[ntp:questions] NMEA ref.clock better than my ISP's timeserver?
unruh-spam at physics.ubc.ca
Thu May 28 16:49:50 UTC 2009
"Richard B. Gilbert" <rgilbert88 at comcast.net> writes:
>E-Mail Sent to this address will be added to the BlackLists wrote:
>> Unruh wrote:
>> > Note that my checking with something in germany is not going
>> > to give me very good timing. the earth is a large place.
>> Isn't the round trip delay calc
>> (Client Destination TS - Client Origin TS)
>> - (Server Transmit TS - Server Receive TS)
>> diff supposed to help compensate for that?
>It doesn't "compensate"! It simply tells you the maximum uncertainty
It does "compensate". It says that the best estimate of the time offset
is the difference between the average of the two local times minus the
average of the two remote times. Ie, it assumes that the propagation
delays are symmetric. What you are talking about is the worst case
expected error in that which is half the roundtrip time.
>introduced in transmitting the request and reply. You know that it
>can't be worse than one half of the round trip time. All other things
>being equal, the lowest round trip time gives you the best time.
But all other things are not equal. Thus I have to secondary sources,
one here at UBC and one in Regina. (my primary is a gps receiver) the a
average offset from Regina is about 300usec, with a round trip time of
45ms, while the average ofset from UBC ( less than 1/2Km away) is 4ms
with a round trip time of 6ms.
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