[time-nuts] Terrestrial Tides and Land Movement

Bob Camp kb8tq at n1k.org
Mon May 25 18:05:08 EDT 2015


Hi

If I was going to do it today, I’d do an “all in view” solution with GPS / GLONASS and whatever 
else I could pick up (WAAS etc). I’d grab the data at L1 / L2 / L5. Even in motion, the location 
solution is going to be pretty good. 

If it was a system that made money for people, there are “for pay” services that will feed 
you up to date data (WAAS on steroids …).  You can get an on the fly solution that is as
 good as the normal fast  “self survey position” done by most L1 timing receivers.

That said, even if you don’t get all that fancy, every single position your gizmo calculates comes
with a time solution. Even the cheapest GPS still has an amazingly good time solution 
buried somewhere down in it’s math.  It may be 10 ns, but that’s still a lot better than my wrist 
watch. It’s also better than your un-compensated portable clock ….

Bob

> On May 25, 2015, at 3:02 PM, Bob Stewart <bob at evoria.net> wrote:
> 
> 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 <kb8tq at n1k.org>
> To: Bob Stewart <bob at evoria.net>; Discussion of precise time and frequency measurement <time-nuts at febo.com> 
> 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 <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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>> 
>> 
>> 
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