[time-nuts] Terrestrial Tides and Land Movement

[Bob Camp]

Hi

Add to that the fact that not everybody is moving at an inch per year. Around here the magic number 
is in the 1.5 to 2 mm per year range. It’s enough to be worth correcting survey results  vs benchmarks 
every few years. It’s not enough to get into an L1 timing system any time soon ….

Bob

> On May 25, 2015, at 1:29 PM, Tom Van Baak <tvb at LeapSecond.com> wrote:
> 
> 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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