Tracking satellite retired after 22 years of service
BY STEPHEN CLARK
Posted: December 25, 2011
NASA decommissioned an aging tracking and data relay satellite this month after it linked ground controllers with space shuttles, the International Space Station and launching rockets for more than 22 years, according to agency officials.
"However, it had passed its life expectancy. It was at over 22 years of service," Sobchak said.
Launched by space shuttle Discovery in March 1989, the tracking satellite most recently served over the Atlantic Ocean, supporting the final shuttle flights and relaying data from rockets blasting off from Cape Canaveral, Fla.
Built by TRW, now part of Northrop Grumman Corp., TDRS 4 outlived its design life and saw service in four decades. It provided S-band, Ku-band and C-band communications.
"We used TDRS 4 operationally up until the point in time it could no longer support operations," Sobchak said. "We were using that one for shuttle missions through the end of the shuttle program."
TDRS 4's batteries degraded to the point where they could no longer continuously support the network's customers, according to NASA.
"The batteries on-board had insufficient capacity to support the operational needs of the spacecraft, so at that stage, we evaluated the alternatives and found the best alternative to NASA and the government was to decommission and [raise the orbit of] that spacecraft," Sobchak said.
Engineers turned off the satellite Dec. 9 after boosting its orbit above geosynchronous altitude, which is about 22,300 miles from Earth. Satellites in geosynchronous orbits hover over a fixed position on Earth's surface, allowing stable communications with the ground.
"We went through and executed all those procedures according to plan, including burning all the fuel and deactivating the spacecraft," Sobchak said. "It went well."
Many commercial and government satellites occupy the geosynchronous belt over the equator, so NASA raised TDRS 4's altitude out of the congested orbital space to free up room for new spacecraft.
The TDRS network facilitates voice and data communication with the International Space Station, downlinks imagery from Earth observation satellites and orbiting observatories like the Hubble Space Telescope, and serves the U.S. government's national security satellite fleet.
Stretching almost 60 feet across and covered with seven antennas, TDRS 4 was the second satellite in the constellation to be retired. The TDRS 1 satellite, launched in 1983, was shut down in late 2009 after its communications equipment stopped functioning.
The second TDRS craft was lost in the shuttle Challenger disaster in 1986.
Sobchak said ground controllers activated the TDRS 3 satellite from storage mode to replace the retired TDRS 4 craft covering the Atlantic Ocean region.
Seven satellites remain operating in the TDRS system, and the first of three replacement spacecraft is due to launch in the fourth quarter of 2012.
Two satellites are covering the Atlantic, Pacific and Indian Oceans, constantly linking research craft with ground terminals in White Sands, N.M., and Guam.
Before the Tracking and Data Relay Satellite System, ground controllers could only contact satellites and crewed spacecraft as they passed over a scattered network of ground stations.
The satellite system increased communications coverage of NASA missions from less than 20 percent of each orbit to nearly 100 percent of the time.