[time-nuts] Terrestrial Tides and Land Movement

Attila Kinali attila at kinali.ch
Sun May 24 08:02:40 EDT 2015


On Sat, 16 May 2015 04:41:15 +0000 (UTC)
Bob Stewart <bob at evoria.net> 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?

I guess you are looking for the relationship between earth movement
and GPS/GNSS time transfer? If so, solid earth tides (as they are
commonly called in the timing community) do not become an issue until
a lot of other factors are removed first. First you need a system
that reduces multipath to a minimum, then one that can measure the delay
induced by the ionosphere directly (ie an dual/tripple frequency GNSS
receiver). I am not sure whether tropospheric delay or solid earth tides
come next, my guess would be that both are in the same order of magnitude.

The size of solid earth tides can be up to 30cm (i think i read somewhere
that someone measured 50cm, but not sure), mostly in vertical direction
(horizontal to vertical has about a factor 10 difference in amplitude),
where GNSS precision is quite low (compared to horizontal precision).
It has to be corrected in precise GNSS augmentation systems like IGS 
(see [1, page 12]).

I am not sure whether anyone accounts for continental drift in timing
applications. I would guess that at least people in VLBI have to.
Given that most GNSS high precision time transfer is used rather locally
(a couple of 100km) and that few people are running it for more than
a couple of months without recalibrating the system, i'd say that the
drift rates (which are between 2.5cm(Arctic) and 15cm(Chile) per year)
do not induce much error/jitter.

			Attila Kinali

 
[1] "A guide to using international GNSS service (IGS) products",
by Jan Kouba, Version 2.1, 2009
https://igscb.jpl.nasa.gov/igscb/resource/pubs/UsingIGSProductsVer21.pdf

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