Of the 11 TDRS satellites launched, eight still are operational. Four of those already are beyond their design life. Two have been retired. One was lost in a space shuttle accident. (Images courtesy of NASA)
For over 30 years, NASA’s Tracking and Data Relay Satellite (TDRS) fleet has provided communications support for a variety of missions and launch vehicles. Examples include the Hubble Space Telescope and the International Space Station. Of the 11 TDRS satellites that have launched, eight are currently still in operation. TDRS-L, the second of three replenishment satellites, recently arrived at Kennedy Space Center in anticipation of attachment to a United Launch Alliance Atlas V rocket.
TDRS-L is part of the third generation of TDRS satellites that includes TDRS-K (launched in January 2013, pictured left) and TDRS-M (set to launch as early as 2015). These satellites utilize ground-based beamforming (GBBF) as opposed to on-orbit beamforming of S-band multiple-access return services. With this approach, the location for the communications signal processing of some services changes from the spacecraft to the ground. Reportedly, this approach provides more flexibility and unique tailoring options including unscheduled access on demand.
The satellite payloads contain multiple antennas to provide near-continuous high-bandwidth telecommunications service. A phased-array antenna at S-band multiple access is designed to receive signals from up to five spacecraft simultaneously. It can transmit to one spacecraft at a time through 32 receive and 15 transmit elements. Two mechanically steerable, or gimbaled, antennas provide high-gain support at S-band, Ku-band, and Ka-band single access. Operation at Ku-band enables the support of two-way high-resolution video and customer science data. Ka-band operation allows data transmission at 800 Mbps as well as international compatibility. Forward-omnidirectional and AFT-omnidirectional antennas provide S-band telemetry, tracking, and command.
The TDRS Project was established in 1973 to provide continuous, around-the-clock communications services to critical low-Earth-orbiting missions while improving the amount of data that could be received. The building of all three next-generation satellites is contracted to Boeing Space Systems as a part of NASA's Space Communications and Navigation Program.