The Senate Armed Services Committee yesterday authorized a record $51.9 billion for development and procurement of new weapons next fiscal year -- almost $5 billion more than what President Carter requested in March.

Chairman John C. Stennis (D-Miss.) said the committee believes the higher spending for fiscal 1981 is necessary "in light of the uncertain world situation and the repaid pace of Soviet military modernization."

He said his panel deliberately went beyond the White House proposals and added more ships, planes and weapons "to further strengthen over-all U.S. military capabilities. The approved programs and funding give a new, powerful and necessary momentum and direction to America's defense efforts."

The committee also voted to authorize $1.5 billion in a separate account for military pay and benefit increases that would help over an 11.7 percent increase in military basic pay, subsistence and quarters allowances effective Oct. 1.

The House has already passed a $53 billion version of the procurement authorization bill, $6.2 billion over President Carter's budget.

Stennis said yesterday that his committee hasn't had time to figure out the exact differences in programs between the two legislative bodies, but those differences will be settled in conference.

Stennis said his committee "was particularly concerned about the numerical strength and rate of modernization of the U.S. Navy," and for these reason added $2.2 billion to the president's request for Navy ship-building.

This amounts to a 35 percent increase and includes money for construction of an additional nuclear attack submarine, two additional frigates, development work on a new class of light aircraft carriers, and a controversial move to take on old battleship and aircraft carrier out of mothballs and out them into fleet service.

Also citing military manopower problems as a "serious concern", the comittee approved a number of actions aimed at what Stennis called "our current manpower ills."

Included -- aside from the pay raise and the beginning of registration of young men for the draft -- were a new one-year test of increased educational benefits for uniformed personnel, a restriction on the percentage of recruits that can be taken in the lowest mental category and a 25,000-man reduction in Army strength contingent upon the Army's ability to recruit a greater percentage of male high school graduates.

Stennis said that the net effect of his committee's additions in aircraft purchases would increase the number of aircraft and helicopters to be bought next year by more than 10 percent.

The committee also approved $1.55 billion for continued development of the Air Force's controversial MX mobile intercontinental missile. However, the committee tacked allow only half of the proposed missile force to be based in Utah and Nevada, where there is resistance to the massive project.

The committee gave a boost to the administration's newly forming rapid deployment force, stressing that "there is a real need to improve the mobility of our crisis response force." But here, too, the panel went beyond White House requests for these intervention forces by authorizing purchase of eight new containerships to give a quick boost to U.S. sealift capacity and new lightweight armored vehicles for the Marines that can be airlifted quickly into a battle zone.

"Conditional approval" was also given to the new CX transport plane the Air Force wants to improve emergency airlift capacity.

Included in the $16.9 billion authorized for research and development were a number of controversial decisions.

The committee authorized $10 million to resume development of the ELF, which stands for an extremely low frequency communication system meant to improve radio links with submerged submarines. The system, which requires huge land use for underground transmitting antennas, has been consistently blocked by local opposition in the past.

The committee added $15.8 million for another kind of exotic submarine communications system using blue-green laser light beams which hopefully penetrate ocean water.

The panel knocked out the Pentagon's request for money to start a new airplane that would carry cruise missiles, saying the aging B52 bombers could serve well into the 1990s.

Though the Navy has been having a hard time finding and keeping enough sailors to man its existing ships, the panel authorized a 2,500-man increase in strength for the Navy, if it can find the people, to man the old battleship New Jersey and aircraft carrier Oriskany.

In cutting back the Army, a move bitterly opposed by the top Army and Pentagon leadership, Stennis said the service has "just got to be more severe and demanding" and not take in those who "just can't make the grade."

He said the reduction could be temporary and that Army strength would be allowed to climb back by 1,250 troops for every percentage point in the proportion of high school graduates taken in above the current 52 percent. The Army's stated goal is 72 percent.

The amendment limiting initial deployment of the MX missile is among the most important committee actions. The total MX plan involves 200 missiles and 4,600 underground concrete shelters, orginally all to be in Utah and Nevada. The idea is to constantly move the missiles among these shelters so the Soviets would never by sure they would be able to knock out most of them in a surpirse attack.

The amendment by Sen. Howard Cannon (D-Nev.) would limit the deployment in those two states to 100-missiles and 2,300 silos and would be essentially a compromise meant to keep the program going while hoping to ease local opposition.