[ntp:questions] Loop Frequency and Offset

Miguel Gonçalves mail at miguelgoncalves.com
Mon Sep 26 23:05:58 UTC 2011

Hi Richard!

On 26 September 2011 22:39, Richard B. Gilbert <rgilbert88 at comcast.net>wrote:
>  What is a typical offset of the loop without using special oscillators? Is
>> less than 1 us achievable?
> I don't believe that accuracy of 1 microsecond , or less, is obtainable
> without without installing a GPS Timing Receiver or an atomic clock of some
> sort.

I have a Motorola Oncore UT+ that I'll be testing soon. I've just averaged
the position of the antenna during 72 hours.

Besides correcting the CPU clock frequency what can I do to improve the
performance? The machines I have at the moment are showing worse than 1 us
performance perhaps because I am not using GPS timing receivers.

Your chances of obtaining 1 microsecond accuracy using clocks on the
> internet is just about nil!  OTOH very few people actually need
> microsecond accuracy for anything other than "bragging rights"!

I know... :-) My cable connection is really bad for NTP... I can't get
better than 3 us. That is why I have 2 stratum 1 GPS receivers onsite.

Would you really care if you missed the first 75 microseconds of your
> favorite TV program?  I didn't think so!

This is mainly a personal project and if it goes as intended it will be the
first public stratum 1 available in my country. It is sad but even our
National Observatory doesn't care much about time as a public service...
Take a look at one of their stratum 2 public servers:

tick# ntpq -p ntp02.oal.ul.pt
     remote           refid      st t when poll reach   delay   offset
+ntp01.oal.ul.pt .GPS.            1 u  235  512  377    5.139   -0.158
 ntp03.oal.ul.pt .INIT.          16 u    - 1024    0    0.000    0.000
 ntp04.oal.ul.pt    2 u  444  512  377    0.376   -0.518
+ntp05.oal.ul.pt .GPS.            1 u  255  512  377    0.318    0.163
*ntp06.oal.ul.pt .IRIG.           1 u  483  512  377    0.303    0.169

160 us offsets?? Now, take a look at my VoIP server from which all the desk
phones in my company synchronize time:

tick# ntpq -pn asterisk
     remote           refid      st t when poll reach   delay   offset
+       .GPS.            1 u    7   16  377    0.191    0.001
*        .GPS.            1 u   16   16  377    0.187   -0.002

:-) For me this is a good stratum 2 server :-)

> The telephone companies, wired or wireless, rely on extremely precise
> timing.  Radio astronomers really care about nanoseconds.  I'm sure that a
> few others care about the nanoseconds.  Those who do need the nanoseconds
> generally have the specialized equipment and know how required.

And I care about microseconds just as a proof of concept, as a personal
challenge. I've built two stratum 1 servers to be used in my company and
saved thousands of dollars. Obviously Symmetricoms and Meinbergs have bells
and whistles that ease administration but in the end they just provide time
service. My two boxes do the same for a fraction of the cost.

I was just wondering if there's anywhere else I should look at in order to
improve performance.


More information about the questions mailing list