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I want to know, what is the difference between inertial pointing and nadir pointing for a satellite?

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    $\begingroup$ Please do a bit of research first. What does "inertial frame" mean? What would a satellite have to do if it always pointed at the nadir? $\endgroup$
    – uhoh
    Jan 31, 2022 at 11:11

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Inertial pointing uses devices such as gyroscopes to maintain an absolute orientation "with respect to the fixed stars", while nadir pointing uses some other mechanism (typically optical, but sometimes magnetic) to maintain an orientation with respect to the center of the body being orbited. "Nadir" means "straight down" and generally refers to the direction toward the center of the body.

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To supplement VoyagerPWS's answer, there's another sense of the meaning. Nadir pointing means to orient your spacecraft towards the ground at all times (and that can be independent of the type of attitude sensors used). For example, the Landsat satellites are nadir pointing because they are supposed to image the, duh, land. But they use star trackers to get very accurate knowledge and control of where they are pointing (ie, which particular patch of ground).

Inertial pointing refers to spacecraft like space telescopes which point in one specific direction (that particular star) for long periods of time. Such spacecraft almost always use star trackers for their primary attitude sensor.

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