[time-nuts] GPS accuracy specs

Tom Van Baak (lab) tvb at leapsecond.com
Sun Feb 16 20:19:54 EST 2014


http://www.leapsecond.com/pages/m12/sawtooth.htm

/tvb (i5s)

> On Feb 16, 2014, at 4:46 PM, Bob Stewart <bob at evoria.net> wrote:
> 
> 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.
>> 
>> Jim...
>> N5SPE
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