Serious misconceptions regarding the Reagan administration's overdue program to "Rearm America" continue to surface in public debate, in Congress and in the press. Some of these misconceptions appeared on The Post's op-ed page of Nov. 11 in an article ("Wrong on the B1, Wrong of the MX") by former Defense Secretary Harold Brown, for whom I have high esteem. His misconceptions should be corrected.

The administration was given a sweeping mandate by the American electorate last autumn to carry out a massive, admittedly costly, defense program. The reason, I believe, is clear. In the 1970s, the nation, trusting overly in the spirit of "d,etente," debated--while the U.S.S.R. continued, undramatically but inexorably, to arm. The Soviet Union today not only has matched our previous nuclear superiority; it has exceeded our conventional military strength and is rapidly projecting its power beyond the needs of legitimate defense. I need only cite Afghanistan, Ethiopia, Angola, South Yemen and Cuba.

Unless we move promptly, the U.S.S.R.'s military buildup will carry it past us, leaving us in the position of permanent, dangerous inferiority. The United States could be subject to nuclear blackmail; cowed from moving boldly to defend itself and its allies in a crisis. This we cannot--and will not--risk.

In this dangerous--if not yet critical--situation we must modernize our aging bomber fleet and our increasingly vulnerable intercontinental ballistic missile force. The administration has no intention of abandoning the time-tested "Triad": land-based missiles, sea-based missiles and air- breathing strategic systems. We need all three.

The B52 bomber, workhorse of our bomber fleet, is more than 25 years old. While still useful, it is increasingly costly to maintain. Its large radar "cross-section" and slow speed make it vulnerable to Soviet air defenses, and because it cannot be "hardened" to withstand nuclear blast, or take off quickly, it is increasingly vulnerable to Soviet submarine-launched missiles.

By contrast, the new, highly improved B1B bomber not only has one-one hundredth the radar cross-section of the old B52 but no more than one-tenth the cross section of even the first B1 model cancelled by the Carter administration in 1977. The new B1B employs extremely sophisticated "avionics"--electronic countermeasures that will make it capable of foiling advanced Soviet defense/detection measures well into the 1980s, as William Casey, director of central intelligence, and I jointly notified Congress last week.

Eventually, when Soviet defenses do catch up, it will still be highly effective, armed with cruise missiles for nuclear, or conventional, bombing missions.

Early replacement of the B52 by the B1B will give us vital protection during the dangerous "window of vulnerability" in the mid-1980s and will not delay development of the advanced technology, or Stealth, bomber. The latter is still in its early stages. Potentially, we believe, it will revolutionize air warfare, but it would be risky to force a "crash" program, which might fail, and to be forced to depend, meanwhile, on obsolescent B52s. Such a course might prove more costly than the projected combination of B1B followed by the Stealth system that the administration is recommending.

We must modernize our land-based missiles no less than our bomber force. In our judgment the MX is the answer: in throw-weight, accuracy and flexibility. It provides us with a needed "counter- silo" capability.

The question is, of course, where to base it. Initially we intend to base it in specially hardened Titan and Minuteman silos as a credible, necessary and immediately available deterrent to Soviet attack. This will give us breathing time to devise a permanent, survivable basing mode: either in continually patrolling aircraft, in deep shelters under mountains protected by far more effective ballistic missile defense than we have now or in other combinations as research and technology suggest.

The Carter administration's multiple basing system was rejected by this administration, after prolonged study, as both unduly expensive and vulnerable. The Soviets would need only to increase the number of nuclear re-entry vehicles to saturate it.

A "mix" of B1B penetrating bombers and MX missiles will force the Soviet Union into a costly restructuring of its entire defense system, including command, control and communications networks, thus diverting into defense monies that might otherwise have been available for offense. This will cover our vital defense needs until, by the end of the decade, we will have devised a permanent survivable basing mode for the MX.

National defense is, admittedly, expensive. As a former secretary of health, education and welfare, I understand the need for social programs. But we do not live in an ideal world. For 10 years, we have been force-feeding social programs and starving defense. We cannot risk further delay. The question should not be whether we can "afford" to defend our nation--but whether we can afford not to.