[time-nuts] GPS accuracy specs

Richard H McCorkle mccorkle at ptialaska.net
Sun Feb 16 20:17:57 EST 2014


The 5ns time stamper data attached shows a 500 second hang in the
1PPS phase from an M12+. A graphic example of why including
sawtooth correction in a GPSDO is a good idea. With 500 samples
having an offset even long filter times create a source control
error unless the offset is corrected in the samples before
entering the control loop.


> The smart guys have already contributed to this thread.  What I have is an example
> taken when I was doing some testing.  The plot at the top is the DAC on my GDPSO. 
> It is moving the oscillator back and forth about 4.6E-4 Hz.  Not a lot. 
> The bottom plot is the interesting one, as it is the 1PPS from my Adafruit Nav
> Receiver plotted with respect to the OCXO.  The labeling is unfortunate for this
> discussion.  Just think of "-10" as being +180 degrees of phase, and -30 as being
> -180 degrees, for a total phase of 360 degrees between them..  Notice the plot at
> about 10:40.  It points out that the 1PPS error isn't nicely spread out.  Rather,
> it moves around as the difference between the time signal being received from the
> satellite and the free-running oscillator in the GPS receiver
> http://www.evoria.net/AE6RV/GPSstd_PLL/Plots/PPS%20Anomaly%202.png
> Bob
>> From: Jimmy D. Burrell <jimmydburr at gmail.com>
>>To: Discussion of precise time and frequency measurement <time-nuts at febo.com>
>>Sent: Sunday, February 16, 2014 1:55 PM
>>Subject: [time-nuts] GPS accuracy specs
>>I've looked at several different manufacturer GPS datasheets now regarding the 1
>> PPS output in an attempt to compare apples to apples. Some of them rate their 1
>> PPS output as something on the order of "PPS signals have an accuracy ranging
>> 10ns" which seems ambiguous. Does that mean the leading edge of their 1PPS is
>> within 10ns of the GPS clock? Or simply that the stability of their 1 PPS is
>> within 10ns? Or both?
>>Perhaps there's an industry standard for these specs of which I'm unaware?
>>The datasheet for my (presumably much older) Globalsat ER-102 seems, to me at
>> least, to be much more clear stating "time reference at the pulse leading edge
>> aligned to GPS sec., +/- 1 us". Which I interpret as the leading edge of my
>> receiver's 1PPS is aligned with the GPS's clock to within +/- 1 us.
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