[time-nuts] Allan Deviation recipe?
Tom Van Baak
tvb at LeapSecond.com
Tue Jul 19 15:47:12 EDT 2016
> Check it out:
> That is for a SiRF III (MR-350P) with a bad sky view.
> How does that look? Totally wrong? Somewhat wrong?
> Once I get this packaged up I'll have lots more similar graphs.
Looking good. Not sure how artistically picky you are, but:
- consider changing 'PPS' to '1PPS'
- consider changing 'GPS' to 'NMEA' (at least I think that's what you mean)
- the y-axis title is partially cropped off the chart; center the plot to get the same amount of border as top, bottom, and right sides
- for mobile or printed usage, the axis titles (powers of ten) font size is too small by maybe 2 or 3 points
- consider using a greater depth of adev5 points (e.g., 10 or 20 or 50 per decade) and skip the solid line; or just try both ways to see if one looks better
Also try plotting TDEV instead. In a case like this an ADEV plot is somewhat boring or even misleading -- it's just a -1 slope going down forever. It's not quite what ADEV was meant for. The slope suggests you have a bounded amount of white phase noise and that's that. So, for example, the entire PPS red trace can thought of as 6e-4/tau. In other words, you can represent the performance of the receiver with a single rms number, instead of a featureless ADEV plot. Similarly the blue trace is essentially 0.6/tau.
The weird jump in the NMEA blue trace between tau 1000 and 2000 should be investigated. Increasing the point density (using adev5) or eliminating the false line interpolation or using TDEV may help you look into this.
The other thing, that Bob just alluded to, is that while your NMEA measurements are probably legit, your PPS measurements may be totally skewed by the fact that you're using a plain PC and its operating system (and NTP, and drivers, interrupts, BIOS, caches, etc.) as a measuring instrument. The actual 1PPS out of a typical GPS receiver is orders of magnitude more precise than the "tool" you're using to measure it.
Therefore it's possible that the red trace is more a measurement of how bad your PC/NTP measurement setup is rather than a measurement of how good the 1PPS is. In other words, you have accidentally used 1PPS to measure the limits of your measurement system, rather than use your measurement system to measure the 1PPS. Does that make sense?
----- Original Message -----
From: "Gary E. Miller" <gem at rellim.com>
To: "Tom Van Baak" <tvb at LeapSecond.com>
Cc: "Discussion of precise time and frequency measurement" <time-nuts at febo.com>
Sent: Tuesday, July 19, 2016 11:42 AM
Subject: Re: [time-nuts] Allan Deviation recipe?
With help from Tom Van Baak and Bob Stewart, I have 6 weeks worth of
GPS Serial and PPS timing data in a nice ADEV chart. I think.
Check it out:
That is for a SiRF III (MR-350P) with a bad sky view.
How does that look? Totally wrong? Somewhat wrong?
Once I get this packaged up I'll have lots more similar graphs.
Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97703
gem at rellim.com Tel:+1 541 382 8588
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