[time-nuts] Terrestrial Tides and Land Movement

Bob Camp kb8tq at n1k.org
Sun May 24 20:46:33 UTC 2015


Hi

If you believe the 3 ns / M applies in this case, tides will give you about 
1 ns or so. If the geometry of the motion (vertical) and the orientation
of the sat’s is not same / same, the impact will be a bit less. 

On an L1 system without some sort of ionosphere “help” and working just
off of broadcast data, you can / will / may see > 10 ns per day.  Troposphere
is the same sort of thing only a bit less per day. 

There are a whole raft of interesting relativity issues that get into GPS. Again
the L1 stuff does not do much beyond the in-system corrections. 

The biggie is ionosphere ….

Bob


> On May 24, 2015, at 8:02 AM, Attila Kinali <attila at kinali.ch> wrote:
> 
> 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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