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

Gom gom at gom.invalid
Wed Mar 21 15:54:55 UTC 2012


The thread priority is already maximum.

The program has no problem disciplining the clock, as it's able to keep 
the calculated offset to under 30 µs with the precise PPS.

What I wanted to show is that if you use the basic NMEA you will get 
about 50ms error on the arrival of the first NMEA sentance.

That is confirmed here : http://www.leapsecond.com/pages/MG1613S/
Check paragraph "NMEA latency" , you will see the dots are scattered in 
about a 50 ms windows. If you do the math on the distribution this 
represents a 10 ms jitter (standard deviation).

Le 21/03/2012 15:49, Ron Frazier (NTP) a écrit :
> 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

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