Encyclopedia Astronautica
1999.04.07 - ISS Status Report: ISS 99-14


Flight controllers in Houston and Moscow indicate that the Unity module of the International Space Station, in its present position in space, can be adequately heated prior to the Space Shuttle's arrival.

This determination follows completion of power tests last Friday. In and around ongoing system monitoring, engineers and managers have overseen the first of three tests scheduled to be completed prior to Discovery's launch to the station on the STS-96 mission late next month. This first test demonstrated that higher power usage in the current configuration 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 climbs inside. Test one involved gradually 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. The tests are being conducted during a period of time when the station's orbital environment is similar to that in which it will be next month when Discovery arrives. Analysis of this first test indicates that Zarya can deliver at least 900 watts of continuous power to Unity in the present attitude - or position in space - which simplifies the operations planning underway for the shuttle flight. The next test aboard the ISS will begin April 14 and will involve repositioning the station for further power tests. 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. A third test, scheduled just prior to the start of Discovery's mission, will test a software update to Zarya's computer to permit use of only the module's small thruster jets for control. Meanwhile, flight controllers continued to analyze a persistent low signal strength reading on the right-side Omni antenna on Unity. 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. Flight planners for the STS-96 mission are budgeting time in the mission to survey the antenna with the shuttle's robotic arm prior to the spacewalk by Tammy Jernigan and Dan Barry. No repair hardware is planned to be taken aboard Discovery since 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, including a Russian communications system onboard Zarya. The International Space Station is in an orbit with a high point of 251 statute miles and a low point of 240 statute miles, circling the Earth once approximately every 92 minutes. The station has completed 2,150 orbits of Earth since its launch.

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