The US space agency Saturday postponed the launch of the space shuttle Atlantis for a third consecutive day and announced plans to strengthen safety rules governing launches.
Problems with fuel gauges on the shuttle's external tanks had forced NASA to put off launches scheduled first on Thursday and then Friday.
Liftoff is now set for 3:21 pm (2021 GMT) on Sunday, Kennedy Space Center spokesman Allard Beutel said.
A NASA meteorologist said that there is a 70 percent probability of favorable launch weather Sunday.
"If we find that this proposal allow us to fly with an acceptable risk we'll try to launch Sunday," Wayne Hale, director of the shuttle program, told reporters.
The problems with gauges started on Thursday, when Atlantis's fuel tanks were being filled with hydrogen and liquid oxygen.
The monitors indicated that the tanks were empty when in fact they were full, a malfunction that under the rules the National Aeronautics and Safety Administration prevents a shuttle launch.
Hale said three sensors had failed in a manner that did not allow NASA to launch Atlantis.
"We had this problem way back in March 2005," he added, pointing out that as a program manager he had found this problem "extremely frustrating."
The new safety proposals, according to Hale, tighten up launch criteria and require that all four of the sensors be functioning well.
They also call for use of new instrumentation to measure the voltage of the system, the manager said.
Hale said NASA engineers will gather for a meeting at 1:00 pm (1800 GMT) Saturday to evaluate chances for the planned launch.
He said technicians had already carried out a series of engineering tests, examined every piece of the system, the electronic box that interprets voltage readings coming from the sensor box, the sensors themselves and the wiring between all of the components.
