Controllers in Houston did note an apparent problem, late last week, with one of the two antennas mounted on the exterior of the Unity module used by the U.S. Early Communications system. Controllers noted that the ability of the starboard antenna was impeded when the station is in certain orientations, amounting to about a 15 percent reduction in the total capability of the U.S. communications system to recieve signals. When the change in performance was seen on Friday, flight controllers switched off the starboard antenna and continued using the port antenna only.
Most of the U.S. system's communications is information that is transmitted from the station to the ground, called downlink communications. These communication sessions routinely use only 1 antenna anyway, so day-to-day operations of the station are virtually unaffected. Flight controllers are continuing to analyze the problem and will switch back to the starboard antenna periodically this week for troubleshooting. Plans also are in work to use a television camera mounted on the exterior of the Zarya module to attempt to inspect the antenna. 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 that is used for the primary command and control of the station from a control center in Korolev, Russia.
Also this week, station managers decided to ship 18 small Charge-Discharge Integrated Current units from the Khrunichev Space Center, Moscow to the Kennedy Space Center, Fla., to prepare to replace the units during Space Shuttle mission STS-96 in May. The units, also known by the Russian acronym MIRT, are believed to be responsible for a less than optimum performance of the Zarya batteries that was noted earlier this year. The small, cellular phone-sized units, are part of a system that indicates the level of charge for each of Zarya's six batteries and in turn dictate when the onboard charging system believes the batteries to be fully charged and begins to taper off its supply of power.
Flight controllers have worked around the problem by deep-cycling - fully charging and then fully discharging -- each of the six batteries every week, a procedure that resets the charge indication and maintains the batteries at peak performance. Replacing the units, however, may reduce the need for such frequent cycling and provide better performance from the batteries over long durations as well as providing additional backup equipment capabilities for the station.
STS-96 mission specialists Julie Payette and Valery Tokarev plan to visit the Gagarin Cosmonaut Training Center, Star City, Russia, for several days later this month for training to replace the units in the Zarya module simulator. The units, three for each battery, are located under the floor panels of Zarya and measure 2.5 by 3 by 6 inches.
The International Space Station is in an orbit with a high point of 256 statute miles and a low point of 242 statute miles, circling the Earth once every 92 minutes, 24 seconds.