[time-nuts] Some GPS performance plots

John R. Ackermann N8UR jra at febo.com
Wed Nov 3 17:00:53 EST 2004

Tom Van Baak wrote:
> Your plots are fine. Presumably you're showing how
> one can use GPS to check the rate of the Rubidium.
> In this case, as in many others, the sawtooth has
> little to do with it.
> You might try a plot or two using the AVG mode in
> Stable32. Tom Clark often used N=300 (5 minutes)
> with his 53131 counters. Not only does this clean
> up the phase plots nicely but it also does a good
> job smoothing the sawtooth without embarking on
> a hardware/software project; it becomes what I call
> a "mouse project" (all you need to do is click on avg).

I normally average at 100 seconds for this kind of stuff, but the 
results seem to look smoother when I'm doing the averaging in my data 
capture program.  I think I recall that Stable32 in "average" mode for 
phase data just selects every Nth point rather than averaging the 
intermediate values, so the amplitude of the noise doesn't go down in 
the plots.

I wouldn't bother doing the sawtooth correction for this work, but it's 
an interesting project to see what the noise looks like with that 
applied.  I should be able to just add a serial data routine to my 
current program to interrogate the receiver for the T-RAIM status which 
includes the offset of the *next* 1pps, and apply that to the value from 
the counter before logging.

> One other suggestion, it makes an interesting plot
> to show 10 or 20 minutes worth of sawtooth, not
> just 20 seconds. The point here is to observe that
> there is great variability in the "pitch" of the teeth
> of the saw. If you're lucky you can find periods when
> the sawtooth is fine, coarse, positive, negative, and
> in between like a suspension bridge, or upside down.

Good idea.  I'll see if I can find some plots like that in the data.


> /tvb
>>Let's see.  To do that, you'd have to set the logging program up to 
>>record the time interval via the GPIB port, and then get the correction 
>>value via the serial port, then apply the correction and log the data.
>>Well, there's my project for this weekend...
>>(Actually, for my real purpose which is measuring the offset of the Rb, 
>>it's not worth the effort since I'm averaging over many hours, but it 
>>sounds like a good "science fair" project!)
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