[ntp:questions] NTP absolute accuracy?

RedGrittyBrick RedGrittyBrick at spamweary.invalid
Mon Nov 2 15:04:21 UTC 2009


Unruh wrote:
> John Hasler <jhasler at newsguy.com> writes:
> 
>> Bill Unruh writes:
>>> Sorry, at 10usec, the distance away of the transmitter must be less than 3 km.
> 
>> 10usec at the transmitter.
> 
>>> Also your system needs to see the start of the tone to 10usec which
>>> means that the tone would have to be about 1MHz which is a bit beyond
>>> audio.
> 
>> There isn't any tone.  It's the UK equivalent of WWVB.
> 
> How is the beginning of the second ( an hour) marked?

"The MSF signal carries time and date information that is transmitted in 
a simple binary-coded decimal format by on-off modulation of the 
carrier. The full sequence of information is transmitted each minute, 
using two data bits during every second of the minute except the first.

The MSF time and date code includes the following information:

     * year
     * month
     * day of month
     * day of week
     * hour
     * minute
     * British Summer Time (in effect or imminent)
     * DUT1 (a parameter giving UT1-UTC; "
http://www.npl.co.uk/science-technology/time-frequency/time/products-and-services/msf-radio-time-signal


> The bandwidth of whatever marks it has to be pretty narrow, or the transmitter
> would interfer with everything around it. Ie, that means a large time uncertainty.

Accuracy of the transmitted signal is continuously monitored at other 
locations. E.g. Teddington (~300 miles from transmitter).

http://www.npl.co.uk/upload/pdf/user_guide_bulletins.pdf

MJD   Date        UTC(NPL)-MSF(60 kHz)
                   12h UTC
                   (microseconds)
55075 2009-09-01  -9.5
55076 2009-09-02  -9.7
55077 2009-09-03  -9.9
55078 2009-09-04  -9.8
55079 2009-09-05  -9.8

 From http://resource.npl.co.uk/time/bulletins/msf/msfbul_09_2009.pdf


> Ie how fast is the carrier actually switched off and on. It cannot be
> instantaneous or the signal would spill out badly into neighbouring bands. 

"Simple on-off carrier modulation is used, the rise and fall times of 
the carrier are determined by the combination of antenna and 
transmitter. ... "
http://www.npl.co.uk/upload/pdf/MSF_Time_Date_Code.pdf



-- 
RGB




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