Already far ahead of their work schedule, the space shuttle Discovery's crew members began preparing tools and procedures to be used in a daring and ambitious attempt to revive a U.S. Navy satellite orbiting lifelessly for more than four months.

Mission planners had hoped the repairs to the Leasat communications satellite could be made during a Saturday spacewalk, but they added one for Sunday because ground tests showed that the shuttle's crippled robot arm could not work fast enough to complete the repair job in one day.

"It appears we are faced with a two-EVA extravehicular activity plan," flight director Bill Reeves said. He blamed a failed circuit that prevents Discovery's robot arm from being operated in an automatic mode.

Reeves said tests conducted on the ground showed that the robot arm, when operated manually, will need about 75 percent more time to maneuver the Syncom 3 satellite, which has been lifeless since April.

He said it will take two 6 1/2-hour spacewalks to complete the repair. Originally, one spacewalk of about seven hours was anticipated.

The 50-foot-long boom is to hold the satellite while astronauts William F. Fisher and James D. van Hoften make the repairs.

Reeves said the two-spacewalk plan means that Discovery is now scheduled to land at Edwards Air Force Base in California about an hour before dawn next Tuesday. The shuttle has landed on a runway illuminated by floodlights only once before.

The change in spacewalk plans is another in a series forced on Discovery by equipment problems, and mission officials said more changes may be needed as the problems are studied.

Fisher and van Hoften spent about four hours today checking out electronic gear they will use to repair the Syncom 3. Reeves said the checkout "went extremely well."

The testing came two days earlier than planned, one of the improvisations that resulted from Tuesday's unscheduled doubleheader launch of two satellites.

The only job left for Discovery's five-man crew before the salvage try is deployment of a third communications satellite almost identical to the Leasat. It is to be deployed Thursday morning before Discovery is maneuvered toward the Leasat.

Satellites owned by Australia and the American Satellite Co. were propelled from Discovery Tuesday on their way to permanent positions in orbit 22,300 miles above the equator.

"We're very pleased," said Otto Hoernig, vice president of American Satellite Co. "We had a picture-perfect deployment."

By Friday, crew members Joe H. Engle, Richard O. Covey, John M. (Mike) Lounge, van Hoften and Fisher intend to be well into planning the unprecedented salvage attempt.

Lounge is to work the shuttle's robot arm from inside the cockpit, while van Hoften and Fisher don spacesuits and go outside to work on Leasat.

The robot arm plays a crucial role, holding van Hoften in place while he wrestles with the satellite 35 feet above the cargo bay.

It is also to be used in helping van Hoften turn the satellite so Fisher, standing in foot restraints inside the bay, can remove a panel and disengage a timing lever, then plug two electrical cables into the panel and, in effect, jump-start the satellite. Disengaging the lever bypasses the Minuteman rocket motor and prevents accidental ignition.

Ground rehearsals have indicated that the salvage try will involve the most difficult spacewalk undertaken by U.S. astronauts.

"Whenever we rehearsed in the simulators or in the water tank, it came out to a full 6 hours and 50 minutes, which is our time limit for these things," Reeves said earlier today, referring to limits on amounts of oxygen and cooling fluid in the spacesuits.