[time-nuts] Terrestrial Tides and Land Movement

Björn bg at lysator.liu.se
Mon May 25 16:32:48 EDT 2015


Bob,

Tom says 'almost'. 

In a static location you can improve your timing results by establishing your antenna position as good as possible. It will work almost as good without a survey. 

In a moving scenario you can of cause not do a survey. But try finding another way to do time transfer with better performance.

Btw... pure position averaging over many hours does not seem so optimal by todays standards. Some ppp-version even on L1-only should be more effacient. 

--
   Björn

<div>-------- Originalmeddelande --------</div><div>Från: Bob Stewart <bob at evoria.net> </div><div>Datum:2015-05-25  20:09  (GMT+01:00) </div><div>Till: Discussion of Precise Time and Frequency Measurement <time-nuts at febo.com> </div><div>Rubrik: Re: [time-nuts] Terrestrial Tides and Land Movement </div><div>
</div>Tom said: "The nice thing about GPS, unlike other time transfer methods, is that can handle the case of a moving antenna. As the antenna moves so does the time. This is why GPS timing receivers work (almost as well) on top of your car as on top of your house."
I don't get that.  What's the purpose of doing a survey when you move your antenna if this the case?
Bob


     From: Tom Van Baak <tvb at LeapSecond.com>
To: Discussion of precise time and frequency measurement <time-nuts at febo.com> 
Sent: Monday, May 25, 2015 12:29 PM
Subject: Re: [time-nuts] Terrestrial Tides and Land Movement
   
Attila,

Timing people account for everything that's important. A continental drift of an inch per year acts like a slow phase change over time, which by definition, is a frequency offset. So an inch per year is at most 1/12 * 1e-9 / (365*86400) or 3e-18. For the current precision with which UTC/TAI is calculated this is too small to worry about.

The other way to think of the frequency offset is simply the ratio of speed-of-continent vs. speed-of-light. A continent is slow, about 1e-9 m/s and light is fast, 3e8 m/s. This ratio is about 3e-18.

Note that an inch-per-year is about a nanometer-per-second. I'm also told fingernails grow about an inch a year. How's that for a rule of thumb (literally).

There's a nice (1 inch) 25 mm per year interactive drift map here:
http://www.unavco.org/software/visualization/GPS-Velocity-Viewer/GPS-Velocity-Viewer.html

The nice thing about GPS, unlike other time transfer methods, is that can handle the case of a moving antenna. As the antenna moves so does the time. This is why GPS timing receivers work (almost as well) on top of your car as on top of your house. Just think of continental drift as a slow moving car.

/tvb

See also:
http://www.iris.edu/hq/files/programs/education_and_outreach/aotm/14/1.GPS_Background.pdf
http://www.unavco.org/education/resources/educational-resources/tutorial/how-quickly-are-we-moving-gps-tutorial.pdf


----- Original Message ----- 
From: "Attila Kinali" <attila at kinali.ch>

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

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