I had run across this just yesterday:  
https://www.youtube.com/watch?v=zILwgQhjC_Q

He talks about how crystal oscillators are sensitive to movement and changes in 
gravity.  An interesting watch!

        -Randal
                (at CubeCentral)

-----Original Message-----
From: time-nuts [mailto:time-nuts-boun...@febo.com] On Behalf Of Bob Camp
Sent: Monday, 17 October, 2016 05:48
To: Discussion of precise time and frequency measurement <time-nuts@febo.com>
Subject: Re: [time-nuts] Moving GPSDO

Hi

Is the OCXO in your GPSDO acceleration compensated? If not then the 
acceleration involved in your “moving around” will get into the OCXO at around 
1x10^-9 / g. Some OCXO’s are compensated to the 1x10^-11 / g level. Some low 
cost crystals are up around 2x10^-8. Often vibration is an issue as well, since 
it is an acceleration, the same stuff applies. 

How well temperature controlled is your moving environment? Your oscillator can 
have all sorts of temperature coefficients,. It will respond to dynamic change 
as well as static change. The sensitivity can be just about anything over the 
1x10^-12 / C up to 1x10^-7 / C depending on the part. 

None of that has anything at all to do with GPS, all of it is something the 
GPSDO will have to deal with in addition to any GPS errors. Unless you want a 
very coarse output
(1x10^-9 to 2x10^-8 / second) you will need some sort of loop filter. If it is 
in the traditional 
> 200 seconds range, the stuff above can become very apparent in the 
> output.  The loop
does not eliminate error at the (say) 200 seconds point, it only gets going by 
then. 

Does your moving around include tunnels? If so, how long will you be in a 
tunnel? Urban canyons also count in this respect. Once you go down to < 3 sats, 
you are in big trouble for timing in motion. Even with 3, your fix and time 
performance is degraded. Chugging in and out of holdover has it’s impact. 

Do you want to use multiple systems? If you are running a mixed mode system, 
you can go from 100% GPS to 100% GLONASS to 100% BEID0U due to viewing angle.  
That tosses a whole *bunch* of issues into the mix. 

How fast are you going? Moving around can involve move 100’, stop and wait for 
a day or it can involve something moving at mach 3. By far the most common for 
a GPSDO is some sort of car, truck, or military vehicle on the highway. There 
are many other possibilities.  

Lots of variables. Lots of things to get into the mix in addition to the basic 
nav math.

Bob


> On Oct 17, 2016, at 12:19 AM, Joseph Gray <jg...@zianet.com> wrote:
> 
> I'm curious to know how accurately a GPSDO will keep its 10 MHz output 
> while moving. No initial survey to set a position would be done. I 
> don't care about the UTC time.
> 
> Joe Gray
> W5JG
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