Power

Solar Alpha Rotary Joint

Port SARJ angle S0000004 · in °

Position of the left rotary joint. It completes one full turn every orbit, about 4° per minute.

Starboard SARJ angle S0000003 · in °

Position of the right rotary joint. It reads as the mirror of the port one — the two sum to 360° — because each side counts its angle in its own direction. They can also be driven a quarter turn apart, which NASA has photographed.

Port commanded angle S0000005 · in °

The angle commanded to the port SARJ. Subtract the actual position above and you have the tracking error, which normally runs a couple of tenths of a degree — the joint is where it was told to be, and a growing gap would be the first sign it is not.

Port SARJ mode S0000008 · a state

Autotrack is the normal state — the joint follows the Sun at orbital rate, and both SARJs report it live. The other names are ones Mimic glosses for the thermal joints: directed position holding a commanded angle, shutdown the motor disabled. It carries two fully redundant drive strings, each with its own motor controller and drive-lock assembly, so a failure in one need not stop the wings.

Starboard SARJ mode S0000009 · a state

Same states as the port joint, and normally autotrack too. This is the joint that failed: eleven weeks after it was switched on in June 2007 its drag began climbing, and a spacewalker lifting a thermal cover found fine metal shavings across the race ring. Operation was halted; on STS-126 the crew scraped and greased the ring and replaced eleven of the twelve trundle bearings — the twelfth had come off a year earlier, for examination on the ground.

Beta Gimbal Assembly

BGA 1A S4000007 · in °

Angle of the wing about its own mast, over a full 360° of travel, from the position where its cells face inboard along the truss. Facing the Sun would read 90° minus beta — or 90° plus, 270° minus, 270° plus, as each wing counts its own way — but the station flies them about 45° short of that on purpose, the "sun slicer" drag-reduction bias, and past about 48° of beta parks most of them at 0° or 180° instead.

BGA 3A S4000008 · in °

Same convention as 1A: 0° puts the cells inboard along the truss, and the wing is flown about 45° short of the Sun on purpose. 3A shares the S4 segment with 1A, so the two ride the same rotary joint and see the same beta; only their own gimbals differ. Its roll-out array was added in 2022, in the same pair as 4A.

BGA 1B S6000008 · in °

Same convention as 1A, on the outermost starboard segment, S6. A roll-out array was laid over this wing in 2023, in the same pair as 1A. Past about 48° of beta six of the eight wings stop tracking and sit at exactly 0° or 180°, parked where the bias would have carried them through zero.

BGA 3B S6000007 · in °

Same convention as 1A, on the S6 segment beside 1B. The gimbal exists to follow the seasonal drift of beta, about 4° a day, yet over thirteen minutes in which beta moved 0.05° every BGA moved 1.2° to 1.7° — a small orbital-rate term riding on the seasonal one, below the resolution of anything else published here.

BGA 2A P4000007 · in °

Same convention as 1A, on the port side: even channels are port, odd starboard. The gimbal can be commanded to an angle, to a rate, latched in place or left to be turned by hand from the truss side — but none of that is public. The stream carries a mode and a commanded angle for the SARJ only, so a deliberate bias on the wings cannot be read from here, only measured.

BGA 4A P4000008 · in °

Same convention as 1A, on the P4 segment beside 2A. The wing’s power crosses this joint through roll rings, a rolling electrical contact that works over the full 360° of travel. A roll-out array went on in 2022, in the same pair as 3A.

BGA 2B P6000008 · in °

Same convention as 1A, at the port end of the truss. 2B and 4B are the P6 pair, and P6 spent its first years mounted on top of the Z1 truss, feeding the laboratory directly, before the segment went out to the port end its symbol now names. It received the first of the roll-out arrays, in 2021.

BGA 4B P6000007 · in °

Same convention as 1A, on the P6 segment beside 2B. This is the wing that had to be rolled back into its blanket boxes in December 2006: once P4’s arrays began turning on the new rotary joint they would have struck it where it then stood. A roll-out array was added over it in 2021, with 2B.

Solar beta angle USLAB000040 · in °

Angle between the orbital plane and the direction of the Sun. Beyond 70° the station stays in permanent sunlight, and heats up.

Photovoltaic control units

Array 1A drive voltage S4000001 · in V

This sits at one of two levels, and which one says whether the station is in daylight. In Earth’s shadow it is always about 151 V — the batteries are carrying the station and there is nothing the arrays can do. In sunlight it rises to about 160 V while they charge, and drops back to 151 V once they are full and the surplus is shunted away. So a low reading in daylight is normal: it means the batteries are already topped up.

Array 1A drive current S4000002 · in A

Not published: all eight drive currents sit at exactly zero. NASA’s own ISS Mimic guide marks this reading "not working".

Array 3A drive voltage S4000004 · in V

Same two levels as 1A, about 151 V in shadow and up to about 160 V while the batteries charge. This is the channel that changes last: when the eight were caught apart, 3A was the straggler in 13 of 15 readings, up to 9 V behind — a transient, not a pointing difference. The 2011 catalogue labels it 3B; the S4 segment carries 1A and 3A.

Array 3A drive current S4000005 · in A

Not published: this reads exactly zero like the other seven drive currents, and carries no timestamp at all — never measured, which is what the reading NASA’s Mimic guide marks ‘not working’ looks like from here. The catalogue labels this pair 3B, though the S4 segment carries 1A and 3A.

Array 1B drive voltage S6000004 · in V

Same two levels as 1A. The unit that sets them, the sequential shunt unit, sits on this wing’s own gimbal platform at the foot of the mast: it switches whole strings of cells in and out to hold the bus near 160 V, and in shadow the batteries hold it near 151 V instead. 1B is the outermost starboard channel, on S6.

Array 1B drive current S6000005 · in A

Not published: exactly zero on all eight channels, with no timestamp behind it. The name of the symbol says what it would be — the integrated current through relay 6 of the 1B switching unit, the box that routes this wing’s power to the batteries and the main bus.

Array 3B drive voltage S6000001 · in V

Same two levels as 1A. The digit is the power domain the channel feeds: 3B belongs to the 2/3 domain with 2A, 2B and 3A, the 1/4 domain taking the other four, and each domain has its own cooling loop. The letter only says which of the two wings on the S6 module this is.

Array 3B drive current S6000002 · in A

Not published: it sits at exactly zero, as all eight drive currents do, and publishes no timestamp. The Mimic guide marks the same readout ‘not working’; the voltage beside it is the live half of this pair.

Array 2A drive voltage P4000001 · in V

Same two levels as 1A. Sunlight only permits the high one: across five days every reading taken in Earth’s shadow was below 155 V, without exception, while only about half of those taken in sunlight were — the other half had batteries already full. 2A is on P4, the inner port segment.

Array 2A drive current P4000002 · in A

Not published: exactly zero, with no timestamp, like every other drive current. It stayed at zero through five minutes in which the SARJ turned 19° and the gimbals moved over a degree, so it is not a quiet channel — it is an empty one.

Array 4A drive voltage P4000004 · in V

Same two levels as 1A. What this wing sends inboard is primary power; it is only downstream, in converter units, that it is stepped down to the 124 V every load aboard is built for. 4A is the second wing on P4 and has carried a roll-out array since 2022.

Array 4A drive current P4000005 · in A

Not published: exactly zero and without a timestamp, on this channel as on the other seven. An earlier version of this site promised that a negative reading meant the channel was delivering; no such reading has ever been seen, and the line was removed.

Array 2B drive voltage P6000004 · in V

Same two levels as 1A. When the eight split, they do so in groups about 9 V apart, some near 160 V and some near 151 V, and eclipse has only ever been seen with all of them converged. 2B is on P6, at the far port end, one of the pair that fed the laboratory on its own before P4 came on line.

Array 2B drive current P6000005 · in A

Not published: exactly zero, with no timestamp, like the other seven. Nothing here is wrong with the wing — the voltage beside it is live — the stream simply does not carry this measurement.

Array 4B drive voltage P6000001 · in V

Same two levels as 1A. The batteries this bus falls back on are on the same P6 segment as the wing, six charge-discharge units per module, each able to put 8.4 kW into its battery and take 6.6 kW back out. 4B, with 2B, was the first pair to receive a roll-out array, in 2021.

Array 4B drive current P6000002 · in A

Not published: exactly zero, with no timestamp, on all eight drive currents alike. Mimic’s own deck marks the middle line of each wing, the current, ‘not working’, and keeps the angle above it and the voltage below.

Where it lives on the station

• Port SARJ Solar Alpha Rotary Joint

  A 3-metre rotary joint that turns the entire port outboard truss to track the Sun — one full revolution every orbit.

• Starboard SARJ Solar Alpha Rotary Joint

  Same role as its port counterpart, for the starboard side.

• Solar Array 1A Solar Array Wing 1A (S4)

  A solar wing spanning 34 metres. Its orientation combines the SARJ rotation, which tracks the Sun around the orbit, with the BGA rotation, which compensates for the seasonal tilt.

• Solar Array 3A Solar Array Wing 3A (S4)

  A solar wing spanning 34 metres. Its orientation combines the SARJ rotation, which tracks the Sun around the orbit, with the BGA rotation, which compensates for the seasonal tilt.

• Solar Array 1B Solar Array Wing 1B (S6)

  A solar wing spanning 34 metres. Its orientation combines the SARJ rotation, which tracks the Sun around the orbit, with the BGA rotation, which compensates for the seasonal tilt.

• Solar Array 3B Solar Array Wing 3B (S6)

  A solar wing spanning 34 metres. Its orientation combines the SARJ rotation, which tracks the Sun around the orbit, with the BGA rotation, which compensates for the seasonal tilt.

• Solar Array 2A Solar Array Wing 2A (P4)

  A solar wing spanning 34 metres. Its orientation combines the SARJ rotation, which tracks the Sun around the orbit, with the BGA rotation, which compensates for the seasonal tilt.

• Solar Array 4A Solar Array Wing 4A (P4)

  A solar wing spanning 34 metres. Its orientation combines the SARJ rotation, which tracks the Sun around the orbit, with the BGA rotation, which compensates for the seasonal tilt.

• Solar Array 2B Solar Array Wing 2B (P6)

  A solar wing spanning 34 metres. Its orientation combines the SARJ rotation, which tracks the Sun around the orbit, with the BGA rotation, which compensates for the seasonal tilt.

• Solar Array 4B Solar Array Wing 4B (P6)

  A solar wing spanning 34 metres. Its orientation combines the SARJ rotation, which tracks the Sun around the orbit, with the BGA rotation, which compensates for the seasonal tilt.

Each link opens the 3D twin on that part, with its live readings beside it.