[ntp:questions] Lep seconds

Rob Kimberley time.bandit at btinternet.com
Wed Jan 16 12:13:30 UTC 2008


Have come across requirements in MIL sat comms systems that need GPS time as 
well as UTC. In fact I supplied a system to a MIL customer a few years ago 
with two Zyfer GPS NTP servers - one set to provide UTC time and the other 
set to provide GPS time.

Rob Kimberley


"David L. Mills" <mills at udel.edu> wrote in message 
news:fmij6g$odi$1 at scrotar.nss.udel.edu...
> Joseph,
>
> Please say what GPS equipment is used in the "big radar world" to deliver 
> TAI or even GPS. My expensive GPS receivers have no provision for other 
> than UTC. TAI is of course a constant offset from GPS give or take 
> laboratory nanoseconds. It would be interesting to learn why big radar 
> needs other than UTC.
>
> Dave
>
> Joseph Gwinn wrote:
>> Dave,
>>
>> In article <flli7r$5iu$1 at scrotar.nss.udel.edu>,
>>  "David L. Mills" <mills at udel.edu> wrote:
>>
>>
>>>Joseph,
>>>
>>>Conversely, if a client syncrhonizes to a server strictly running TAI and 
>>>never signals leaps, NTP will deliver TAI. NIST, USNO and I have 
>>>discussed this serveral times and concluded the lessor of two evils is to 
>>>continue with NTP on UTC.
>>
>>
>> Yep.  True enough.  But GPS emits TAI (plus an offset), so one can claim 
>> that configuring the NTP timeserver to emit GPS System Time (not UTC) is 
>> to generate what is essentially TAI.  This is widely done in the 
>> big-radar world.
>>
>> Joe
>>
>>
>>
>>>Dave
>>>
>>>Joseph Gwinn wrote:
>>>
>>>
>>>>In article <T1199401837 at ex.djwhome.demon.co.uk.invalid>,
>>>> david at ex.djwhome.demon.co.uk.invalid (David Woolley) wrote:
>>>>
>>>>
>>>>
>>>>>In article <298286.52982.qm at web55415.mail.re4.yahoo.com>,
>>>>>marknmbox-88 at yahoo.com wrote:
>>>>>
>>>>>
>>>>>
>>>>>>compliant.  Is there a similar mod for NTP.  I am
>>>>>>hoping that there is a mod that will cause NTP to
>>>>>>supply theoretical UTC (even if it is not ascci).
>>>>>
>>>>>Both POSIX and NTP use UTC.  Your problem is that you are not using
>>>>>using UTC, but, rather, using TAI.
>>>>
>>>>
>>>>Actually, POSIX does *not* use UTC in the normal sense of the word, as 
>>>>no leap seconds are applied.
>>>>
>>>>The fundamental POSIX timescale counts what amount to SI seconds from 
>>>>the POSIX Epoch, 0h 0m 0s UTC 1 January 1970.  Every day contains 
>>>>exactly 86,400 seconds.
>>>>
>>>>That said, if one drives a POSIX box via NTP from a GPS timeserver set 
>>>>to emit UTC (versus GPS System Time), time on the POSIX box will be 
>>>>pretty close to UTC.
>>>>
>>>>Joe Gwinn 





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