The Tracking and Data Relay Satellite (TDRS) system represents a new era in tracking Earth-orbiting spacecraft, including the Space Shuttle, and transmitting their data back to Earth. The TDRS concept was conceived following early 1970s studies which showed that a system of orbiting telecommunications satellites, operated from a single ground terminal link, could more effectively support Space Shuttle, scientific and other NASA mission requirements than the nearly 25-year-old tracking and communications network of ground stations located worldwide.

The TDRS network provides almost full-time coverage not only for the Shuttle, but also for typically up to 24 other orbiting spacecraft simultaneously. Services provided include communications, tracking, telemetry and data acquisition.

The TDRS satellites orbit geosynchronously at 22,300 statute miles (35,888 kilometers) above the Earth and look down on an orbiting Shuttle or spacecraft. This means that for most of their orbits around the Earth, these spacecraft will remain in sight of one or more TDRS satellites.

In the past, spacecraft could communicate with Earth only when they were in view of a ground tracking station, typically less than 15 percent of each orbit. The full TDRS constellation enables spacecraft to communicate with Earth for about 85 to 100 percent of the orbit, depending on their altitude. All Shuttle missions and nearly all NASA spacecraft in Earth orbit require TDRS support capabilities for mission success.

The TDRS constellation currently includes four on-orbit satellites positioned over the Equator. Two are stationed 130 degrees apart. TDRS-5 functions as TDRS-West at 174 degrees west longitude, while TDRS-4 at 41 degrees west longitude is known as TDRS-East. Both are fully functional.

The remaining two spacecraft, TDRS-1 and TDRS-3, are on- orbit backups. TDRS-1, at 171 degrees west longitude, has exceeded its design life of seven years and provides limited services. TDRS-3, at 62 degrees west longitude, also has limited capability.

In the event of a malfunction of one of the fully operational TDRS spacecraft, the absence of a third fully operational satellite in ready reserve would severely impact orbiting spacecraft for nearly a year before an emergency replenishment launch could take place.

A fifth TDRS spacecraft, TDRS-F, is slated for launch in early 1993. Successful deployment of this TDRS, which will become TDRS-6 in orbit, will fulfill the requirement for two fully operational satellites and a fully operational on- orbit, ready-reserve capability, and ensure that communications, telemetry and data acquisition capabilities required by space missions will not be jeopardized.