Telemetry · Communications
Communications
The station does not talk to the ground directly: it aims at TDRS relay satellites 35,800 km up. Between two relays, contact drops for a few minutes.
19 of the station's public readings belong here — NASA's catalogue files them under CATO, CATO/VVO. They are live on the console, and each one is explained below.
S-band — voice and commands
- RFG 1 azimuth
- S-band carries voice, commands and telemetry — NASA works 2,025–2,110 MHz up to the station and 2,200–2,300 MHz back down. Two groups are kept, one per side, so the station stays in contact when its own structure blocks one of them.
- RFG 1 elevation
- Up and down, where azimuth is left and right; between them the two gimbals hold the starboard dish on whichever relay satellite is in view. This link outranks Ku-band: it carries the critical commands and telemetry, while the Ku dish carries the non-critical data and the video.
- RFG 1 power
- Read at the remote power controller that feeds the group, not from the radio itself: Off-Ok means the switch is open, Not-Off Ok that it is closed and healthy, Not-Off Failed that it is closed but reporting a fault. Two groups exist and typically only one is active at a time.
- RFG 2 azimuth
- Left and right for the port group, on the P1 truss. The catalogue never says which antenna each group drives; the side was settled from the symbol names, S1 against P1, and from where the two antennas sit in NASA’s own model of the station.
- RFG 2 elevation
- Up and down for the port dish, the twin of the starboard reading. Typically only one of the two groups is carrying the traffic at a time and the other is the spare; the angles alone do not say which, so read them alongside the active S-band string below.
- RFG 2 power
- The same switch reading for the port group, from a power controller on the P1 truss — the symbol name carries the segment. An old timestamp on it dates the last time the state changed, not the last time it was checked: a group reading Not-Off Ok for weeks is a group that has stayed on.
- Active S-band string
- The station keeps two complete S-band chains — baseband signal processor, transponder, radio frequency group — and this names the one carrying the traffic. It is an identifier, not a measurement: the timestamp dates the last swap, and an old one means the string has not had to change.
Ku-band — video and data
- Ku-band transmit
- Ku-band carries the heavy traffic: high-definition video, experiment data, file transfer. NASA holds 13.4–14.3 GHz and 14.5–15.2 GHz for the relay satellites, and this dish aims at one of them 35,800 km up rather than at the ground.
- SGANT elevation
- Zero points straight up, −90° towards the back of the station, +90° straight ahead. Near the extremes the dish must reset to catch the next relay satellite, and the link drops.
- SGANT cross-elevation
- The dish’s second axis, swung across the line of elevation; the two together keep it on a relay satellite. The catalogue leaves the unit blank, so the application shows degrees because the elevation axis of the same dish is published in degrees.
- Video downlink 1
- Ku-band carries several video channels to the ground at once, and this is the first of the four the station reports. Active means a picture is going down it now — a camera view, an experiment feed or video from a crew member outside on a spacewalk — and Inactive that the channel is idle.
- Video downlink 2
- The second of four. Each channel reports its own activity, so several can be Active at once — one carrying a live event, another an experiment’s camera — and the count of busy channels is a rough measure of how much picture is leaving the station.
- Video downlink 3
- The third of four. Mission Control and NASA TV are at the far end: the live views of Earth and of the crew that the public sees come down these channels, through the relay satellite the Ku dish is aimed at rather than straight to the ground.
- Video downlink 4
- The last of the four. The same Ku-band link also carries the crew’s internet access and file exchange, and the experiments’ own data, so a channel reading Active is only one part of what is passing through the dish.
UHF and audio
- UHF radio 1
- Around 400 MHz — the short-range link to spacewalking astronauts and to vehicles on final approach. It also carries each suit’s own telemetry. Both radios on at once very likely means a spacewalk is under way.
- UHF radio 2
- The second space-to-space radio. The two are fed from different power controllers — LA1B against LAD52B in the symbol names — so the pair does not hang on one switch, and the reading here is that switch’s, not the radio’s: Off-Ok is the switch open, Not-Off Ok closed and healthy, Not-Off Failed closed but reporting a fault.
- UHF frame sync lock
- Locked when receiver and transmitter are talking to each other properly.
- Internal audio controller 1
- The station’s voice switchboard: it routes audio between the crew, the S-band link to the ground and the UHF link to a spacewalker, which is how Mission Control hears a crew member outside. Two controllers are kept; one is Active and the other stands by as Backup, and a swap between them shows here.
- Internal audio controller 2
- The second audio controller, the twin of the first. Read it against controller 1: the two are a pair, and whichever is not Active is the Backup. Like every enumerated state its timestamp dates the last change, so an old one means the roles have not swapped for that long.
Where it lives on the station
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Ku-band Antenna Space-to-Ground Antenna (SGANT)
The large dish that sends video and high-rate data up to the TDRS relay satellites.
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S-band Antenna SASA-2 S-Band Antenna Support Assembly 2
One of the two low-rate links carrying voice and commands to the ground.
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S-band Antenna SASA-3 S-Band Antenna Support Assembly 3
The second S-band assembly, redundant with the first.
Each link opens the 3D twin on that part, with its live readings beside it.