[ntp:questions] Questions about joining pool.ntp.org

unruh unruh at wormhole.physics.ubc.ca
Wed Aug 31 00:22:10 UTC 2011


On 2011-08-30, Chris Albertson <albertson.chris at gmail.com> wrote:
>> If you care about microseconds or nanoseconds, shortwave radio is a poor
>> choice.  The length of the radio propagation path changes constantly! Not by
>> much but if you care about the nanoseconds. . . .
>>
>> If you REALLY NEED the nanoseconds, buy an atomic clock and have it
>> calibrated by your national standards people.  If you don't REALLY NEED
>> the nanoseconds you can almost certainly save $50K - $100K.
>
>
> GPS, eBay and the cell phone industry have changed that.   Prices have
> fallen to well below $1K  or even below $100 if you can do some soldering.
>  For example Synergy's "12MT+" receiver has a 2 nanosecond (1 sigma) error
> and sells for $60 new.   It is easy to find a good working Rubidium
> oscillator (a kind of "atomic clock") on eBay for $100 to $150.
>

I am not sure that I believe that stated accuracy of 2ns. It would seem
others do not as well-- I found a paper by Mumgford in 2003 who compared
the performance of the Oncore M12+ , with a Leica CRS1000 survey grade gps
unit, and found that the Motorola was about 600ms late compared to the
Lassen, and had about a 60ns jitter. Of course since this is a
comparison, it is possible that the Leica was 600ns too soon, and the
jitter was all its jitter. 

Mind you even 600ns is pretty good and almost impossible to get into
your computer. 


>
>
>
> Chris Albertson
> Redondo Beach, California




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