When Boeing’s CST-100 Starliner failed to reach the International Space Station in December, initial reports pegged part of the problem to NASA’s Tracking and Data Relay Satellite System (TDRSS).
Later analysis revealed software issues led to Starliner’s abbreviated test flight. Still, the mission raised the profile of TDRSS and underscored the need for robust communications in low Earth orbit.
Since the 1970s, NASA has updated its Space Network repeatedly to keep up with demand. Seven satellites and four ground stations provide the bulk of communications for more than 40 missions including the International Space Station, NASA’s Hubble Space Telescope and Transiting Exoplanet Satellite Survey.
Some Space Network satellites are more than two decades old. Nevertheless, the Space Network continues to offer reliable global coverage 24-hours a day for missions led by NASA, the National Oceanic and Atmospheric Administration and other government agencies, said Ted Sobchak, NASA Space Network project manager. The satellites generally have plenty of fuel and when battery power runs low, NASA works with experts to manage it, he added.
Meanwhile, the space agency continues to update ground stations. “The magic happens on the ground,” Sobchak said. “As modern technology and waveform processing improves, we are able to add new capabilities and come up with new ways of solving problems.”
In the past, satellites in certain regions of Earth orbit, including over the Indian Ocean, couldn’t reach the Space Network. Those issues have largely been resolved with satellites over the Indian Ocean and a gateway in Guam.
“As long as you are [at an altitude] over about 73 kilometers, you’re always in view of a Tracking and Data Relay Satellite,” Sobchak said.
That’s not to say spacecraft never experience lapses in Space Network coverage. The Space Station’s signal drops when part of the outpost’s structure blocks its antenna. In addition, Space Network satellites are in inclined orbits, meaning other satellites passing over Earth’s poles sometimes move out of TDRSS’ view.
Regarding Starliner, “it was in view of TDRSS all the time,” Sobchak said. “Our system performed as expected and we pointed appropriately.”
Boeing reached the same conclusion. The Starliner flight didn’t reveal any deficiencies with the Tracking and Data Relay Satellites or the Space Network, Boeing spokeswoman Jessica Landa said by email. “There are many factors that impact space-to-space connections, such as relative geometry between spacecraft, antenna boresight angles and dynamic ranges, as well as environmental interference.”
Nevertheless, there was a brief loss of signal during the Starliner mission.
“We can confirm that at the time of the dropout, our system correctly selected the right antenna pair pointing at the right Tracking and Data Relay Satellite, and that throughout most [of] the mission we were able to maintain constant communication and signal lock,” Landa said. “We did observe external signal interference over certain geographies, which may be attributable to a high noise floor because of cell towers operating at similar frequencies.” Boeing is continuing to investigate the matter and to make its system more resilient, she added.
This article originally appeared in the March 16, 2020 issue of SpaceNews magazine.
