[ntp:questions] Re: distance to dcf77 transmitter: how to set the fudge?

Johan Swenker no_spam_please at swenker.xs4all.nl
Fri Jan 28 22:11:04 UTC 2005


On Wed, 26 Jan 2005 17:37:03 +0100, Folkert van Heusden wrote:

> Am I right that when I want to tell ntpd that it has to take care of the 
> distance to the dcf77 transmitter that I have to do that like this?
> 
> # DCF77 receiver
> # "Der DCF77 Sender steht in Mainflingen (50 0 Nord, 09 00 Ost), etwa 25 km 
> sdstlich von Frankfurt/Main."
> # http://www.maporama.com/
> # Mainflingen: 50 1' 54" , 9 1' 33" 50.0318 , 9.026
> # Gouda/2804 RR: 52 1' 37" , 4 42' 15" 52.0272 , 4.7044
> # http://www.fcc.gov/mb/audio/bickel/distance.html
> # -> 374.766km
> # speed of light: 299 782.458 km/s
> # delay: 1.25012652ms

And you have to add to this the effects of the serial line between your
reciever and your PC. The cheap recievers sample the erial line with 50
baud. That is 20 msec between each of the following ||

   |StartBit|bit 0|1|2|3|4|5|6|7|Parity bit|Stop bit|Stop bit|
    ^                               The serial interface     ^ sends
    |                               the data to the PC
    | The beginning of the start bit is sampled with 16 times
      the baud rate (or was it 8 times?)

Adding everyting together 
- 12 times 20 msec = 240 msec (which is about your 246.13)
- inaccuracy of at least 20/16 = 1.25 msec 


> server 127.127.8.0 prefer mode 14
> fudge  127.127.8.0 time1 0.00125012652
BTW is time1 a time in seconds or in msec?

> 
> I ask this question as suddenly the offset for my DCF receiver sky-rockets:
> 
>       remote           refid      st t when poll reach   delay   offset 
> jitter
> ==============================================================================
>   GENERIC(0)      .DCFa.           0 l   77   64   16    0.000  -246.13  11.484  <---
>   keetweej.intran 130.149.17.8     2 u   71   64   66    0.063    0.488  23.017
> 
> Folkert van Heusden

Groeten Johan Swenker



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