Western Union Telegraph Co., principal subsidiary of financially troubled Western Union Corp., signed a letter of intent with the People's Republic of China to launch the sixth in a series of communications satellites begun in 1974. State-owned China Great Wall Industry Corp. is to launch the satellite by March, 1988. A definitive agreement requiring approval of both governments is expected by December. Westar had been scheduled to be launched by the U.S. space shuttle last June but was postponed after the loss of the Challenger spacecraft. The satellite will be owned and operated by a joint venture with Unilease Aerospace.