[time-nuts] Terrestrial Tides and Land Movement

[Lamar Owen]

On 05/16/2015 12:41 AM, Bob Stewart wrote:
> I did some idle searching trying to see if there was a relationship between terrestrial tides and timing receivers. I couldn't find anything useful, but I did discover that the Jersey Village area, about 2 miles northeast of me, is sinking about 2 inches a year.  So, my question is what effect do either of these, terrestrial tides or this local sinkage, have on timing accuracy?
>
If you're interested in finding out more about this sinkage and plate 
movement, we are affiliated with the EarthScope system and have one of 
their instruments here onsite.  The system map is at 
http://www.earthscope.org/science/maps/current-status-map/ and details 
about the plate boundary observatory component are at 
http://pbo.unavco.org/  with the instrument at our site at 
http://pbo.unavco.org/station/overview/P779

Positional changes of 2 inches per year should create sub-nanosecond 
differences in signal arrival times.  The survey receivers used by the 
PBO utilitize several techniques for measuring these very small changes 
in position; however, as you can see by looking at the position data 
(such as http://pboshared.unavco.org/timeseries/P779_timeseries.png ), 
it's a long-term averaging that is optimized for position, not timing, 
data, and the precision in the z axis is not as good as that in the x 
and y axes by a factor of four or more.  For any given GPS frame, the 
uncertainty of arrival time is more determined by ionospheric effects 
than by absolute position.  Meteor scatter and multipath also play a role.

Now I am by no means a GPS expert, but I would think a receiver 
optimized for timing would need different techniques than a receiver 
optimized for survey, but I reserve the right to be wrong.