Telemetry · Attitude & orbit
Attitude & orbit
The station holds its orientation with four 100 kg gyroscopes spinning at 6,600 rpm. They absorb disturbing torques without burning any propellant.
49 of the station's public readings belong here — NASA's catalogue files them under ADCO, ADCO/VVO, ADCO/TOPO. They are live on the console, and each one is explained below.
Control moment gyroscopes
- CMGs online
- Four gyroscopes are fitted, all in the Z1 truss, and this count is the sum of the four flags below: when the stream was checked both read 4. The momentum capacity further down is that of the active set, so it falls when a gyroscope drops out.
- CMG-1 online
- IN USE means this gyroscope’s stored momentum is part of the set the controller steers with. A wheel in use must be spinning, so the verification run pairs each flag with the wheel speed below it and warns when they disagree.
- CMG-2 online
- The second of four. Each carries a 98 kg flywheel at 6,600 rpm, and NOT IN USE removes it from the set the controller steers with; the verification expects that flag to go with a stopped wheel, and IN USE with one at speed.
- CMG-3 online
- The third of four. The Reference Guide calls the gyroscopes the preferred method of attitude control because they run on electricity from the arrays; when too few are in use to provide the momentum needed, the Russian segment’s thrusters take over.
- CMG-4 online
- The fourth of four. The operational gyroscopes sit inside the Z1 truss and are not visible from outside, which is why these channels attach to Z1 in the twin; the two CMGs modelled on the ELC platforms are stowed spares, not this one.
- CMG-1 wheel speed
- The flywheel spins continuously at about 6,600 rpm; it is the tilt of its axis, not its speed, that steers the station.
- CMG-2 wheel speed
- All four wheels read 6,600 to 6,601 rpm when the stream was verified, their documented nominal speed. A wheel that is online must be spinning, so this reading is checked against the CMG-2 online flag above.
- CMG-3 wheel speed
- The 98 kg flywheel stores its angular momentum by spinning fast, and the speed is held constant: steering comes from the gimbals that tilt the wheel’s axis, not from speeding it up or slowing it down.
- CMG-4 wheel speed
- Because the speed is meant to be constant, a wheel far below 6,600 rpm is spinning down and out of service, not steering harder. The verification treats anything under 6,000 rpm as a stopped wheel and expects its online flag to say NOT IN USE.
- CMG-1 vibration
- Read in g, fractions of Earth’s gravity. A 98 kg flywheel turning at 6,600 rpm would be felt through the Z1 truss if it ran rough, and the Reference Guide counts smooth control among the reasons the gyroscopes are preferred to thrusters on a station kept for microgravity.
- CMG-2 vibration
- The four vibration sensors are alike, so compare this one with its three neighbours rather than against a limit: none of the documents this site cites publishes a vibration threshold for a CMG.
- CMG-3 vibration
- Vibration is health telemetry for the ground; a gyroscope is taken out of the control set through its online flag, not by this reading. Should one need replacing, two spare CMGs are stowed outside on the ELC platforms.
- CMG-4 vibration
- Each of the four is a separate unit with its own vibration sensor, mounted together in the Z1 truss. The Mimic telemetry guide describes readings like this one as the detailed health status of each of the four CMGs, which is why they are published at all.
- CMG-1 spin motor temp
- The bearing the 98 kg flywheel turns on, at 6,600 rpm without pause while the gyroscope is in use. The catalogue files the unit as Fahrenheit, but its own description says degrees Celsius, and Celsius is what is shown.
- CMG-2 spin motor temp
- The second gyroscope’s spin bearing. The spin motor it names is what the arrays’ electricity feeds, which is the Reference Guide’s point about the CMGs: they hold the station’s attitude on power the station makes, not on propellant it must have delivered.
- CMG-3 spin motor temp
- The third gyroscope’s spin bearing. Temp1 in its name is the motor end: the catalogue lists a second bearing temperature per CMG, at the Hall resolver end, which this site does not subscribe to. With the online flag, wheel speed and vibration beside it, it is the detailed health status the Mimic guide describes.
- CMG-4 spin motor temp
- The fourth gyroscope’s spin bearing. It is a temperature in degrees Celsius despite the catalogue’s Fahrenheit tag, like the other three; the value is never converted here, only its label corrected.
Momentum
- CMG momentum saturation
- As the gyroscopes approach saturation, the station must desaturate them using the Russian thrusters.
- Active CMG momentum
- The momentum the active gyroscope set is holding at this moment, in newton-metre-seconds. Divided by the capacity below, it gives the saturation percentage: when the stream was checked, 14.36 % published against 14.36 % computed. The catalogue’s ft·lb·s tag is wrong; the description says N·m·s.
- CMG momentum capacity
- The ceiling the saturation percentage is measured against. The Reference Guide notes the gyroscopes are limited in the momentum they can hold; when they can no longer provide what is needed, the Russian thrusters take over. It is the capacity of the active set, so it depends on how many are online.
- Desaturation request
- Whether the US controller may ask for a thruster firing to unload the gyroscopes. ENABLED lets the request go out when the stored momentum needs resetting; INHIBITED blocks it, and the gyroscopes are left to hold what they have accumulated.
- Control torque — roll
- The torque the controller asks the gyroscopes to apply about the station’s long axis, the body X axis that in normal flight lies along the direction of travel. It is produced by tilting the flywheels, not by changing their speed. The catalogue labels it ft·lb; the description says N·m.
- Control torque — pitch
- The torque asked for about the body Y axis, which runs along the truss. It stays small when the station sits at the attitude where the natural torques balance, which is why the gyroscopes have little to fight. It updates every two seconds, like the rest of this section.
- Control torque — yaw
- The torque asked for about the body Z axis, the one that points at the ground in the usual XVV ZNADIR attitude. Roll, pitch and yaw here are the three body axes in turn; the ops name says so, InBody X, Y and Z.
Orientation
- GNC mode
- The station flies in a local frame: its belly stays towards Earth, its nose towards the direction of travel.
- Attitude controller type
- ATTITUDE HOLD keeps a commanded orientation whatever it costs the gyroscopes. TEA, the torque equilibrium attitude, lets the station settle where the natural torques balance, so the gyroscopes have less to fight and store momentum more slowly.
- Attitude reference frame
- LVLH is the frame that turns with the orbit, belly to Earth and nose forward, and the errors below are measured in it. Inertial holds the station fixed against the stars instead. XPOP keeps the long axis perpendicular to the orbit plane, an attitude flown in the early years because it kept the P6 arrays facing the Sun all the way round.
- Attitude manoeuvre in progress
- TRUE while the station is being turned from one attitude to another rather than holding one. The Reference Guide gives the docking of visiting vehicles and debris avoidance as reasons for a manoeuvre; the errors and control torques above move while it is set.
- Roll error
- Zero on all three axes means the station is flying exactly along its velocity vector. It normally holds an attitude that minimises natural torques, so the gyroscopes have little to fight.
- Pitch error
- The angle about the truss axis between where the station points and where the controller wants it: nose up or down of the target. The catalogue files it in radians, but the description says degrees, and degrees is what is shown.
- Yaw error
- The angle about the vertical axis between the attitude held and the one commanded: nose left or right of the target. The three errors are what the three control torques above answer, and all of them update every two seconds.
- Inertial rate X
- How fast the station is turning about its long axis against the stars, measured by the rate gyros on the S0 truss. In the usual Earth-facing attitude this one should sit near zero: only the pitch axis carries the once-per-orbit turn.
- Inertial rate Y
- The rate about the truss axis, and the one that is not zero: a station that keeps its belly to Earth turns once per orbit against the stars, 360° in about 93 minutes, which is 3.87° a minute or about 0.065°/s in the unit shown.
- Inertial rate Z
- The rate about the vertical axis. The catalogue tags all three rates rad/s, but the description says degrees per second, which is what is shown; like the errors above, the three rates update every two seconds.
- LVLH quaternion q0
- Four numbers that describe one rotation, from the orbit-following LVLH frame to the station’s body axes, and mean nothing apart. Their squares must sum to one, which tests four sensors at once: on the live stream the norm came to 1.000000.
- LVLH quaternion q1
- The second component. The site does not plot the quaternion, on purpose: one component alone is not a curve anyone can read, and together the four are an attitude, which the roll, pitch and yaw errors already show in degrees.
- LVLH quaternion q2
- The third component. LVLH is the frame in which the belly points at Earth and the nose along the direction of travel, so the quaternion says how far the body axes are turned from that ideal, and normally not far.
- LVLH quaternion q3
- The last of the four. Read with the other three it fixes the station’s orientation in the frame that turns with the orbit; it is refreshed with the rest of the attitude data, every two seconds.
State vector and mass
- Total station mass
- The station itself is about 420 tonnes; the published figure includes every docked vehicle and its propellant, so it runs higher. It has to stay accurate — the thrusters need it to control the station’s orientation. Check its age: this one is updated only occasionally.
- J2000 position X
- One of three coordinates in J2000, an inertial frame pinned to the stars at the start of the year 2000. The three together give the distance from Earth’s centre: 6,790.5 km when the stream was checked, 0.07 km from what Celestrak’s orbital elements gave for the same moment.
- J2000 position Y
- The second coordinate. The catalogue lists the unit as feet, but the description says kilometres and the values are kilometres: a radius near 6,790 km is the station’s orbit, and the same numbers in feet would put it inside the Earth.
- J2000 position Z
- The third coordinate, along Earth’s polar axis as it stood in 2000. It swings between north and south every orbit, and because the orbit is inclined 51.6° it never reaches the full radius: the station never flies over the poles.
- J2000 velocity X
- One of three components of the station’s velocity in the J2000 frame. Together they came to 7,662.2 m/s when checked, within 0.07 m/s of an independent propagation from Celestrak, and the speed and radius satisfy the vis-viva equation to 3.3 m/s.
- J2000 velocity Y
- The second velocity component. The catalogue tags it feet per second; the description says metres per second, which is what is shown. The Reference Guide gives the same speed as 28,000 kilometres per hour.
- J2000 velocity Z
- The third velocity component, along the polar axis. Position and velocity together are the state vector, which the station propagates itself; the site does not plot the six components because the orbit panel already presents them as a position and a speed.
- GPS 1 status
- DOING POSITION FIXES is the healthy state: the receiver, in the US Lab with its antennas on the S0 truss, is fixing the station’s position. The other states are degradations, from timing-only modes to too few usable satellites (SV) to a fix the receiver will not trust.
- GPS 2 status
- The second receiver; the Reference Guide lists GPS receivers, plural, in the US Lab. SV in several states is a GPS space vehicle, and the ONLY 1, 2 or 3 USABLE SVs states say how many the receiver can hear, fewer than a position fix needs.
Alarms
- Loss of CMG attitude control
- TRUE means the gyroscopes have stopped holding the attitude, and control passes to the Russian thrusters, the fallback the Reference Guide describes for when the CMGs can no longer provide what is needed. It is filed as a caution where the alarm beside it is a warning, and latched: it stays raised until cleared.
- Loss of ISS attitude control
- The graver of the two: the station as a whole, not just the gyroscope set, is no longer holding its orientation. The catalogue’s own name for it ends in Warning where the CMG alarm is a Caution. FALSE is the normal state, and it decoded to a known state when the stream was verified.
Where it lives on the station
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Z1 Truss Z1 Integrated Truss Segment
The first truss piece launched, mounted above Unity. It carries the four control moment gyroscopes that point the station, and the Ku-band antenna.
Each link opens the 3D twin on that part, with its live readings beside it.