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More Details for 1999-04-14

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Today's test is the second of three planned before STS-96, now scheduled for launch about 8:30 in the morning on May 20 from the Kennedy Space Center in Florida. The test involves repositioning the station using its motion control system by sending commands from the Russian Mission Control Center in Korolev outside Moscow. The position is one with the ISS positioned sideways, rather than perpendicular to the Earth.

The test's primary objective is to demonstrate the ability of Zarya to deliver 1,500 watts of power to Unity in the attitude required for Shuttle docking. The test was to run until the evening followed by a maneuver of the ISS back to its normal operating attitude with Unity pointed at the Earth.

Test one, completed April 2, involved increasing the power used aboard Unity by turning on several heaters to gather insight to plan the best method for warming the module prior to Discovery's docking with the station. Analysis of the first test indicated that Zarya can deliver at least 900 watts of continuous power to Unity in its normal operating position. Since launch, the station systems have been operating on about 600 watts of power.

In and around ongoing system monitoring, engineers and managers have overseen these tests, which are designed to demonstrated that higher power usage provides adequate battery margins in the Zarya module while warming Unity's shell temperatures, which is necessary before the shuttle docks to the station and the crew enters the modules.

A third test, scheduled for next month, will test a software update to Zarya's computer to permit use of only the module's small thruster jets for control.

No further work has been done on the persistent low signal strength reading on the right-side Omni antenna on Unity and the plan remains to survey the antenna with the shuttle's robot arm after docking. The antenna, one of two used by the U.S. early communications system, has shown degradation in its ability to receive signals from the ground when the station was in certain orientations. The slightly reduced communications capability has had minimal impact on operations.

The U.S. communications system, installed on Shuttle mission STS-88 last year, is one of two complementary communications systems on the station. The second is onboard Zarya.

The International Space Station is in an orbit with a high point of 251 statute miles and a low point of 239 statute miles, circling the Earth once approximately every 92 minutes. The station has completed 2,257 orbits of Earth since its launch.