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A Stack Exchange answer illustrates reference frame and observer as follows:

A frame of reference means a co-ordinate system and an observer is someone using that co-ordinate system. For example I could define cartesian co-ordinates $(x, y, z, t)$ and define myself to be at the point $(0, 0, 0, 0)$ and since I'm always at the origin that means I'm at rest. So when I measure your position and speed I'm the observer and I get some value in my frame i.e. my co-ordinate system.

Conclusively, observer uses a reference frame (coordinate system) to perform measurement, but this reference frame can be in motion relative to another frame. My question is, is an observer always stationary in the reference frame that it uses?

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  • $\begingroup$ Yes, if someone says "A's reference frame", they mean "a frame in which A is stationary". $\endgroup$
    – m4r35n357
    Feb 7 at 12:34

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Not necessarily. You can pick any reference frames you like. However, if you are considering things in relative motion, it usually simplifies the problem if you pick a reference frame in which one of the things is at rest at the origin. That's why you will often see thought experiments described from the vantage point of an observer at rest, where their position is taken to be the origin of a frame of reference.

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