[ntp:questions] YEA! My Sure Electronics GPS just arrived.

Ron Frazier (NTP) timekeepingntplist at c3energy.com
Sat Mar 24 14:30:10 UTC 2012

On 3/24/2012 7:05 AM, David J Taylor wrote:
>> I now have the PPS circuit working on the Sure board.  I have not 
>> soldered it yet.  I just taped a jumper wire between the PPS test 
>> point at the edge of the board and the DCD pin 1 on the RS-232 port.  
>> The serial data is coming in through the Trendnet TU-S9 serial - USB 
>> converter, which is passing DCD.  I'm getting + .5 / - 1.5 ms 
>> offsets. The PPS is nowhere to be seen on the statistics screen, but 
>> it is obviously working.  I don't know why it's not more centered 
>> around zero, and maybe that will change.  However, my total peak to 
>> peak range of offset variance is 2 ms, and that's coming through 
>> USB.  If I can maintain that level of accuracy, and it's consistent 
>> with UTC, then I'm very happy.  That's plenty good for my purposes.  
>> I still may try to run it through a real serial port on another 
>> machine just for kicks.
>> Sincerely,
>> Ron
> Ron,
> Yes, those were the sorts of figures I was seeing.
>  http://www.satsignal.eu/ntp/NTP-on-Windows-serial-port.html#usb
> The jitter I saw was about 45 microseconds for the USB/PPS against 2.3 
> - 3 microseconds for various serial-port/PPS.  All figures from 
> Windows XP. If the graphs there are anything to go by it seems that 
> the offset varied between -0.15 to +0.3 milliseconds (very 
> approximately).

Hi David,

You appear to be up early.  I'm curious to know what time this email 
says it arrived.  If it says it arrived at about 1030, then that's my 
time.  If it says it arrived at about 14:30, then that's your time.

Since I wrote that, it seems to have centered itself around zero.  I now 
have a very nice + 1.2 ms / - 1.2 ms offset pattern.  Since I've been 
struggling to get anything under 50 ms with other technology, this looks 
really sweet to me.


Conversion of these images to jpeg reduced the clarity a bit, but you 
can still see what's happening.

MICROSECONDS, did you say?  I'm nowhere near that territory with 
everything going through a serial - USB converter.  However, I'm quite 
happy with 1.2 ms under the circumstances.

I am NOW assuming that my clock is more accurate than the internet 
clocks.  I am NOW hoping that neither will appear to be drifting away 
and that nothing in the system will be having routine heart attacks.

I notice there is a difference between my clock and the average internet 
clock reading.  Hypothetically, even though mine is probably closer to 
UTC than those readings, if I wanted to shift my offset to match them, 
so NTP won't clockhop, how as long as the GPS is working, how would I do 

Here are my config lines:

# COM5 57600 windows lines for testing gps selected as main source - 
gpgga 57600 baud
server                        minpoll 3 maxpoll 3          
fudge flag2 0      refid PPS                              
# PPS standard polarity
server                 prefer minpoll 3 maxpoll 3 mode 66  
fudge time2 0.0000 refid GPS1                             
# use WITH PPS

Also, why doesn't the PPS show up in my status screen anywhere?  I know 
it's working, based on the graphs.




(PS - If you email me and don't get a quick response, don't be concerned.
I get about 300 emails per day from alternate energy mailing lists and
such.  I don't always see new messages very quickly.  If you need a
reply and have not heard from me in 1 - 2 weeks, send your message again.)

Ron Frazier
timekeepingdude AT c3energy.com

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