[time-nuts] Moving GPSDO

[Bob Camp]

Hi

In addition to TimeNuts, there are actually *are* people who get nutty about measuring 
changes in gravity. That’s a bit different than what he’s talking about in the video. Local
variations in the earth’s mass cause variations in gravity (think mountains or similar…). 
They also are one of the things that cause GPS orbits to be “bumpy” rather than the nice
smooth curves we tend to think of them as being. 

Bob


> On Oct 17, 2016, at 9:25 AM, Cube Central <cubecentral at gmail.com> wrote:
> 
> 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-bounces at febo.com] On Behalf Of Bob Camp
> Sent: Monday, 17 October, 2016 05:48
> To: Discussion of precise time and frequency measurement <time-nuts at 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 <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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