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
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.