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" <[email protected]> >To: <[email protected]> >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 > > > >_______________________________________________ >time-nuts mailing list -- [email protected] >To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >and follow the instructions there. Jerry Mulchin _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
