Cape Canaveral, Florida - NASA will try to launch the space shuttle Atlantis on Sunday under restrictive rules to deal with problem fuel sensors and an unusually tight liftoff schedule.

The flight, originally scheduled for Thursday and postponed twice as engineers assessed the sensor issue, was set for liftoff at 3:21 p.m., officials said Saturday. However, if all four sensors in the bottom of the liquid hydrogen propellant fuel tank do not operate properly during fueling, the flight will be scrubbed.

"If everything works perfectly, we'll go fly," N. Wayne Hale Jr., the shuttle program manager, said at a news conference.

The main goal for the Atlantis and its crew of seven is to deliver and install a large European science laboratory on the International Space Station. NASA has until Dec. 14 to get the shuttle off the ground before the last launching window of the year closes.

During fueling operations Thursday, technicians found problems with three of the four sensors and their wiring system.

Recent flight rules require at least three working sensors for launching. For this mission, officials decided to require four good sensors, citing reliability reasons.