[time-nuts] GPS message jitter (preliminary results)

[albertson.chris at gmail.com]

To answer about how they get good timing. Many times you run a loop that runs off a timer at say exact 1000 times per second. Having something like a 1KHz loop gives very deterministic timing. Lots of ways to do it. One is to run some RTOS. I had to make all the axis of a machine tool move at exact timing to cut a curve. Not hard just have decide to do it. I'm sure most engineers looked at the NMEA spec and read "with the second" and stopped at that. 

> On Jul 20, 2016, at 5:40 PM, Bob Camp <kb8tq at n1k.org> wrote:
> 
> HI
> 
>> On Jul 20, 2016, at 7:51 PM, Mark Sims <holrum at hotmail.com> wrote:
>> 
>> I added the ability of Lady Heather to calculate the time offset of the timing message from "wall clock" time.  It calculates the difference between  the system clock time to the time that the (end of) the timing message arrived.   The result is only as good as your system clock, so the system clock really needs to be synced to something like NTP (otherwise it does a decent job of showing how much your system clock is drifting).  Heather can plot the results and calculate xDEVs on both the message jitter and the message offset time in real time.
>> 
>> On another note,  I did some jitter measurements on  Jupiter-T and Jupiter-T Pico receivers.    I can't imagine how they do it, but those things are insanely good.   Running at 9600 baud,  their message jitter into a hardware serial port is less than a millisecond peak-peak!  Somebody paid a LOT of attention to getting the message timing consistent…
> 
> Just pre-load it all into a big shift register and let the PPS output gate the clock to the beast.  There’s not a lot of doubt about what the label on the next second going out will be :)
> 
> Bob
> 
>> 
>> 
>>                         
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