[time-nuts] GPS 18 behavior

[David J Taylor]

I don't have a GPS-18 in front of me, and I'm modifying some software
remotely, and I ran across an issue that someone on this list probably
knows off the top of their head.

Does the GPS-18 put out 1pps pulses even if it hasn't got a fix yet?
That is, when you apply power, does it just start putting out 1pps
pulses (at some random phase), and then when it gets a fix, it snaps to
the correct timing.

The manual says that it puts out 1pps pulses until power is removed, so
I assume that when the GPS signal goes away, it keeps on as best it can.
  I would assume that the clock isn't great, but it's reasonably close
to 1pps.

What's the behavior when it reacquires?  Say the free-running 1pps is
something like 1.1 Hz, so in 10 seconds, we've got an extra 1pps. Does
it then skip pulses, or what?

And a sort of background hardware question...

The manual is also not very forthcoming on the electrical
characteristics.. is it a pull up with a switch pulling it down to
ground? or does it source significant current? That is, if you wanted to
"wire or" the output from the GPS-18 with another 1pps source, would it
be "diodes" or "resistors" you'd use.
=============================

Jim,

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 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.

Cheers,
David
-- 
SatSignal Software - Quality software written to your requirements
Web: http://www.satsignal.eu
Email: david-taylor at blueyonder.co.uk