[time-nuts] can of worms: time-of-day in a community radio station

[Hal Murray]

> With this setup you have no single point of failure, and even if the
> connection to internet fails, they can still provide time as they are peering
> and synchronizing with each other. 

No, it doesn't work that way.  You need connectivity to at least one stratum 1 
server.

There is an option to elect one, orphan mode, if you get disconnected from the 
net.  That keeps all the local clocks close together.  They will all drift 
along with the chosen leader.


> You can use this list to pick stratum 1 servers:
>                           http://www.advtimesync.com/docs/manual/
> stratum1.html 

There is no date on that list.  At a quick glance, a few systems I'm familiar 
with are way out of date.  I wouldn't trust any of the details.  Same for 
other lists of stratum 1 servers.  They might be a good starting point.  In 
particular, many servers that say "open access" on lists like that have 
changed their rules, often going off the air.


> server time.nist.gov

That's a bad example.  NIST has servers at several locations.  That name 
rotates across them.  You might get a good one, or you might get one on the 
other side of the country.  If it works well today, it may not after you 
reboot and pick a different server.
  https://tf.nist.gov/tf-cgi/servers.cgi


> just a cheap GPS receiver (serial is best, but USB should be OK

There are 3 levels of GPS receiver.  GPSDO is best.  GPS with PPS is second - serial prefered, but USB works.  Low cost GPS (without PPS) on USB is last.

USB is polled -- no interrupts.  That adds a millisecond of jitter.  That's probably not an issue if your goal is 100 ms.  (If you are a geek, be sure you understand hanging bridges.)

The timing on the serial port from low cost GPS receivers is often crappy.  This is a horrible example:
  http://users.megapathdsl.net/~hmurray/ntp/GPSSiRF-off.gif
Anyway, you need to check the device you select.  (Or get suggestions from people who are using them for NTP.)

The PPS signal will fix that.

The GPSDO will continue providing decent time if your GPS stops working.  Holdover is the buzzword.


-- 
These are my opinions.  I hate spam.