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I understand ISS is using gyroscopes (CMG) to maintain its attitude. At the same time, I understand that ISS maintains constant attitude relative to Earth, i.e. its Z axis always points toward the center of the planet. Given the station's speed, it means ISS has to continuously change its attitude with speed of 4 degrees per minute (see 1).

This is where I am missing something. Gyroscopes, by definition, maintain certain attitude. Being stabilized by gyroscopes, shouldn't the station always be in the same "absolute" position, which would mean it has to be spinning relative to Earth?

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    $\begingroup$ Note that the moon isn't spinning relative to earth, and it uses no gyroscopes. Thus we can see that an orbit can be maintained with minimum energy input with the orbiting object always facing one side toward earth. Since the ISS is optimized for minimum energy expenditure, this is one of several possible attitudes it can maintain relative to earth with minimal energy use. $\endgroup$ – Adam Davis Dec 1 '15 at 17:17
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    $\begingroup$ @AdamDavis 's comment is really fundamental. SF.'s says it clearly but in some of the other answers it doesn't really pop-out yet. Once a satellite is revolving around the earth, you can give it a little torque so it slowly rotates at the same rate. So except for small corrections, you don't have to do anything further to keep it pointing "down." $\endgroup$ – uhoh May 17 '16 at 0:35
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The term "gyroscope" is a bit general and applies to two devices in the context of spacecraft orientation: a gyroscope sensor and a control moment gyroscope. A gyroscope sensor is measuring the rotation rate of the spacecraft meanwhile a control moment gyroscope is controlling the rotation rate (by generating torque).

Spacecraft like the ISS require a complete attitude control system that uses sensor measurements to estimate the true orientation, compare that to the desired orientation, and then apply the required torque. The answer to your question really lies in how you define that desired orientation. Yes, the ISS is rotating to be aligned with the Earth, but it is also being controlled to maintain that desired orientation. It is no problem that the desired orientation is changing, you don't need to be stabilized to a "certain attitude" when using gyroscopes. You can just as easily track a spinning desired attitude using gyroscopes, and you can also track a moving target with changing spin as well.

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  • $\begingroup$ When referring to the "monitoring gyroscope" you'll usually encounter the term "gyrocompass." When you encounter the term "gyroscope" unqualified, in spacecraft context, you should assume it's the actuator device that sets the attitude of the craft; also known as "reaction wheel". $\endgroup$ – SF. Dec 1 '15 at 9:06
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    $\begingroup$ Sure, but reaction wheels and control moment gyroscopes work on entirely different principles, so that isn't really accurate! $\endgroup$ – Brian Lynch Dec 1 '15 at 10:20
  • $\begingroup$ @SF this is improper although common. The gyrocompass is a different mechanism operating on a more complicated principle. Gyrocompass is only usable on a rotating celestial body (planet) and uses a gyroscopic precession effect to point towards a pole of said body. Gyroscope uses gyroscopic effect to point to a fixed direction and works anywhere in space. $\endgroup$ – kubanczyk Dec 4 '15 at 22:09
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Let me add:

The ISS without gyroscopes would follow Newton's 1st law of rotary motion - maintain its spin; if the spin speed is 1 revolution per orbital period, it will face Earth with the same side at all times.

CMGs with axis that are perpendicular to the rotation plane would not be affected, maintaining their speed; their torque doesn't act in that plane if their speed is unchanged.

CMGs with axis in the rotation plane though would exert powerful torque if they are only spinned up - flipping a spinning gyroscope upside down in a straight arc requires quite a bit of force.

But they don't need to be flipped. The CMGs are gimballed. So while the station spins (just following its own spin inertia), the CMGs that would exert torque, remain in a fixed orientation, slipping in their gimbals relative to the station. Only when the station's spin needs to be changed, the motors of the CMGs and the motors of their gimbals are engaged to provide the desired torque - outside of that, they only account for friction/losses.

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ISS uses a torque equilibrium attitude which by definition requires the minimum input from the control moment gyros. http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20100024204_2010020499.pdf

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The four control-moment-gyroscopes employ a double-gimbal design that allows the spin axis to maintain an appropriate orientation that is not fixed in the station frame of reference. Although not usually 'free to swing around', this allows electric motors to create torque by acting against the spin axis momentum vector.

The software controller uses the motors to overcome 'resistance torques' to keep the station stable and rotating to track orbital motion around the earth. In effect, the station uses energy from the sun to drive the motors and generate the required torques to overcome resistance and maintain the orientation in orbit. Z-axis of the station points along the radial line to the earth. Occupants get best view of the planet throughout the orbit.

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