Shareholders of Comsat Corp., the satellite services company, yesterday overwhelmingly approved a $2.8 billion purchase offer from aerospace giant and Bethesda neighbor Lockheed Martin Corp.

Comsat said 99 percent of the votes cast, representing about 74 percent of the company's outstanding shares, favored the deal.

Lockheed, the country's largest defense contractor, agreed last September to buy up to 49 percent of Comsat at $45.50 a share, conditioned on the Federal Communications Commission approving Lockheed as a "common carrier" under the nation's telecommunications laws.

Once that is achieved, Lockheed said it would move forward with plans to buy the rest of Comsat if it is successful in convincing Congress to pass legislation amending the 1963 law that created Comsat and limited outside ownership. The Senate has voted overwhelmingly to lift the limits on outside ownership, but there has been little action in the House.

Lockheed gained an important edge this week when Sen. Conrad Burns (R-Mont.), chairman of the Senate subcommittee on telecommunications, encouraged the FCC to proceed with its action on the common-carrier issue. The FCC is now considering Lockheed's purchase of Comsat.

"Granting common-carrier approval should be a simple pro forma matter," said George Dellinger, a telecommunications analyst at Washington Analysis Group.

The Comsat shareholder vote comes as a welcome boost to Lockheed, which has suffered a series of setbacks this year, including a loss in the second quarter, delays and failures in its satellite-launching business, and a House vote to cut $1.8 billion in defense spending for the company's F-22 jet fighter.

Lockheed scored another victory Thursday when the Pentagon decided to go ahead with the company's troubled missile-defense system. The antimissile program recently completed two successful test firings after a series of failures; the program could be worth as much as $4 billion to Lockheed.

The aerospace giant hopes to add Comsat to its Global Telecommunications subsidiary, a business set up last year to compete in the rapidly growing market for international telecommunications services built around wireless technology. Lockheed is one of the largest providers of communications satellites and the rockets on which they are launched into space.

"We are gratified by the Comsat shareholders' endorsement of this transaction, said John Sponyoe, chief executive of Lockheed's telecommunications unit. "This completes one major milestone and we eagerly await the day when this merger is completed."

Lockheed's stock closed at $37.75 a share yesterday, up 43 3/4 cents, while Comsat shares closed at $35.87 1/2, down 50 cents.