[time-nuts] Re: NTP servers

[Andy Talbot]

GPS with any serial port can be used (along with PPS) for accurate time.
You write a routine for a microcontroller that maintains time  keeping and
clock it from the 1 PPS signal.  That gives you correct display of accurate
time.   You read the NMEA from the GPS and use it to set your registers on
a regular basis.  I usually do every minute, at second number 58.  The NMEA
read can be done in time so the next PPS increments to the correct
setting.   NMEA string is always less than one second long

The only time this falls over is for leap seconds when they don't show on
the display for up to one minute after they happened.

Andy
www.g4jnt.com



On Mon, 22 Nov 2021 at 10:42, Hal Murray <halmurray at sonic.net> wrote:

> time.isanapp at gmail.com said:
> > but I am curious to see if I could sync up with some of you guys who
> seem to
> > have some pretty cool set-ups.
>
> GPS has taken over the time-distribution business.
>
> If you want to use GPS, there are 2 ways to go.
>
> You can get a simple GPS receiver and plug that into your PC.  For decent
> time, you need a PPS signal and a real serial port, not USB.
>
> The other approach is to get a GPSDO -- GPS Disciplined Oscillator.
> That's a
> box with a GPS receiver and a good crystal and some software.  It smoothes
> out
> the rough edges in the timing from the GPS signal and/or keeps going
> (holdover) when the GPS signal fades or dies.
>
> GPS is very very good in the long term but noisy in the short term.  Short
> means seconds to minutes.  Long means days/months.  A GPSDO will get rid
> of
> most of the short term noise.
>
> There are/were several models available at relatively low cost after they
> were recycled from cell phone towers.  HP Z3801A and Trimble Thunderbolt
> are popular.  "GPSDO" will get lots of hits on eBay.  I don't know how good
> the recent ones are.
>
> Just reading the info available can be fun if you like that stuff.
>   http://www.realhamradio.com/GPS_Frequency_Standard.htm
> The Z3801A manual is a good read.
>   http://www.leapsecond.com/museum/z3801a/097-z3801-01-iss-1.pdf
> Time sink warning!
>
> GPS works much better if you have a good antenna location.  Modern GPS
> receivers are sensitive enough that they sometimes/often work indoors,
> maybe better on a window sill, maybe even better if the window faces
> south.  Just because it's working now doesn't mean it won't glitch often
> enough to cause problems.  Mumble.  Try it and see what happens.
>
> ---------
>
> There is actually a 3rd way: buy a box that does it all.  But that's not
> much
> fun, at least for most of the people on this list.
>
> ----------
>
> > there are plenty of good stratum 1 NTP servers open to the public (e.g.
> > NIST's servers),
>
> If you have a good local NTP setup, you can measure how good other servers
> are, or more likely how good the network connection is.  NTP assumes the
> network delays are symmetric.  Often, that's not true.  So "how good" turns
> into measuring network (a)symmetries.
>
>
> --
> These are my opinions.  I hate spam.
>
>
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