Designation of a TDRS satellite depends on
whether it is on orbit or still on the ground. On-the-ground TDRS
spacecraft receive a letter designation: TDRS-A, TDRS-B, TDRS-C, etc. Once
on orbit, this designation changes to a numeral: TDRS-A becomes TDRS-1,
etc. Even though the TDRS-B spacecraft never achieved operational orbit
because it was lost in the Challenger accident, the TDRS-2 designation it
was to have received was not reassigned.
The concept of a multi-satellite
constellation has engendered another way of referring to the TDRS
spacecraft. The TDRS occupying the 41 degrees west longitude slot also is
referred to as TDRS-East, and the spacecraft in the 174 degrees west
longitude location occupies the TDRS-West slot.