[ntp:questions] basic questions about the leapsecond

Martin Burnicki martin.burnicki at meinberg.de
Mon Dec 15 15:05:28 UTC 2008


Hi Antonio,

Antonio M. Moreiras wrote:
> Hi...
> 
> I'm a bit worried regarding the leapsecond, since I'm not finding much
> documentation about it... I have a simple (but unanswered) doubt: Do I
> have to manually configure something?
> 
> Could you point some docs?

Here's some basic information:
http://www.meinberg.de/english/info/ntp.htm

> We have the following structure repeated 3 times:
> 
> +------- --+ 1 PPS +--------+  IRIG  +-------+   +-----------+
> |Atomic Ref|------>|IRIG Gen|------->|NTP st1|-->|pub NTP st2|
> +----------+ 10Mhz +--------+ + 1PPS +-------+   +-----------+
>                                         ^            ^
>                                         |            |
> 
>                                      other st1    other st2
> 
> - 1 st1 server is freebsd + ntpd + IRIG audio
> - 1 st1 server is symmetricom syncserver S250
> - 1 st1 server is spectracom netclock 9283
> 
> At the IRIG generator we have to manually set the leapsecond bit.
> 
> Could anyone tell me if these servers could read the leap bit correctly
> from IRIG signal? Specially IRIG audio?
> 
> How could I test it at NTP? (Since I have made the adjust at IRIG gen
> how could I tell if ntp can read it correctly?)
> 
> We have also 2 systems with GPS:
> 
> +---+  NMEA  +-------+
> |GPS|------->|NTP st1|
> +---+ + 1PPS +-------+
> 
> One of them is a Trimble Acutime 2000, and the other is a Garmin 18LVC.
> Do I have to worry about these ntp servers?

Please note your NTP servers need to receive a leap second announcement e.g.
one day *before* the leap second actually occurs, otherwise they can not
handle the leap second correctly.

Leap second announcements can be received from an upstream NTP server, from
a reference time source, (IRIG, GPS, ..), or from a leap second file. It is
important to check whether *both* the reference time transport format (IRIG
frame type or NMEA format) and also NTPs reference clock driver configured
for that refclock support leap second announcements.

Most commonly used IRIG frame formats have not specified a leap second
announcement flag, but some formats like IEEE1344 support this. So it
depends on the IRIG format you are using if a leap second announcement
arrives at your IRIG receiver.

I'm also not sure whether NMEA supports leap second announcements. E.g. the
parse driver (type 8) does. For an NTP server it is *not* sufficient if
only the leap second is transmitted as second 60 in a time string).

If your NTP servers run NTP v4 then the best way is to install the NIST leap
second file. See:
http://support.ntp.org/bin/view/Support/ConfiguringNTP#Section_6.13.

Don't forget to enable autokey on your servers since otherwise the leap
second file will not be evaluated.


Martin
-- 
Martin Burnicki

Meinberg Funkuhren
Bad Pyrmont
Germany




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