[ntp:questions] NTP and GPS

Miguel Gonçalves miguel.barbosa.goncalves at gmail.com
Tue Apr 30 21:08:14 UTC 2013

Hello fellow NTP users!

I have a few topics I would like to discuss with you guys. I describe my
configuration first and then I wrote some questions. This is a lengthy
message but I believe that some topics are interesting for other users.

In my network I have 2 dedicated low powered i386 boxes with GPS receivers
from different brands (Garmin 18 LVC and Sure Electronics GPS Evaluation

At the moment both servers only have their GPS clock as sole reference.

I have a Cisco router that gets the time from these two local stratum 1
servers and from more 5 stratum 2 servers (close to me) on the Internet. I
know routers aren't good timekeepers but this is intended as a backup as
you'll see further down.

Every device in my network runs NTP and requests the time from the 2
stratum 1 servers and from the Cisco router.

Now the questions...

If one of the stratum 1 goes nuts the Cisco will mark it as a falseticker
and discard it's data, right? I believe the Cisco will then choose the
other stratum 1 to sync from. Right?

If the Internet connection is overloaded (not usual here) the Cisco will
choose predominately the stratum 1 servers. Right?

Is this circular topology (a client using a server that in turn references
the other two sources the client is using) a good idea?

I know a better approach would be to add another stratum 1 (having 3 in
total) and ignore completely the Internet servers. Can I really get away
with the above configuration? I am not looking for the ultimate precision
but I intend to use these two receivers I have to their fullest.

How good is NTP detecting a problem with a reference clock? Let's imagine
there's a problem with one particular satellite and makes the time jump a
few mili-seconds. With only one reference clock will NTP follow blindly or
ignore it? Adding another GPS would be pointless but a radio receiver would
be OK I guess. The problem is that I don't have any radio signals in my
country (I could get a GLONASS receiver). GPS receivers designed for timing
usually employ the TRAIM algorithm and can discard bad data. Does NTP do
something like this? I mean, if my NTP is synched and running fine I would
expect NTP to discard obviously bad data. OTOH, temperature can have a huge
impact on this and NTP doesn't know how hot the server is running.

In the end, I might buy a Meinberg NTP server for my network and have 3
sources and then I will be able to completely ignore the Internet servers
and validate my servers among each other.

Thanks for reading and I will appreciate your contributions.

Kind regards,

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