[Thumbgps-devel] performance testing sure gps board via usb serial converter

Ron Frazier (NTP) timekeepingntplist at c3energy.com
Tue Mar 27 00:59:12 PDT 2012

Hi guys,

I thought you'd like to see this.  I'm definitely getting performance 
from my GPS connecting through a Prolific based serial - USB converter 
that is within the specs that you proposed for your project - ie + / - 
1ms offsets from GPS time.  I posted this on the NTP Questions list 
tonight.  Some parameters apply only to Windows and some apply only to 
Linux.  The testing reported was done on Windows.  Season to taste for 
your OS.




I've been doing some performance testing with the Sure GPS board 
connected via serial - USB converter, with PPS to DCD.  I'm running 
Win7.  Performance is limited by the USB interface, but I've observed 
some interesting trends.  I have email periodically checking for 
messages in the background, and I have 5 browser windows open with about 
48 tabs.


Thanks to Alby for showing me how to use flag1 on the 20 driver to turn 
on pps without using the 22 driver separately.  I don't know if there 
are any performance differences between the two.

Thanks to Dave Hart for letting me know that flag3 to turn on kernel 
discipline doesn't apply to Windows and could cause problems.

All these parameters and permutations are confusing, particularly if 
experimenting with Windows and Linux.

Thanks to David Taylor for encouraging me to experiment with 
interpolation, using the environment variable 
turn off.

Does that apply to Linux by the way?

In the past, I thought realtime priority for the NTPD process was 
causing problems.  So, I've been experimenting with both priority and 
interpolation.  Realtime is the default priority.

The most accurate time source I have is my GPS.  Internet, in my case, 
doesn't even come close.  So, I'm testing min and max offsets of my 
computer's clock to the GPS polling every 8 seconds.  The tests weren't 
too scientific nor too long, but I still saw some interesting results.

1) Interpolation ON , Above Normal Priority, + 1.00 / - 0.75 ms, Total 
Range 1.75 ms
2) Interpolation ON , Realtime     Priority, + 0.99 / - 0.67 ms, Total 
Range 1.66 ms
3) Interpolation OFF, Above Normal Priority, + 1.21 / - 1.19 ms, Total 
Range 2.40 ms
4) Interpolation OFF, Realtime     Priority, + 1.13 / - 1.02 ms, Total 
Range 2.15 ms

Comparing lines 1 and 2, going from Above Normal to Realtime priority 
with interpolation on reduces range by .09 ms.

Comparing lines 3 and 4, going from Above Normal to Realtime priority 
with interpolation off reduces range by .25 ms.

Comparing lines 3 and 1, in that order, turning interpolation on at 
Above Normal priority, reduces range by .65 ms.

Comparing lines 4 and 2, in that order, turning interpolation on at 
Realtime priority, reduces range by .49 ms.

Conclusion, I'm leaving interpolation on, and I'm leaving the process at 
Realtime priority.

OK.  It's 3:34 AM and I have to get some sleep.  I really have to get 
UN-obsessed with this.  I really do like to fully understand the 
technology I'm using, but GPS + NTPD has turned out to be a real 
challenge.  Signing off for now and, if there are typos, to bad. 
Hopefully, someone will find this useful.




(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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