[ntp:questions] NTP and GPS

David Woolley david at ex.djwhome.demon.invalid
Wed May 1 07:11:18 UTC 2013

Miguel Gonçalves wrote:

> 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?

Noting the lack of sufficient votes for a falseticker, already 
mentioned.  ntpd will normally synchronise to both stratum one servers, 
and, with a lower weighting, to any higher stratum servers, although it 
will only report the error statistics and source identity of one of them.

> 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

GPS time solutions are normally made from multiple satellites (although 
in pre-surveyed mode, a time receiver could use just one.

> 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

Stratum one servers normally set a root dipersion that would mean that a 
1 millisecond error would still be within the allowable error bounds, 
even without the increases to those error bounds to allow for time since 
last measurement and network propagation delays, so it is very unlikely 
that a clock that differed by only or two milliseconds.

In any case, if the radio clock reports a fault, the stratum one server 
will continue to serve time, but with an increasing root dispersion.  It 
will get ignored when that root dispersion gets too high.

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