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This answer mentions that the Wikipedia article Soyuz MS (Союз МС, the latest revision of the Soyuz spacecraft, evolution of the Soyuz TMA-M) says:

Instead of relying on ground stations for orbital determination and correction, the now included Satellite Navigation System ASN-K (Russian: АСН-К, Аппаратура Спутниковой Навигации) relying on GLONASS and GPS signals for navigation. It uses four fixed antennas to achieve a positioning accuracy of 5 m (16 ft), with the objective to reduce that number to as little as 3 cm (1.2 in) and an attitude accuracy of 0.5°.

However, I am so far unable to verify from the references currently linked in that paragraph that the plan is to derive "attitude accuracy of 0.5°" from GLONASS and GPS signals.

If this is the plan, could someone help find a reliable source that states this explicitly, and hopefully explains at a least a little bit how that works?

It's certainly true that with a multi-antenna configuration GNSS signals can be used to determine attitude, but I'd like to confirm that Soyuz is set up to test or use this.

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A few degrees, not a half. The 0.5° accuracy is the goal of the attitude determination system for the in-development Federatsiya (Federation) spacecraft. This is based on the system already in use by the Soyuz MS. The paper Determining the attitude of the advanced crew transportation spacecraft federatsiya from measurements of global navigation satellite systems discusses both the Soyuz MS and the proposed Federatsiya attitude determination system. The Federatsiya system will operate in three modes, described below. Note well: The mode names are mine, not theirs.

  • A lost in space attitude determination mode whose goal is to obtain a very rough attitude estimate with an error of 20-30° within a few seconds. This mode is entered on system startup or when more refined attitude determination modes lose attitude. There is no comparison of signals from the same satellite between pairs of antennae in this mode. Instead, this mode uses the known orientations of the antennae in the spacecraft body frame combined with the reported positions from the navigation satellites to provide a rough estimate of attitude.

  • A coarse attitude determination mode whose goal is to refine the initial 20 to 30° error down to 2 to 3°, with occasional departures to 6°. This refinement takes a hundred to a few thousand seconds to achieve. This mode operates by computing a vector of first-order differences of integral phases between satellites viewed in common by various pairs of antennae for use as the measurement vector in an attitude determination filter.

  • A fine attitude determination mode whose goal is to further refine the 2 to 3° attitude error from the coarse mode down to a half of a degree. This refinement will take a few tens of seconds to achieve. This mode operates by computing a vector of second-order differences of integral phases between satellites viewed in common by various pairs of antennae for use as the measurement vector in an attitude determination filter.

It appears from the paper that this final mode may be new to the Federatsiya spacecraft and is not currently used by the Soyuz MS.

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  • $\begingroup$ This is excellent, thank you very much. I don't know what "integral phase" means, but luckily I've postponed my library run until tomorrow, as it seems I now have more homework. $\endgroup$ – uhoh Oct 9 '18 at 12:46

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