[time-nuts] Moving GPSDO

Bob Camp kb8tq at n1k.org
Mon Oct 17 07:48:07 EDT 2016


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.


> On Oct 17, 2016, at 12:19 AM, Joseph Gray <jgray at 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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