[time-nuts] M12+T sawtooth correction and additional filtering

Jerry Mulchin jmulchin at cox.net
Wed May 9 00:52:10 UTC 2012


Tom,

So I'm thinking that as long as I'm putting in sawtooth correction, and I'm using a
0.25ns/step delay device (DS1123L part), then it seems to me that at some point
in the kalman filtering algorithm I should be able to decide that instead of delaying
1ns all the time, that I should be able to delay 0.5ns, or maybe 0.25ns. Giving a
smoother sawtooth correction to the PRS10.

But does that really improve anything over 1ns delay corrections given by the M12+T
receiver. I don't want to go off and try and convince myself that this is a real improvement
when in fact it may not be. So the real question is; is this going to work or am I
wasting my time?

Jerry


At 04:19 PM 5/8/2012, you wrote:
>Jerry,
>
>It depends on what your goal is. Hardware sawtooth correction is
>good at improving the short-term jitter of the 1PPS output.
>
>But if you're using the M12+T with a TIC simply to measure the
>performance of some time/frequency standard, then software
>correction is easier and gives slightly better results. This assumes
>you have a PC or equivalent, and that you like coding more than
>soldering.
>
>You'll note that both Trimble's Tboltmon and Rick's Tac32 programs
>allow you to combine sawtooth corrections and TIC readings into
>a single value by using two serial ports.
>
>Note there is a point where averaging lots of raw 1PPS data and
>sawtooth corrected 1PPS data is pretty much the same. Before
>you jump into this -- try a minute, an hour, and a day of data and
>see what you get.
>
>But there's more...
>
>As long as you're playing with @@Hn messages, Kalman filters,
>and software sawtooth correction, you might want to consider
>looking at the per-SV "fractional GPS local time estimates" in the
>Hn message. It's the mean of these values that determines the
>virtual 1PPS (= the physical 1PPS + sawtooth correction).
>
>It's my guess that taking a number of factors into account -- such
>as lat/lon, az/el, svn, snr, and fractional stddev -- that one could
>have a history-driven "intelligent weighted mean" instead of the
>simple realtime simple mean that is currently used in the M12.
>
>The M12 uses a fixed global elevation mask to decide if a SV is
>in the solution; so it's all or nothing. With your Kalman filter, over
>a few days or weeks, your software would have a more general
>equation with which to solve for the 1PPS each second. Each
>SV clock would have its own weight, each Az/El square degree
>of each SV would have its own weight. You can also weight by
>SNR or DOP. Or even temperature. Not only should this give a
>more precise virtual 1PPS but you also get wonderful 2D or 3D
>plots showing how weight varies with each factor.
>
>I have some data on this I can share; contact me off-line.
>
>/tvb
>
>----- Original Message ----- From: "Jerry Mulchin" <jmulchin at cox.net>
>To: <time-nuts at febo.com>
>Sent: Tuesday, May 08, 2012 3:20 PM
>Subject: [time-nuts] M12+T sawtooth correction and additional filtering
>
>
>>OK, so before I commit to my Sawtooth correction design, I had a thought about adding
>>additional filtering to the sawtooth corrections using a Kalman filter. 
>>The Kalman filter would be driven using the @@Hn data coming from the M12+T reciever.
>>Is this something worth doing or am I expecting too much from the GPS system timing or
>>the M12+T receiver?
>>Advice and comments welcomed.
>>Thanks
>>Jerry
>
>
>
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Jerry Mulchin





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