[ntp:questions] Re: Bought a GPS receiver--What next?
John R. Ackermann N8UR
jra at febo.com
Tue Nov 2 13:30:57 UTC 2004
Richard B. Gilbert wrote:
> David Schwartz wrote:
>> "Richard B. Gilbert" <rgilbert88 at comcast.net> wrote in message
>> One millisecond is pretty poor PPS accuracy for a GPS receiver
>> intended for timing applications. One microsecond is typical.
> Evidently I don't write any better than Motorola did!! The leading edge
> of the PPS is exact! In the Oncore M12 it is specified to be within 50
> nanoseconds of the UTC second!
Coincidentally, I'm using the 1pps output from an M12T right now to
calibrate an HP 5065A Rubidium frequency standard (comparing 1pps
outputs via a time interval counter with 20ps resolution to determine
the frequency offset of the Rb).
I was hoping I'd be able to post a graph of the performance right now,
but I'm having a problem with format conversion -- the analytic software
outputs a 3MB bmp file and my tools won't successfully convert it to
something more web-friendly. I'll work on that problem tonight.
But the short story is that over 12 hours, and after removing the Rb's
frequency offset (about -1.5x10e-12), the "noise" on the 1pps output
from the GPS is +/-60ns, with a small number of peaks that extend a few
nanoseconds further -- the absolute maximum deviation over the period
was about +/-75ns, with only a couple of samples hitting that extreme.
Averaging over 100 seconds reduces the noise to the +/-15ns range. Oh,
and I'm not using the sawtooth correction capability of the M12T -- I'm
just looking at the raw PPS output.
I'll try again tonight to generate a plot and will post the URL here.
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