[ntp:questions] good paper on timing and delay

Ron Frazier (NTP) timekeepingntplist at techstarship.com
Tue May 22 14:07:06 UTC 2012

Hi Dave T, and others,

(I'm cross posting my reply to the NTP questions list since I think they'd be interested too. The original message was from the Thumbgps-devel mailing list.)

I enjoyed that article. I'll admit to not spending 4 hours studying it, and sometimes my eyes glazed over, but I enjoyed it. It brought up some, perhaps simplistic, questions, but I'll pose them anyway.

The essential problem of time sync is to observe one or more remote time servers, with variable and asymmetric propagation delays between you and them, and choose what the best time to set your clock is. Obviously, not simple.

However, it occurs to me that the GPS receivers are doing essentially the same thing via radio. They are observing numerous satellites at various locations in orbit. Those satellites have precision clocks all calibrated to within 100ns (or .1 us) of true time on Earth. The GPS receiver, cannot "poll" the satellites, but it can observe their broadcasts. Each satellite has a variable and at least somewhat asymmetric propagation delay. That propagation delay can be in the range of .25 sec or 250,000 us. Yet, the GPS receiver can routinely output a PPS pulse with an accuracy of 1us or better, taking the Garmin 18 as an example.

So, the possibly simplistic question is, if our network time sync programs used the same algorithm that the GPS receivers use to read their "servers", ie satellites, which all have variable and perhaps somewhat asymmetric propagation delays, which can be substantial, would we be able to achieve much greater levels of accuracy doing synchronization via the internet?

Thanks in advance for any replies.




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Ron Frazier
timekeepingdude AT techstarship.com

Dave Taht <dave.taht at gmail.com> wrote:


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