The Joint Chiefs of Staff have asked Defense Secretary Caspar W. Weinberger to set aside $7 billion over the next five years to improve one of the most obscure and least glamorous, yet most vital aspects of the U.S. defense structure: the command-and-control system.

In a recent letter the Joint Chiefs asked Weinberger to give the highest priority to protection of this chain of command, through which the president would issue the orders to retaliate in the event of a nuclear attack.

Up to $25 billion may eventually be needed for this purpose, according to senior military officers, making the search for security for the president and the communications system he and the military would need in a crisis as expensive as the search for a new bomber or missile.

The command-and-control problem has been around for years but has generally been shunted aside because it is costly and a lot less glamorous than new missiles, bombers or submarines.

The problems are awesome, assuming that the Soviets can do what U.S. defense planners assume they can.

For example, if World War III ever starts the opening shot could be a nuclear-tipped missile fired from a Soviet submarine off the East Coat of the United States that would hit Washington within 10 to 14 minutes. It could hit even sooner if the submarine had sneaked through U.S. defense and gotten closer than the normal patrol distance from the coast.

Unless the president or his 16 legally designated civilian successors were somehow protected, the entire so-called National Command Authority -- meaning those authorized to order U.S. missile, bomber and submarine forces to retaliate -- could be wipe out or thrown into chaos.

Instead of striking Washington directly, a Soviet atomic weapon launched from a submarine could be exploded high in the atmosphere only three to five minutes after launch. This could cut down ever further the warning time for U.S. leadership to act by creating enough atmospheric and electrical distubrbances over the East Coast to play havoc with the communications systems that the president would use to send messages to the U.S. strategic forces.

The likelihood that such attcks will take place, of course, is very, very small. They would probably lead to nuclear suicide for both countries because the United States, experts believe, would still be able to retaliate with devastating force, even though it might take a while to get organized.

Still the question of the vulnerability of the U.S. military and civilian command-and-control system -- the warning, communications and intelligence network that would be necessary to respond to and try to control any nuclear conflict -- is of deeping concern to national leaders.

The nation has invested hundreds of billions of dollars in atomic weapons, and at the same time has developed the most sophisticated strategies to fine-tune their use to avoid a holocaust. Et the system that is meant to activate them is the weakest link in the chain, the one most vulnerable to being knocked out first and fast in an attack.

At a privately sponsored week-long conference on U.S. strategic forces this month, attended by many high-level military and civilian specialists in and out of government, a session on command and control attracted more attention and provoked more pessimism that any other.

One participant, a politican who has served at the top of government, reminded the gathering that if an attack comes, an American president will have to make one of history's most fateful decisions, and said that much more thought must be given to making sure the president is around to make it. Today, he said, the chances of the president and vice president's survival are very limited indeed; they might have as few as three to 10 minutes to respond if they were in Washington.

Staying in the White House was described as hopeless. Underground shelters in the Maryland suburbs are vulnerable to missile attack. Both of these assessments were publicly verified last year in a report by the chairman of the Joint Chiefs to Congress.

The only alternative that offers any chance of escape is to get the president into the air from Andrews Air Force Base aboard one of four special E4 emergency command post aircraft. Yet only one of these four planes has been "hardened" or modified to protect against the electromagnetic pulse that would accompany an atomic blast in the atmosphere and disable the presidential communication equipment. Though this pulse problem has been known for 15 years, the hardening of all of these jets has been delayed repeatedly.

Another participant, who had a ringside seat during the 1962 Cuban missile crisis, pointed out, however, that keeping the president in Washington during a severe crisis was also important. The president needs to be at the corner of the situation and for him to leave could panic the population. More thought, he suggested, should thus be given to perhaps getting the vice president away, perhaps under the guise of some other necessity.

Several participants noted that, late in the Carter administration, the command and control finally began to get some of the attention its urgency warrants. And the Reagan administration is reported to be determined to do more than its predecessors to fix whatever can be fixed.

But many specialists felt there was a limit to what could be done, in part because of the openness of American society generally. Much of the communications capability, for example, depends at some point on the Bell telephone system, which is commercially run. It would be extremely difficult to figure out how much of that system, how many telephone operators and switching centers, could be reconstituted in the aftermath of an attack.

The U.S. military has literally dozens of ways to communicate with its nuclear forces, and that diversity has spawned confidence the military can respond to atomic attack, assuming the initial orders from civilian authority arrive. Although it is never discussed publicly, there is at least reason to believe that if the civilian head of government was cut off, Navy submarine commanders almost certainly, and Strategic Air Command bomber and missile commanders, would at some point be able to retaliate using what is called "premissive action."

Those closest to the existing command-and-control system say there is not doubt that the United States has the technical capacity to launch its retaliatory force while it is under attack. But they acknowledge that the situation deteriorates seriously and quickly after the first incoming missiles land and suggest that there is no time for much more than one considered decision by the president in the first phase of such an emergency.

One expert summed things up by saying the command-and-control system might endure for weeks if an enemy attacked using only "tens" of weapons, days or hours if hundreds of attacking weapons were involved, and if thousands of atomic weapons were heading toward this country, there would be time for at least a response.

These officials say the United States, through satellites, has an execllent ability to detect enemy missiles within seconds of launching, and thus make sure all U.S. land-based missile forces are on maximum alert and as many bombers get into the air is possible. The crisis atmosphere itself would cause a force alert.

However, these satellites detect the booster rockets only and do not see the separation from the boost4er of the numerous individual atomic bombs carried on each missile. Other U.S. ground-based early warning radars can count the warheads but they becomes quickly saturated or are easily knocked out. Thus, the United States does not now have a good system which would tell the president how many individual warheads are coming and where they are likely to land.

As explained by insiders, the U.S. strategic forces have always been pre-targeted to hit thousands of military and urban-industrial centers in the Soviet Union in response to a first strike.

It is also assumed, however, that both sides might place some value on restraint if it ever came to a nuclear exchange. Thus, sources say, the president does have options more limited that an all-out response to any atomic provocation.But it was stated that these limited options would have to be exercised very quickly so as not to lose the command-and-control ability to launch the massive strike if needed.

In the mid-1970s, Defense Secretary James R. Schlesinger talked publicly about the need to be able to respond in kind to a variety of possible Soviet attacks as a way to ensure deterrence of all attacks. In 1980, President Carter signed Presidential Directive 59, aimed at fine-tuning nuclear responses, especially against military targets, to convince the Soviets that they could never win a nuclear war in any military sense.

Participants in the recent conference, including the insiders, left no doubt however that the U.S. is still perhaps a decade away from achieving a command-and-control system that id durable enough actually to fight either a prolonged or limited atomic conflict.

What is needed, aside from better protection of the president, are satellites that cannot be blinded during the first stage of an attack, thus removing even the warning time we now have. Specialists said they had little doubt that if the Soviets ever decided to attack, they would strike the warning satellites immediately, even though this act itself would alert Washington. The United States, however, might not know for sure whether the satellites were malfunctioning or under attack. Thus, some suggested that what may be needed are warning systems for the warning systems.

Other things needed are mobile command posts and more secure two-way communications that would allow U.S. forces to be used more effectively after the damage from initial strikes is assessed.

A non-governmental specialist who takes the grimmest view of the consequences of continued vulnerability of the command and communications system, believes that it will lead to extraordinary pressure within the U.S. military either to strike first in a crisis or immediately launch as massive a response as possible.

In his view, the Soviets do not believe that any military command structure, including theirs, can stand up to a moder atomic attack and thus there is not likely to be a limited attack option for the United States. Any limited U.S. response, he feels, would probably be met by an overwhelming Soviet attack, with heavy emphasis on knocking out the U.S. command system.