[ntp:questions] PPS time1 and reported offset

A C agcarver+ntp at acarver.net
Tue Oct 11 22:24:30 UTC 2011


On 10/11/2011 13:52, David Woolley wrote:
> A C wrote:
>>
>> Let us assume that the pulse from the GPS receiver is off from the
>> master UTC clock tick by about 3 ms due to a variety of delays in the
>> overall system starting from the satellite and working its way down to
>> ntpd. (No arguments about how I get to that number, we're just
>> assuming this is the case).
>>
>>
> What's your basis for assuming all the error is attributable to the PPS,
> rather than the other sources contributing to ntpd's time solution.
>
> The GPS receiver should have solved for the time of flight from the
> satellite to better than 100ns for GPS to be able to achieve its stated
> accuracy. If the time of flight solution was out by 3ms on just one
> satellite, your position error would be about 500km. Even if was out by
> the same fixed amount for all satellites, you'd have a position error of
> around 30 metres.
>
> Nearly all the GPS offset will be attributable to the last few feet -
> most of it within the PC.

I'm asking based on an assumption that this is the delay end to end from 
the satellite to ntpd's final acceptance of the PPS tick.  This is why 
I'm saying not to argue this point of the delay calculation. I want to 
know what the expected behavior of ntpd is given the conditions outlined 
where a time offset is set into time1 on the PPS refclock.  What the 
actual value of that is doesn't matter to my question other than to 
provide a fixed number for the operation.

I'll gladly discuss the reasoning for a delay later but I first want to 
have the operation question answered.  Note that this is also only the 
PPS refclock, not a NMEA refclock.  NMEA is not being used in this case, 
only PPS.  The actual time is being derived from pool servers.


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