[ntp:questions] Jitter on $GPGGA of Sure board

Ron Frazier (NTP) timekeepingntplist at c3energy.com
Wed Mar 21 14:49:19 UTC 2012


Hi Gom,

I just posted a discussion on the PSYCHO PC thread about all the 
different "jitter" type data we can measure and discuss.  We'll see what 
the replies are.  Dave Hart said that jitter is an RMS value of the 
offsets.  So, technically, I don't think that's what your talking 
about.  You're talking about a short term variance in when you get your 
NMEA data.  I also thought that was jitter, but I may have been wrong.  
Take a look at this graph.

http://dl.dropbox.com/u/9879631/drifting02c%20-%20gps%20NOT%20insane.jpg

Ignore the part at the end.  This graph shows the offsets from my PC 
clock to the GPS NMEA only data polling it every 8 seconds.  I have a 
USB based device, which is sending data through an internal serial / USB 
converter.  Note that the peaks are in the -7 / + 10 ms range, for a 
total range of 17 ms.  Going through a serial port might give better 
performance.  There is no serial port on my PC that I'm testing with.

Are you running Windows, and are you running NTP?  I got that graph 
using David Taylor's NTP Plotter and graphing the loopstats file that 
NTP generates with the GPS as the only selectable clock.  I'm not sure 
why you are showing a variance of 50 ms.  It may be in the way you're 
measuring it.  If you're using your own program, there are going to be 
timing delays in that which are different from what NTP does.

You could boost your program's process priority to above normal or 
high.  Warning, that can destabilize or lock up your system.

Another thought is, what is your baud rate.  Mine's at 9600.  I had it 
up to 57600 at one time, but being a USB connection, I'm not sure what 
that means anyway.  Some people said that could destabilize the system.  
Boosting it didn't seem to improve my short term variance much, but you 
could try boosting yours, both on the GPS and the PC, and see what happens.

Maybe a stupid question, is the GPS moving at all?  Any changes in 
position data could increase variance as the GPS composes the NMEA data 
string.

You could try using the GPZDA string instead.  It reports only time, and 
shouldn't vary in length.  However, it does not have a data validity 
field.  So, NTP might be more likely to fail to know it if the GPS data 
is bad.

Sincerely,

Ron


On 3/21/2012 4:49 AM, Gom wrote:
> Only $GPGGA.
>
> I changed things slightly, in previous version time was queried when 
> $GPGGA was found after some processing, now it is queried as soon as 
> the serial port interrupt arrives.
>
> Serial interrupt arrived between 267 ms and 320 ms, still providing a 
> 50 ms jitter.
>
> 08:46:04.267.945.0      LastUpdate:     0.268265 s
> 08:46:05.000.018.2      PerfCntScale:   0.99997059      offset: 24.2 us
> 08:46:05.277.034.1      LastUpdate:     0.277070 s
> 08:46:05.999.075.4      PerfCntScale:   0.99997054      offset: 5.3 us
> 08:46:06.297.092.4      LastUpdate:     0.297696 s
> 08:46:06.999.132.6      PerfCntScale:   0.99997099      offset: 36.6 us
> 08:46:07.295.149.5      LastUpdate:     0.295680 s
> 08:46:07.999.189.8      PerfCntScale:   0.99997041      offset: -33.9 us
> 08:46:08.277.217.6      LastUpdate:     0.277788 s
> 08:46:08.999.289.8      PerfCntScale:   0.99997047      offset: 0.6 us
> 08:46:09.287.306.3      LastUpdate:     0.288098 s
> 08:46:09.999.347.0      PerfCntScale:   0.99997045      offset: -15.5 us
> 08:46:10.268.373.9      LastUpdate:     0.269155 s
> 08:46:10.999.447.0      PerfCntScale:   0.99997040      offset: 8.3 us
> 08:46:11.288.463.5      LastUpdate:     0.288641 s
> 08:46:11.999.504.2      PerfCntScale:   0.99997039      offset: -6.4 us
> 08:46:12.311.535.4      LastUpdate:     0.312251 s
> 08:46:12.999.604.2      PerfCntScale:   0.99997032      offset: 15.4 us
> 08:46:13.307.621.8      LastUpdate:     0.308169 s
> 08:46:13.999.661.4      PerfCntScale:   0.99997030      offset: -1.3 us
> 08:46:14.295.691.0      LastUpdate:     0.296053 s
> 08:46:14.999.761.4      PerfCntScale:   0.99997033      offset: 31.3 us
> 08:46:15.318.779.6      LastUpdate:     0.319420 s
> 08:46:15.999.818.6      PerfCntScale:   0.99997031      offset: 15.2 us
> 08:46:16.320.837.0      LastUpdate:     0.320896 s
>
>
> Le 21/03/2012 00:56, Ron Frazier (NTP) a écrit :
>> I'm curious. Are you outputting ONLY GPGGA, or other sentences as well.
>> Other sentences will increase jitter. I'm getting about 5 ms of jitter
>> on my USB based BU-353 outputting GPGGA only at 9600 baud. I'll be
>> testing the Sure board once I get it.
>>


-- 

(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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