Telemetry · Command & data
Command & data
Around fifty redundant computers run the station, in three tiers. The top tier is two-fault tolerant: three identical command-and-control machines, one hot, one warm backup processing data without commanding, and one cold.
16 of the station's public readings belong here — NASA's catalogue files them under ODIN, ODIN/VVO, CDH. They are live on the console, and each one is explained below.
Station status
- Station mode
- Standard most of the time. It switches to proximity operations while a spacecraft docks, to external operations during a spacewalk, and to reboost when the thrusters fire to raise the orbit.
- Crew laptops connected
- The Portable Computer System laptops are how the crew command the station: each plugs into a data bus through a receptacle and talks to the command-and-control computer directly, at most one command a second. The architecture paper allows up to eight of them on the control buses under that computer.
- Standard commands received
- Every command the command-and-control computer accepts adds one, whether it came from a crew laptop or from Houston over S-band. The count changes only when a command arrives, so an old timestamp here dates the last command, not a stalled sensor; it sat still through a five-minute capture.
- Data load commands received
- A second tally, kept apart from the standard commands. The architecture paper lists a data load as a transaction of its own, distinct from a standard command, and describes what the ground uploads to this computer besides commands: files that reconfigure its telemetry tables, sent over S-band. Like the other count, it moves only when one arrives.
- Onboard time (coarse)
- The station’s own clock as the command-and-control computer keeps it. Every reading on this site is dated by the station’s clock rather than by when the packet arrived: the age beside a value is the station’s, so a sensor that stopped weeks ago reads as stopped however recently the stream re-sent it.
- Onboard time (fine)
- Read with the coarse count above, it gives the computer’s time. On each data bus one computer is the controller and broadcasts a time-synchronisation message at the start of every 100 ms processing frame; the others set their clocks to it and correct for the message’s travel time down the bus.
Time
- GMT
- The station runs on Greenwich time — the compromise between Houston and Moscow. It counts days from 1 January, so day 243 is 31 August; the year comes from a second reading, TIME_000002.
Computers
- C&C MDM 1
- One of three identical command-and-control computers in Destiny: the hot one commands, the warm one processes the same data without placing commands on the bus, and the third is powered off. The status is read at the remote power controller feeding the machine, so Off-Ok is the cold spare, not a failure; only Not-Off Failed reports a fault.
- C&C MDM 2
- The second of the trio. A 2002 architecture paper gives these computers an Intel 386SX at 32 MHz with 8 MB of RAM, and remarks that the next crew laptops would be far more capable than the machines they command; some have since had the EPIC processor upgrade the 2015 reference guide describes.
- C&C MDM 3
- The third of the trio, which is what makes the top tier two-fault tolerant: two of the three can fail before the station loses command. The tiers below generally settle for a pair, one hot and one powered off.
- GNC MDM 1
- One of the two local-tier computers that host the guidance, navigation and control software. Commands from the command-and-control computer pass through it on the way down to the gyroscopes and sensors, and their telemetry climbs back the same way.
- GNC MDM 2
- The other half of the GNC pair, one-fault tolerant rather than two: in the local tier one machine is hot and the other is powered off, so a healthy pair reads one Not-Off Ok and one Off-Ok.
- Internal control zone 1
- One of the two computers of the local tier that the catalogue names the internal control zone, as against the external one on the truss. Its status is the state of the remote power controller feeding it, the switch named in its nomenclature, rather than a word from the computer itself.
- Internal control zone 2
- The other internal control zone computer, normally the reverse of its twin above: whichever is hot reads Not-Off Ok and the powered-off spare Off-Ok. Being a state rather than a measurement, its timestamp dates the last time that changed.
- External control zone 1
- One of the two external control zone computers, mounted outside on the S0 truss rather than inside Destiny. The architecture paper notes that a computer outside is exposed to micrometeoroid strikes and slightly more prone to radiation-induced upsets than one in a module; one of the pair is hot, the other powered off.
- External control zone 2
- The other external computer on S0. Like every entry in this list its status is read at the remote power controller feeding it: Off-Ok is a switch open on a healthy spare, Not-Off Ok a powered machine, Not-Off Failed a powered one reporting a fault.
Where it lives on the station
-
S0 Truss S0 Integrated Truss Segment
The centre segment, bolted to the Destiny laboratory. Everything else on the truss extends from here.
-
Destiny U.S. Laboratory
The American laboratory and nerve centre of the US segment: the station is commanded from here, and most experiments run here.
Each link opens the 3D twin on that part, with its live readings beside it.