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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