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I'm looking at some old satellite documentation which mentions the use of a "Q frame" coordinate system to keep track of the satellite attitude. What does this "Q frame" generally mean in the field of satellite localization?

Edit: The documentation can be found here: https://store.cloudeo-ag.com/sites/default/files/product_details/EROS%20METADATA%20v3.0.pdf, page 11, items 18 and 19 in the table.

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    $\begingroup$ @uhoh, I don't think the Q means quaternion. Unit quaternions are simply a handy way to represent rotations and transformations in three dimensional space. $\endgroup$ – David Hammen Dec 11 '16 at 11:05
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    $\begingroup$ Ang Zhi Ping: DO you have a reference? Google searches for "Q frame", "satellite Q frame", and "spacecraft Q frame" come up empty. The closest is the orbital frame plane, which some call the PQW frame. Another possibility is the G frame for a spacecraft with a single control moment gyroscope. The "G" stands for gimbal, but is written in a script font that looks a bit like a Q. $\endgroup$ – David Hammen Dec 11 '16 at 11:09
  • $\begingroup$ @DavidHammen added a link to the reference. $\endgroup$ – Ang Zhi Ping Dec 11 '16 at 11:44
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    $\begingroup$ @uhoh -- I rhetorically asked your question: What are quaternions and how are they used to represent spacecraft dynamics. In the meantime, I have to walk my dogs, feed my dogs, feed myself, wash my hot car, etc. If someone beats me to the answer to my rhetorical question, that's great. I will answer my own question if someone hasn't beat me to the punch. A proper writeup will take several hours. Don't hold your breath. $\endgroup$ – David Hammen Dec 11 '16 at 13:10
  • $\begingroup$ @DavidHammen I've been scratching my head for a couple years already so waiting for a several hours or a few days is well worth the wait! $\endgroup$ – uhoh Dec 11 '16 at 17:12
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In section 3 of your reference, it states

The EROS-A legacy pass-file contains all metadata in one file, but requires a lot of exceptional processing associated with [sic] customized coordinate system (so-called Q-frame).

Satellites usually have many frames associated with different aspects of the body of the satellite. For instance, there will be at least one star-tracker reference frame, so that star measurements can be easily computed in that frame. If the satellite is pointing a laser towards earth, there will be another frame associated with the direction of the laser. There will be another frame associated with the ACS, and indeed every other portion of the satellite from which calculations need to be made. For sharing data between different components of the satellite, everything is usually converted into one frame that is of primary importance. Going back to the laser, if the primary mission for the satellite dealt with the laser, the laser reference system would probably be the most important frame, and thus most measurements would be converted to that.

Simply put, this "q-frame" is almost certainly just a reference frame associated with an important part of the satellite. All measurements are represented in it to limit ambiguity.

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