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I am certain that I read a year or three ago about an unmanned satellite that was going to be testing a method of attitude determination that relied upon two (or more) separate antennas on one spacecraft, and used the difference in arrival times of GPS signals at the two antennas to calculate an approximate attitude.

I am not sure if it used a standard "relative GPS" implementation (see also this answer and this mathematical discussion), or if it was more advanced because all signals were available simultaneously to one correlator cluster, whereas relative GPS relies on the exchange of digital information using a link between two systems.

@DavidHammen points out in comments that this has been done before. This is not new. It seems examples include the Soyuz spacecraft, the ISS, and even much earlier experiments.

Question: However I'm looking for a recent unmanned satellite and if possible an understanding if it uses relative GPS or a more advanced technique running the output of all correlators from all antennas through a single fitting procedure.

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    $\begingroup$ Highly related, if not a duplicate: Do any spacecraft use GNSS for attitude determination? The two answers to that question are incomplete; this idea was tried over 20 years ago. $\endgroup$ – David Hammen Oct 8 '18 at 23:09
  • $\begingroup$ @DavidHammen Thanks for finding that. I see some of my up votes there which means I have read those in the past. But I don't believe that Soyuz is the instance I'm thinking of (I'm still trying to verify that it's actually GPS that gives it 0.5 degrees attitude as Wikipedia claims), and certainly the ISS isn't what I'm remembering. I'm not interested in when it was first used I'm interested in current status, and the example I remember is an unmanned satellite. $\endgroup$ – uhoh Oct 9 '18 at 0:37
  • $\begingroup$ @DavidHammen I'll look more closely into this today to try to clear this up. In the mean time I've asked Does the Soyuz spacecraft really try to achieve attitude accuracy of 0.5° from GLONASS and GPS signals? and edited the question to make sure it's not a duplicate. $\endgroup$ – uhoh Oct 9 '18 at 0:55
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    $\begingroup$ This is put to test repeatedly for the same reason laser propulsion is. Both are sexy, both appear to be high tech, both represent large sunken investments, and both have backers who are very good at selling the ideas to politicians. That both are losers is irrelevant. $\endgroup$ – David Hammen Oct 9 '18 at 2:20
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    $\begingroup$ Another way to look at it: Would SpaceX use this? I doubt it. Star trackers aren't useful when a rocket is still in the atmosphere, but when a rocket is in the atmosphere the atmosphere tells the spacecraft part of its attitude. The unknown part is subject to gyro drift, but how much do gyros drift in the five minutes or so before a star tracker can see the stars? $\endgroup$ – David Hammen Oct 9 '18 at 2:28

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