Communication is the linchpin of safe, reliable International Space Station operations, and the world's window on research progress and results. But it's a tough job with complex requirements. Consider:
The Communication and Tracking System must provide two-way audio and video communications among ISS crew, between crew and Mission Control, and between crew and Earthbound scientists via Ku-band, S-band and UHF frequencies.
It must permit Mission Control to send commands directly to ISS flight controllers or relay them through the Space Shuttle while providing system, experiment and payload data to Mission Control and the Payload Operations Center.
And the System is tasked with allowing the European Space Agency, National Space and Development Agency of Japan, and Mission Control Center-Moscow to communicate with the ISS via Mission Control Center-Houston.
Here is how it works:
Mission Control Center, Houston sends commands and receives telemetry via the 60-ft. diameter, high-gain microwave ground terminals at NASA's White Sands Test Facility near Las Cruses in southern New Mexico. It uses the S-band and UHF frequencies, as well as the early ISS communications system.
The terminals relay signals to and from a pair of Tracking and Data Relay Satellite System satellites orbiting at an altitude of 22,300 miles. The satellites pass the signals directly to and from the U.S. segment of the ISS at a rate equivalent to sending the contents of the entire encyclopedia Britannica every second.
Commands can also reach the station through a docked Space Shuttle and through the Russian communications system. Russian coverage is nearly continuous while using Russian ground stations, which are available for a portion of an orbit. Russia's launch tracking and data relay satellite provides coverage for about 45 minutes per orbit.
In addition to carrying commands and telemetry, the S-band is used for voice communication and file transmission. UHF is used during spacewalks and other relatively short-distance communication.
Inside the ISS, an Internal Audio Subsystem serves as the intercom, telephone and alarm system for the pressurized elements of the U.S. segment. Working in tandem with Russia's Service Module, the Subsystem is capable of serving the entire Station.
The Internal Audio Subsystem also transmits to the Space Shuttle, spacewalkers, and ground stations. And it will feed into on-board videotape recorders for the recording and playback of audio. Without this capability, ISS videos essentially would be silent movies. On-board video cameras will not provide audio. Video on the ISS is recorded on external, internal and payload rack cameras.
The high-bandwidth Ku-band is used for video transmission, as well as for the two-way transfer of files. It also carries data from experiments and other payloads to Mission Control, the Payload Operations Integration Center, and scientists on the ground.