Bill,

My question was more generally about any GPSDO, but it's good to know
about the Motorola/Lucent.

What about this scenario - the GPSDO has a fixed position on initial
powerup, but then it is moved periodically or even continuously,
without updating the GPS coordinates. The distance from the original
coordinates could be anywhere from close by, to across the state.
Periodically, a new survey could be run to keep things from getting
too far out in left field.

At some point, I'll have to do some field testing, but I wanted to get
the list wisdom first.

Joe Gray
W5JG

On Sun, Oct 16, 2016 at 11:08 PM, Bill Hawkins bill.iaxs@pobox.com wrote:

Joe,

In T-mode, the location in X, Y and Z is assumed known so those values
is used as constants in producing the T difference between a satellite
and the GPSDO. As you now move your receiver from the original X, Y and
Z those location errors will translate into time errors. The worst of
this will the T-RAIM hopefully clip out, but for the remaining the
errors might drive the receiver all over the place, and as it shifts
around so will the frequency. It won't be a pretty picture.

What you should do, if you can, is to go out of T-mode into normal
tracking. Choosing the speed-grade of your movement can be worth some
effort. Far from all GPSDOs can do this, since they typically is
intended for fixed locations.

Potentially can aiding by IMU help some, but very few GPSDOs come close
to support it. When moving you have to consider other factors of a
dynamic environment.

I know of only a handful of GPSDOs for dynamic platforms.

Cheers,
Magnus

On 10/17/2016 08:02 AM, Joseph Gray wrote:

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

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

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@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

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

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and follow the instructions there.

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 Mon, 17 Oct 2016 07:25:34 -0600
"Cube Central" cubecentral@gmail.com wrote:

As usual with EEVblog, there are several things wrong with what he presents.

1. As Bob wrote, the acceleration sensitivity of crystals vary a lot more
   than the rough number Dave gave.

2. The acceleration sensitivity has very little to do with the cut.
   Even though there is some cut related effect on acceleration, it is
   very very small. The by far most dominant effect comes from the
   mechanical stress of the holder onto the crystal. It is probably
   obvious that the crystal and the holders bend upon acceleration.
   This bending will cause the crystal to change its frequency due
   to stress on the lattice. Now, why are AT cuts worse than SC cuts?
   The reason is simple, SC cuts are usually ment for high stability
   applications and people tend to take more care with the mounting
   and the crystal shape, than for regular AT cuts.

3. Rb vapor standards have very low acceleration effects. As Dave
   said, the control loop will kill most of the effects inside the
   loop frequency. The loop frequency is relatively fast, in the
   order of couple of 100Hz to a couple of 10kHz, depending on the
   exact build of the Rb standard and what type of crystal they used.
   Contrary to what he says, there are no acceleration dependent
   diffusion effects. The vapor cell is is heated and the gas atoms
   in there are pretty fast, any acceleration of them will be negligible
   compared to what they do upon collision. The relativistic red shift due
   to acceleration is way too low to measure (much lower than the intrinsic
   noise). The only mechanical effect that is possible, is bending of
   the microwave cavity or the parts inside. But given that the cavity
   is several mm thick aluminium and everything is mounted rigidly,
   I very much doubt that there is any significant effect.

4. If you think that the tilting your frequency counter will have
   a significant effect, you are severly underestimating temperature
   and aging effects. Just in comparison, a good SC cut DOCXO has an
   aging of a few parts in 10^-9. So each week will affect on your crystal
   more than a 1g of acceleration. And that's with a slow aging DOCXO.
   Temperature effects are in the order of 1x10^-10/°C to 1x10^-7/°C.
   So depending on how good the OCXO is, a single degree of temperature
   will cause a larger shift than 1g of acceleration. Etc.pp.

    	Attila Kinali
   

--
It is upon moral qualities that a society is ultimately founded. All
the prosperity and technological sophistication in the world is of no
use without that foundation.
-- Miss Matheson, The Diamond Age, Neil Stephenson

Speaking for my GPSDOs specifically, you can’t disable the survey mode with the Venus838 receiver (well, you can, but it requires you to talk to the module with SkyTraq’s software, which requires disabling the GPSDO’s controller - possible to do, but annoying). I haven’t attempted to see what it would do if it were moved after the survey is completed (if you move during the survey, then the survey will fail and repeat).

The older GPSDOs that had PA6H receivers were navigation receivers that didn’t have a survey mode.

In either case, without a surveyed position, PPS stability will suffer if/when the constellation degrades due to poor reception, which can happen if, say, the unit is in a moving vehicle in interesting terrain. Poor PPS stability will have an impact on the PLL, obviously.

Hi

Survey is pretty much a bad thing while in motion :)

Some (but not all) GPS modules allow you to set up in a “mobile / do not survey” mode. This is one area that the
newer ( = not 1997 era) devices do much better at. It also is something that SBAS / EGNOS / WAAS may indeed
help you out on. You may be changing many settings on the part to make it behave in a mobile setting. There
are a few bugs you just may turn up while doing this ….

Bob