Most of the spacecraft are well beyond their 10-year design life, he added.
Initially developed to support the space shuttle and space station programs, the TDRS network now serves a variety of NASA spacecraft and commercial users such as Space Exploration Technologies and foreign space agencies flying cargo ships to and from the station, a $100 billion research laboratory staffed by rotating crews of astronauts and cosmonauts.
The new spacecraft, which cost between $350 million and $400 million, will take about 10 days to reach its intended orbit. It will then go through a three-month checkout before it is put into service, Gramling said.
The 12th and 13th TDRS satellites are targeted for launch in 2014 and December 2015.