The ISS is using the Control Moment Gyroscopes to maintain its attitude (the Cupola pointing towards Earth). That means the ISS is slowly rotating with respect to distant stars. What happens during docking of a visiting spacecraft (Soyuz, Progress, ATV, Dragon, etc)? These spacecraft should approach the ISS from one direction towards a designated docking port. If ISS would be keeping its nominal attitude, it would be very difficult for the incoming craft to keep the port in its approach trajectory. Is the ISS attitude changed during docking? Does the ISS have to "catch-up" (rotate to point the Cupola again towards the Earth) after the docking?


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There are multiple things going on here. First, the ISS does maintain a constant attitude in a rotating LVLH (Local-Vertical-Local-Horizontal) frame (cupola pointing towards earth), but it uses a combination of CMGs and the jets on the Russian segment for attitude control.

The attitude of the ISS does change during docking, but that attitude is still constant in a rotating LVLH frame. It is changed to an attitude that has minimal torque from drag so that the CMGs can hold the attitude for as long as possible (minimum 180 seconds). Usually what happens is the CMGs are controlling attitude, but due to non-secular torques, the CMGs have to be desaturated by the jets on the Russian segment every so often. This causes non-desired effects during berthing and docking, and so the jets are not used during capture (hence the 180 second rule of only using the CMGs)

To hold a constant attitude in a rotating LVLH frame, the vehicle will rotate once per orbit, so the attitude rate is nearly constant. This is not difficult to maintain for a vehicle as long as there are no external torques.

The docking port is in the orbital trajectory of vehicles that dock from the rear (Progress, ATV, etc), but in a curvilinear LVLH frame, not a rectilinear LVLH frame. Once the vehicle is close (around 500m or a kilometer), the differences between the two reference frames are negligible. Farther than that, the docking port can't be "seen" anyways.

But neither the station nor the approaching vehicle maintain a constant attitude in an inertial frame at any time during docking (or berthing).


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