Hi Bob,
How is this achieved? Is there a coupled dead-reckoning system that updates
the timing location, or something else?
Bob
From: Bob Camp <[email protected]>
To: Bob Stewart <[email protected]>; Discussion of precise time and frequency
measurement <[email protected]>
Sent: Monday, May 25, 2015 1:53 PM
Subject: Re: [time-nuts] Terrestrial Tides and Land Movement
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 <[email protected]> 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 <[email protected]>
> To: Discussion of precise time and frequency measurement <[email protected]>
> 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" <[email protected]>
>
> 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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