkers in particular, he said, with fiber optic antennas radiating out along the earth's surface, hold promise.

"There are commercial tunnel-boring machines that will drill 30-foot-diameter, completely smooth holes right through hard rock at 20 feet an hour, and they make beautiful holes," Latham said. "The technology to dig deep and do it fast is here today."

To keep some generals alive throughout a nuclear war, there are other ideas the Pentagon won't discuss.

"Mobility is one way to do it, deception is another, covertness another," Latham said. "Add all those together, and that's what we're doing to approach this." Network of 130 Towers

The Defense Department knows that surviving generals aren't much good unless they can communicate with their forces, and that is where GWEN comes in.

A network of 56 radio towers will be in place by the end of the year, and 130 to 150 will be built in the next few years, Salvucci said. Although any single tower can easily be destroyed, Latham said, the system can switch messages through alternate routes, so that most of the system would have to be destroyed to knock out GWEN.

"Is the Soviet rich enough in warheads?" Latham asked. "If he's going to take down GWEN, he's got to put a lot of weapons on it . . . . My feeling would be that that is not on the top of his priority list by any means."

Still, GWEN carries only short messages typed on a keyboard. The eight Milstar satellites and hundreds of Milstar radio terminals on planes, subs and bases, by contrast, will provide the "ultimate global connectivity," Latham said, a system designed to operate for months after a nuclear war begins.

Officials are reluctant to discuss why they believe Milstar, scheduled to be operational in the early 1990s, would survive. But some of their ideas to protect satellites from nuclear attack include launching decoys and spares in deep space, "hardening" solar panels and instruments against nuclear effects and building satellites that can maneuver or shoot back.

At the same time, satellites are being built to work autonomously for months, in case ground control stations are destroyed; the ground stations, in turn, are being installed in mobile vans. Even the Pentagon's weather satellite station near Fairchild Air Force Base, Wash., "will be hardened against collateral damage . . . caused by strikes on Fairchild AFB and Spokane," according to congressional testimony.

Col. James H. Heilman testified this year that the Air Force wants to outfit satellites with nuclear power to allow more on-board processing, reducing "reliance on vulnerable ground stations during stressing periods" and aiding the "timely reconstitution of ground operations."

A Reagan directive also mandates that commercial satellites be made more survivable to support the "continuation of government functions during all phases of conflict," Lt. Gen. Winston D. Powers, director of the Defense Communications Agency, said this spring.

Besides Milstar, the satellite most important to the military's nuclear war strategy is the 18-craft constellation carrying the Nuclear Detection System. NDS is built to function for six months without ground commands and to provide generals with information on the progress of the war.

"If you can't contact the governor of New York, does it mean you have no communication, or that Albany has been destroyed? NDS will tell you that," Miller said.

"If the war continued," he added, "you would know what you didn't have to hit again . . . . Some would say that's a nuclear war-fighting mentality. I would argue those are the logical kinds of questions that have to be answered if deterrence fails." Retargeting Missiles

Those questions are forcing dramatic changes in U.S. nuclear forces. To take advantage of their new command and communications systems, the generals need something to control as the "protracted" war grinds on -- and that means missiles that can be retargeted, bombers that can be reloaded, ships and submarines that can operate in the midst of nuclear war.

Lt. Gen. William E. Odom, now chief of the Defense Intelligence Agency and then a member of Carter's National Security Council staff, emphasized at a Harvard University seminar in 1980 the need to "be able to conduct a long campaign in which you may choose new targets."

To meet that need, Lt. Gen. Richard Saxer told Congress that his Defense Nuclear Agency is developing a "field-deployable" computer facility that could instantly reprogram nuclear missiles to attack a changing array of targets.

The Pentagon will only hint at how long it expects to keep fighting. Latham told Congress in 1984 that a "strategic reserve force" of nuclear missiles would be left aboard specially designated Trident submarines, "so that you really want to have an assured connectivity to them over a period of weeks or months."

A research plan this year notified Congress of efforts to support strategic bombers and tankers "for restrike during the extended conflict phase of strategic nuclear war (i.e., days to months after an initial exchange)."

How could bombers return from the Soviet Union, refuel, reload and take off again with new instructions in the midst of nuclear war?

"It's being very smart in how you handle the spares, the logistics, the stationing of equipment," Latham said in an interview. "There's lots and lots of things you can do if you're very clever about it, and we're looking at all of those."