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On 2018-11-07 16:15, Brad Morris wrote:
> Okay, so now I am just a bit confused.  The geoditic monuments that
> Peter mentioned must have their asigned latitude and longitude changed
> as they move about due to plate tectonics, or so it is stated. But are
> they also reassigned as a function of pole wander?

No. In fact, the x and y polar motion values published by the IERS are
measured with respect to the ITRS pole (which for practical purposes is
identical to the WGS84 pole). For instance, the wandering pole in the
diagram below is depicted on the fixed grid of the ITRS.

http://hpiers.obspm.fr/eop-pc/index.php?index=pm&lang=en

> If not, then how are the two systems aligned.  Example of confusion.
> I take my GPS receiver to a geoditic monument.  The GPS ephemeris
> updated to include pole wander, as per Paul.  So the geoditic latitude
> will NOT match the GPS latitude.

To the contrary, the coordinates *will* match due to the peculiar way
the quasi Keplerian orbital elements of the GPS satellites are
constructed. Normally, Keplerian elements yield a satellite position in
some inertial (non-rotating) coordinate system. But the GPS "Keplerian"
elements yield a satellite position with respect to the WGS84, which
rotates in space once per day. Furthermore, it's a wobbly rotation since
the WGS84 pole is misaligned with the axis of rotation by tenths of an
arc second.

I had never heard of Keplerian elements used this way and wouldn't have
thought it possible. The rotation of Earth and the pole offset are baked
into the orbital elements so the solution comes out in the WGS84. What
would Kepler have thought?

   

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