[time-nuts] PPS sync
jerry at hanler.com
Mon Jun 5 15:51:51 EDT 2017
Chris, I think you are onto something. Running Lady Heather on this unit I see a line under “receiver” with the term “SawT” and a parameter of 24ns. So if we combine this information with what you teach below, it’s starting to look like maybe the M12 unit is doing something different than the Lucent. I am watching it today and I see 5 PPS that vary from the lucent pulse (used as a trigger) each about +20ns until it resets.
Mark Sims, can you comment on the SawT parameter, I assumed being reported by the M12 GPS, displayed on Lady Heather?
> On Jun 5, 2017, at 10:15 AM, Chris Albertson <albertson.chris at gmail.com> wrote:
> I did not finish the sawtooth explanation. One of the units is designed
> such that the PPS is always on the raising edge of an internal 10MHz clock.
> If the 10MHz clock were perfect this means the maximum error is 1/2
> cycle. The software in the GPS choose site best edge to minimize error.
> the jump you see it when it selects a different cycle and jumps The
> software tracks the error and outputs an estimate of the root on the serial
> You can verify this by plotting the sawtooth correction vs.time and see
> that it lines up with your observation of the jump back to zero error.
> There might still be errors cause by other things, like improper self
> survey but the results you reported are exactly like what one would expect
> from a unit that uses an oscillator edge to trigger PPS. It other words
> what you see is a design feature not an error.
> On Mon, Jun 5, 2017 at 6:40 AM, Jerry Hancock <jerry at hanler.com> wrote:
>> It was off 7.5KM, that’s a little beyond groggy, no? More like a trip to
>> I’ll let it rerun the survey and see if it gets closer.
>>> On Jun 4, 2017, at 9:50 PM, Mark Sims <holrum at hotmail.com> wrote:
>>> Be careful when using one unit's location to set a different model's
>> location... particularly the altitude. Some devices report altitude in
>> MSL, others in AGL... and different units may use different models for the
>> ellipsoid. You are always better off using coordinates generated by the
>> particular device (either the built-in self survey or something like Lady
>> Heather's precision survey).
>>> I did some tests while developing Lady Heather's precision survey code.
>> I got pretty consistent lat/lon/alt results on the same units but
>> consistent offsets between different models of receivers.
>>> I would suggest letting the Motorola re-surevy itself... it may have
>> been a bit groggy after first being powered up after a few years of
>> sleep... I know I am...
>>>> I then set the M12 reference location to the Lucent location as I know
>> it to be correct.
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> Chris Albertson
> Redondo Beach, California
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