[time-nuts] Re: NTP servers

[Hal Murray]

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.

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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.

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> 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.