2 p.m. CDT, Thursday, April 22, 1999
Mission Control Center, Houston, Texas
International Space Station Status Report #99-16
The second planned power test aboard the International Space Station was completed late last week setting the stage for the arrival of Discovery next month on the first logistics flight carrying hardware and supplies to the outpost, which celebrated its fifth month in orbit two days ago.
The test involved repositioning the station parallel rather than perpendicular to the Earth's surface by using its motion control system. The test's objective demonstrated the ability of Zarya to deliver 1,500 watts of power to Unity - a requirement for the docked phase of the STS-96 mission scheduled for launch May 20. Prior to the shuttle's arrival on May 22, the inside temperature of the station will be raised to about 66 degrees F (18 degrees C), which is necessary before the crew enters the modules.
Evaluation testing - also known as 'characterization' testing - of the station's two high-gain antenna was underway earlier this week when communication was lost through the Early Communications System during the test on the right antenna on Unity.
This loss of communication is under evaluation, but a faulty transmitter box may be the culprit. A spare transmitter is already aboard the ISS and other spares were already planned for delivery to the station next month. In the meantime, communication and command of the ISS is being conducted in the primary mode through Russian ground stations. The potential also exists to issue commands using the Early Communications System's two low-gain antennas, or Omni antennas, but with reduced insight into on-board activities.
The 'characterization' testing of the high-gain antenna is designed to provide ground controllers data for future use in calibrating or measuring changes to the signal strength over long periods of time.
Early this week, an erroneous command was sent to the station by the Russian flight control team and a procedural review is underway to understand the circumstances for the errant command that ultimately had no impact on ISS systems. This command was issued from a ground station when the direct link from the Mission Control Center in Korolev outside Moscow, was not working. In order to send the command, the three-digit number identifying the command was voiced to the ground station from which it was then uplinked to Zarya's computer. One of the three numbers was not received correctly by the ground station and resulted in uplinking a command telling the onboard computer to power up one of the module's solar array retraction motors. This command is the equivalent of enabling power to the system, but not actually retracting the array. When the onboard computer recognized the discrepancy, it automatically disregarded the command and shut off the power to the motor. Subsequent communications passes verified that the motor never ran, thus the solar array never moved. Meetings are underway to understand the sequence of events to improve processes and prevent any reoccurrence.
All station systems are in good health as the first Shuttle visit of the year nears. A third test is planned shortly before Discovery's arrival May 22. This will test a software update to Zarya's computer to permit use of only the module's small thruster jets for control.
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.
The International Space Station is in an orbit with a high point of 252 statute miles and a low point of 238 statute miles, circling the Earth once approximately every 92 minutes. The station has completed 2,382 orbits of Earth since its launch.
ISS viewing opportunities from the ground can be found on the Internet at: http://spaceflight.nasa.gov/realdata/sightings/
Updates on preparations for the launch of Discovery can be found in the Kennedy Space Center's Shuttle status report located on the Internet at: http://www-pao.ksc.nasa.gov/kscpao/status/status.htm
The next International Space Station status report is planned to be issued on Thursday, April 29, 1999.
Note: For further information, please contact the NASA Public Affairs Office at the Johnson Space Center, Houston, Texas, 281-483-5111.
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