[time-nuts] Terrestrial Tides and Land Movement

[Bob Camp]

Hi

If you happen to *need* precise time on a moving platform, then GPS can 
do that as well. There are a number of military systems that have this need.
There are also some things like mobile direction finding by TDOA that have
multiple use cases.

Bob

> On May 25, 2015, at 2:09 PM, Bob Stewart <bob at evoria.net> wrote:
> 
> 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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