[PROJ] Dealing with epoch and transformations

[Jack Riley - NOAA Federal]

>
> Thinking about how the network operates, they need to do static
> solutions to determine coordinates, and those could reasonably be in
> either ITRF2020 2015.0 or ITRF2020 day they do the observations.
> (Really, they should be and probably are doing static solutions
> repeatedly.)  With either, they could transform to 2015.0 or today's
> epoch.  Then, they might need to apply station velocities and load the
> reference station coordinates.
>

I just posted a question with Swift Navigation Support to ask this.
I'll update this thread when/if I hear back from them.


> It's interesting to consider what GPS does, which as I understand it is
> to compute reference station coordinates once a year and use them over
> the year.  But the link from reference station coordiantes to
> clocks/orbits to user coordinates has a big enough error budget that
> this 1 year step of a few cm -- in varying directions for varying
> reference stations -- is almost certainly not detectable.
>

GPS WGS84 reference frames that are tagged with a datum realization
of GPS week may be associated with an ITRF or IGS realization and its
attendant reference epoch and ECEF frame accuracy.
NOAA/NOS/NGS uses the following associations in their VDatum software:

WGS84(G730): ITRF91
WGS84(G873): ITRF96
WGS84(G1150): ITRF2000
WGS84(G1674): ITRF2008
WGS84(G1762): IGS08
WGS84(G2139): IGS14


> > If I want to transform from that reference system (ITRF2020) to, let's
> say,
> > ETRS89 or NAD83(2011), should I add an epoch? which one? where? what
> > happens if I do not set any?
>
> My somewhat hazy understanding is that ITRF2020 without a specified
> epoch means 2015.0.
>
> Does proj have plate motion models, to be able to answer not what a
> coordinates a point in space has, but a point fixed to the ground?
>

There are subdirs for the frames which are dynamic in PROJ/data and the
plate motion model parameters are indexed at the bottom; e.g.:
https://github.com/OSGeo/PROJ/blob/master/data/ITRF2014

ITRF2020 is in EPSG and PROJ, but I don't see its definition in PROJ/data.
I'm not sure of the source for official plate motion model parameters.
Here is one reference to a model for ITRF2020:
https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2023GL106373

I found Even's response here re: PROJ transformations between coordinate
epochs to be useful:
https://www.mail-archive.com/proj@lists.osgeo.org/msg00390.html
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