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 > > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. > > > > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
