After acquiring its only significant competitor, Communications Satellite Corp. is reorganizing its Satellite Television Corp. subsidiary, which plans to broadcast television programs from satellites direct to home dishes.

A Comsat spokesman confirmed yesterday that STC President Richard Bodman has resigned. Ron Castell, senior vice president of marketing for STC, has been relieved of his assignment, a source close to Comsat said. But Bodman and Castell may assume other positions with Comsat, the source said.

Last month, Comsat agreed to form a new venture with United Satellite Communications Inc. of New York, which already is broadcasting TV shows from space to more than 11,000 customers.

Prudential Insurance Co. of America, the principal investor in USCI, and the owners of United Press International, Douglas F. Ruhe and William E. Geissler, will become partners in the new company, as yet unnamed.

Comsat initially will own about half of the partnership along with Ruhe and Geissler, Prudential and some previous minority shareholders in USCI. Other partners are being sought for the venture.

"We are, in cooperation with our partners, in the process of giving shape and direction to what we expect will be a new partnership organization," said a Comsat spokesman. "Obviously, in order to establish an organization, we've sought to select the best people available for key positions and ensure that duplication is avoided," he said.

Sources close to the venture said the new company will move to USCI's New York location, and other employes at both companies could be dismissed or could decline to relocate to New York.

A source close to Comsat said that David Gordon, senior vice president of marketing and sales for USCI, would succeed Castell. Other STC employes have been promoted to head finance, engineering, field operations and other key positions in the new organization. The source said that Hazel Kahan, vice president of research and market planning, also has left STC.

No one has been named to serve as president of the new organization, sources report, but a search is being conducted outside Comsat, STC and USCI. Nathaniel Kwit, president of USCI, would not speculate what his role in the new company might be, but said both STC and USCI had been on "very very similar business plans."

Another source close to USCI said that USCI's New York headquarters, and operations and satellite uplink in New Jersey locations would likely be retained, while other cuts would center on STC's marketing department.

"We were in business, and they weren't; we had a very lean group and success in the marketplace," the source said. "Almost all the STC people came from noncompetitive businesses; these were planners." STC has yet to provide a satellite television service, after announcing the project in 1978, while USCI was the first of the so-called direct-broadcast companies to beam programs in 1983 into homes equipped with special satellite antennae.