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

Tom Van Baak tvb at LeapSecond.com
Tue May 8 23:19:14 UTC 2012


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

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.


----- 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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