NASA's Tracking and Data Relay Satellite System (TDRSS) constellation is scheduled to receive its first on-orbit update in a decade Jan. 30, sustaining the workhorse fleet as demands for its services continue to grow.

TDRS-K, the first of three third-generation TDRSS birds, is scheduled to lift off at 8:52 p.m. EST from Space Launch Complex 41 at Cape Canaveral on an Atlas V 401 with a 4-meter-dia. (13-ft.) fairing to contain the large, folded single access antennas that deliver two-way high-data-rate service (see photo).

Although the spacecraft carries the latest in beam-forming equipment, Boeing built it on the same 601 bus it used to build TDRS-H, -I and -J, which were launched between June 2000 and December 2002. After that, the agency ran into funding hurdles.

“Getting funding for us is always the thing that kind of pushes out when we can start building spacecraft,” says Jeff Gramling, TDRS project manager at Goddard Space Flight Center. “As most agencies or companies that maintain constellations of spacecraft, we try to model the health of our on-orbit fleet to predict when it's going to fail. Typically, we've been blessed with our spacecraft lasting well beyond design life.”

NASA maintains seven TDRSS satellites in geostationary orbit, two in each of the three operational “nodes” that collectively cover the planet and one on-orbit spare. Four of those were built for the first-generation TDRSS, which dates to 1983, and the remainder are from the second batch.

That group of three Boeing birds added Ka-band to the lineup, as demand for bandwidth in space continued to grow. The TDRSS supports a variety of spaceflight customers, ranging from the International Space Station (ISS) to the Hubble Space Telescope to dozens of scientific satellites in low-to-medium Earth orbit. They also relay telemetry from some U.S. launch vehicles during ascent, including the new SpaceX Falcon 9, and—as a “critical national resource”—serve some classified spacecraft as well.

The demand will only increase as the ISS swings into full research operations, with requirements for communications links to carry scientific data between the orbiting laboratory and scientists and engineers on the ground, and as new spacecraft are launched for various purposes.

“Typically, the user missions that NASA builds tend not to be constellations, so we're supporting a whole series of one-off spacecraft,” Gramling says. “So those come and they go . . . . And of course, supporting station ops and all the different kinds of vehicles that are supporting station ops, including SpaceX, and launch vehicles, . . . it's like bandwidth on the ground. Bandwidth demand never goes down.”

Procured for $354 million each, TDRS-K and its two follow-ons, -L and -M, will retain the Ka-band capability for the single-access capability, but they will return to the original approach of doing beam-forming on the ground for the multiple-access phased-array antennas that use lower-rate S-band frequencies. Applying demand-access technology on the ground allows low-data-rate users to maintain continuous contact with their spacecraft for anomaly cueing or alerts to interesting scientific phenomena.

“That service evolved while we were building -H, -I, -J, and that said the ground-based beam-forming architecture is something we want to go back to,” Gramling says. “So we had to make that change on these spacecraft [-K, -L and -M], and that meant that we needed to have more power, and the spacecraft got a little heavier because we have to transmit all of the element beams to the ground, instead of just the form beams.”

The new spacecraft's twin three-panel solar arrays are designed to provide 2,850-3,220 watts at the end of their 15-year design life, depending on the time of year. Plans call for the Atlas V to place TDRS-K in a checkout geostationary slot at 150 deg. W. Long., where it can communicate easily with the system's main ground control station at White Sands, N.M. After three months of testing and calibration at that slot, it will be drifted to 171 deg. W. Long. while TDRSS managers decide which of the operational spacecraft it will replace.

TDRS-L is nearing completion at Boeing and is scheduled for launch early in 2014, with TDRS-M to follow in 2015.