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

Richard B. Gilbert rgilbert88 at comcast.net
Tue Aug 30 22:15:35 UTC 2011

On 8/29/2011 11:57 PM, NPG wrote:
> On 8/29/2011 9:47 PM, unruh wrote:
>> On 2011-08-29, NPG<nathan at cmpublishers.com>  wrote:
>>> On 8/29/2011 5:03 PM, Chris Albertson wrote:
>>>> On Mon, Aug 29, 2011 at 12:43 PM, NPG<nathan at cmpublishers.com>  wrote:
>>>>> Stupid noob question alert.
>>>>> Does GPS provide better accuracy?
>>>> Depending on which GPS receiver, GPS can be MANY orders of magnitude better.
>>>>    The best GPSes have a one sigma error on the pulse per second that is on
>>>> the order of 5 nanoseconds or less.  At this level we need carfully measure
>>>> cable lengths and allow for the speed of light delay and "velocity factor"
>>>> of the cable.  We are talking about literally 10,000  or even a million
>>>> times better, literally.
>>> LOL, I only vaguely understand what you are talking about.
>>>> That said if the GPS is only being used to drive NTP we don't need
>>>> nanosecond level accuracy.
>>> OK, I guess I don't necessarily need to understand the above.
>> Not sure what you do not understand, but if nanosecond (1 billionth of a
>> second) confuses you then it is not clear what it is you want to do.
> The time stuff I understand, even the radio wave propagation stuff,
> though obviously not as well as others here.
> I'm not sure what "cable length", "speed of light delay", or "velocity
> factor" is all about.
> My guess is its about where physics start to limit the theoretical
> performance of timekeeping electronics.

The speed of light and/or electrical signals, radio waves, etc. are 
*approximately* the same!  The speed of an electrical signal in copper 
wire is slightly less than the speed of light in vacuum!  It will be 
different again if the wires are aluminum rather than copper.  If you 
are counting nanoseconds the difference may be significant.  A few 
people, e.g. radio astronomers, really care about the difference.  MOST 
of the rest of us are less demanding!

"Get me to the church on time!"

Your doctor WILL be late for your appointment!  It's a law of nature!

More information about the questions mailing list