[time-nuts] GPS 18 behavior

[Jim Lux]

On 7/21/13 6:59 AM, David J Taylor wrote:

> My own notes on the GPS-18 LVC are here:
>
>   http://www.satsignal.eu/ntp/FreeBSD-GPS-PPS.htm
>
> You may be able to gather something about the electrical characteristics
> from that note - the device will happily feed to PC "RS-232" ports
> connected in parallel (driving receive signals, that is) without problem.

I see you're driving a LED through a 1.5k resistor, and still getting 
above the notional 3V threshold for the RS232 input, so that PPS must be 
able to source a fair amount of current.  So it's definitely not a 1 k 
pullup and an open collector driving it.



>
> I recall that during initial testing it takes some time for the PPS to
> start, so think it needs at least a minimal lock.  I have less
> recollection about how long the PPS takes to stop in the event of lack
> of satellite signal, but it does stop at some time and does not continue
> for ever.  Being crystal-controlled, I imagine that the free-running PPS
> would be pretty accurate, not 10% out as you suggest.
>

Oh, I didn't actually think it would be 10% off.. more like a few ppm, 
depending on temperature.  I was wondering more what would happen if you 
were indoors for a couple days, then went back out, what the box does. 
realistically, I don't expect that it smoothly brings it back into line, 
rather, I would expect it to "jump" to the correct time and timing.

I think, realistically, I should be looking at the actual sentences 
coming out and figuring out whether our "GPS synchronized" state has 
changed, and maybe deal with it appropriately.