“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.