With its maiden flight five times postponed and almost three months late, the space shuttle Challenger was being readied today for its first launch on Monday, when it will carry what is being described as the "most complicated and sophisticated communications satellite ever built."

The largest communications satellite ever, the 5000-pound Tracking Data and Relay Satellite (TDRS) was built by TRW Inc. for the Space Communications Co. of Gaithersburg, Md., at a cost of more than $300 million each (three were commissioned).

The satellite scheduled to be lifted into orbit at 1:30 p.m. EST Monday is the first of the trio to be launched in the next 11 months for round-the-clock global communications coverage between the United States and every one of its satellites in earth orbit for the next 20 years.

"The only part of the earth we won't cover is something we call the Indian Ocean gap," said Robert O. Aller, director of the TDRS Division of the National Aeronautics and Space Administration. "This gap covers only 15 percent of the entire earth below orbits of 800 miles from the eastern region of the Soviet Union down to the southern reaches of the Indian Ocean."

Challenger's first flight will be commanded by veteran Skylab astronaut Paul J. Weitz and copiloted by Karol J. Bobko, with astronauts Storey Musgrave and Donald H. Peterson aboard as mission specialists.

The prime responsibility of Musgrave and Peterson will be to get the satellite out of the space liner's cargo bay and on its way to a permanent perch in orbit 22,400 miles above the earth, where it will match the earth's rotational speed and stay for years in the same position.

Musgrave and Peterson also are scheduled to take a 3 1/2-hour space walk on the fourth day of their five-day mission, the first space walk by Americans since the third and last Skylab flight in February, 1974, when two astronauts retrieved film from exterior cameras.

The only concerns that launch directors have expressed about launch time on Monday involve the movements of the jet stream over central Florida and a drop in pressure in one of three hydraulic pumps that maneuver the spaceliner as it rockets away from earth. The pressure drop was described today as an "annoyance" that would not stop Monday's launch unless it worsened.

At midday today, the jet stream that wanders north and south and pushes high and low pressure zones of air eastward across the United States had gone farther south than usual and was almost directly over central Florida. This caused winds of up to 150 miles an hour 41,000 feet above Cape Canaveral; such winds, if present Monday, might prevent a launch.

Maj. Donald J. Green, an Air Force meteorologist, said the jet stream had begun to move north and that computerized forecasts were placing it over southern Georgia by launch time Monday afternoon.

"The upper winds have already begun to lower a bit," Green said at 1 p.m. today, "and we're forecasting acceptable upper-altitude winds for an on-time launch."

Monday's launch would be the sixth space shuttle flight. The first five were made by Challenger's sister ship, Columbia. Challenger, an improvement on its predecessor, has a fuel tank weighing 10,000 pounds less than the one Columbia carried. The light-weight tank allows Challenger to lift into orbit the TDRS satellite, which with its own rocket engine weighs almost 38,000 pounds.

Challenger also is fitted with three new hydrogen engines, whose power levels are 4 percent higher than the engines on Columbia. Challenger's engines will run its pumps faster, increasing the flow rate of fuel into its engine chambers, a move that also increases the weight the spaceliner can lift into orbit.

The new engines are the main reason that Challenger's first flight is almost three months behind schedule.

Four times, leaks were found in oxygen and hydrogen fuel lines, forcing four of Challenger's five launch postponements while technicians repaired the leaks or changed entire engines. At least one of the leaks could have been catastrophic if it had occurred and worsened in flight.

"We found ourselves in a bind because we had to reuse engine hardware during our test period, so that we were short of hardware when flight time came up," Air Force Lt. Gen. James A. Abrahamson, NASA associate administrator, said. "We also lost some engines in testing, and one of the engines where we found a leak had already suffered two test incidents, including a fire on the test stand."

Abrahamson said that the delay in getting Challenger aloft will push back its next two flights, but that launch directors are making up time so quickly that he thinks the shuttle program will be only one month behind by the first week of August, when he expects Challenger to make the eighth flight of the program.

He also said he expects the cost of the delay to be no more than $10 million, in part because the overtime that engineers and technicians have put in on engine testing will be offset by a lack of overtime pay for launch crews.

"I believe this program will be better off because we found these leaks and delayed this flight," Abrahamson said. "The message is that we're being very careful with these astronauts' lives and the investment of the American people."