TDRSS stands for tracking and data relay satellite system. NASA has launched a number of TDRSS communications satellites to act as relay stations to channel information from manned and unmanned spacecraft to the ground.
The system has taken over much of the work of NASA's formerly very extensive worldwide tracking system known as the STDN (space tracking and data network). The TDRSS comprises two satellites and an in-orbit spare, in geostationary orbit 35,900 km above the Equator. The two operational satellites are located at about longitude 41 degrees west (TDRSS east) and longitude 171 degrees west (TDRSS west). The satellites are able to handle incoming data from a shuttle orbiter and up to 25 satellites at the same time. The TDRSS satellites transmit data to a single ground station, NASA's White Sands Test Facility in New Mexico. From there the signals are relayed by the domestic communications network to the Goddard Space Flight Center at Greenbelt, Maryland (for satellites), or to the Johnson Space Center, Houston (for shuttle flights). TDRSS satellites are the largest and most advanced communications satellites yet developed. They are designed for deployment into low Earth orbit by the space shuttle. After deployment, the satellites are lofted to high orbit by an attached booster rocket called the IUS (inertial upper stage). The first TDRSS satellite, TDRS-1, was deployed on the maiden flight of shuttle orbiter Challenger (mission STS-6) on April 4, 1983. Unfortunately, a failure in the IUS caused it to be placed in too low an orbit. But after 58 days of delicate manoeuvres, NASA engineers coaxed TDRS-1 into the correct orbit. It is presently on standby as a spare. TDRS-2 was lost in the Challenger accident in January, 1986. TDRS-3 was launched from orbiter Discovery on STS-26 on September 29, 1988, the first post-Challenger flight. This is currently the TDRSS west satellite. TDRS-4, currently the TDRSS east satellite, was also deployed from Discovery, on the STS-29 flight on March 13, 1989.
