[time-nuts] Best GPS 1PPS Accuracy
Randy at synergy-gps.com
Wed Dec 13 20:32:28 EST 2006
The software does allow several seconds of free running pulses to be
sent out while it verifies the alarm. My guess is that the time error of
the "free running" pulses would not be significant (unless you are a ns
hunter ;-) ). Many users do "flywheel" from these pulses just to keep
some sort of clock source.
I HAVE attached one of my favorite screenshots that shows that you
REALLY have to be careful who you believe when it comes to time
accuracy. We have to remember that the 1PPS is placed at the top of the
UTC second as best the receiver can determine. It can be absolute
garbage, but if one doesn't mind the error flags and alarms, one can get
a lot of useless time data.
Anyway, as with the screenshot I sent previously, this one shows the
sawtooth data from an M12+T. The only difference is that I disconnected
the antenna about a third of the way through the window. Note how
consistent and pretty the sawtooth is? Too bad it's garbage.
Based on some old Motorola stuff about the VP TRAIM, the problem with
severely reducing the TRAIM alarm limit is simply one of irritation. In
their experiments they found that if you tried to reduce the TRAIM limit
to less than 300ns the number of false alarms rose drastically. Remember
this was with an old 8 channel receiver and that SA was on. I think the
plain fact is that 1us works fine for 99.99% of the applications out
there. The telecoms would much rather give the TRAIM alarm lots of room
so that they are not getting constantly bombarded with errant TRAIM
I do have a copy of this old doc that I could send out so that it could
be hosted on one of the timing sites. The information is really
outdated, but it does do a good job of explaining some of the original
reasoning behind TRAIM and timing performance in general. Just keep in
mind that it is historical in nature and not state of the art.
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of SAIDJACK at aol.com
Sent: Wednesday, December 13, 2006 4:56 PM
To: time-nuts at febo.com
Subject: Re: [time-nuts] Best GPS 1PPS Accuracy
slightly off-topic, but has anyone done stability tests with the m12+ or
M12M when the antenna goes bad in TRAIM-enabled mode (e.g. less than 4
good sats being received by i.e. disconnecting the antenna)?
I see that the 1PPS output goes away only after about 5 or so
additional pulses have been generated, so these 5 pulses are
These pulses are somewhat inaccurate, and I am wondering if anyone has
done a max/min analysis on the errand pulse's offsets?
I wonder if the TRAIM limits can be set to say +-50ns effectively to
kill these pulses and improve overall accuracy?
I know lot's of folks run the M12 TRAIM limits at 1us, that seems
excessive on a receiver that is supposed to do 10ns 1-Sigma, doesen't
Thanks for your feedback,
time-nuts mailing list
time-nuts at febo.com
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