Communications Satellite Corp. yesterday won a major boost for its $600 million plan to launch a satellite to compete with the nation's television networks, getting the preliminary go-ahead for the system from the Federal Communications Commission.

Over the objections of representatives of the television and microwave radio industries, the FCC formally accepted a December 1980 application of Comsat, The District-based satellite technology firm, and opened the door for the development of rules to govern the new technology. Comsat has set up a subsidiary, Satellite Television Corp., to provide the service.

As stake is a Comsat plan to offer -- for the first time -- satellite television service beamed directly to antennas atop homes. The plan is designed to offer three channels of partly original television programming to subscribers, bypassing the current alternatives of over-the-air or cable-transmitted signals. The satellites that will beam the signals will not be ready for launch until at least three years after final FCC approval.

Before endorsing the Comsat proprosal, the FCC also began what is likely to be hotly contested proceeding to determine the rules that will govern the new service. At issue is whether the FCC will treat satellite television as a satellite, broadcasting or transmission service or a combination of the three.

In addition, FCC members noted yesterday that the spectrum slots sought be Comsat are likely to force corporate users, such as railroads and newspapers, to find other frequencies for these fixes radio transmission services.

But while STC Chairman John A. Johnson said the company is "encouraged" by the virtually total victory over critics of the system, another applicant for direct broadcast satellite (DBS) Service -- the Direct Broadcast Satellite Corp., a Bethesda firm -- Monday, became the second company to tell the FCC that it wants to get into the market.

Its plan is different from the Comsat proposal because the company, whose president is former Comsat scientist Wilbur Pritchard wants to lease space on a satellite to anyone offering programming and therefore hopes to regulate as a common carrier, rather than a broadcasting outlet. By doing so, the company could avoid the regulatory responsibilities that television broadcasters must face.

The Senate Commerce Committee may take a look at the public-policy questions raised by DBS and the Comsat proposal as part of a broader look at television licensing and the programming responsibilities faced by broadcasters, but the Comsat preliminary approval is unlikely to be reversed.

"The idea of a national station or station sitting up in the sky nearly boggles the mind," said FCC Chairman Robert E. Lee. "In the years to come, the effect of a national broadcaster on our existing broadcast stations will have to be examined. . . . I welcome this debate and the guidance, which I am sure the Congress will give us."

But Vincent Wasilewski, president of the National Association of Broadcasters, the leading Washington arm of the broadcasting industry, called the FCC's action a "cart-before-the-horse situation."

Wasilewski said the decision could have "profound implications" in the nation's consideration of its position in setting aside frequency space to be allocated at 1983 and 1984 international conferences on spectrum allocation.