[time-nuts] Some results of PRS10 and Trimble Resolution
Tom Van Baak
tvb at leapsecond.com
Fri Jun 30 13:03:15 EDT 2006
> Hello Tom,
> I had a conversation with Sam S. from TSC the
> other day, and he said that it's probably not
> possible to get <10ns GPS accuracy anyways
> due to the multipath issues, Ionospheric issues,
> antenna survey issues, thermal issues etc.
Yes, with a standard GPS receiver or GPSDO, I
very much agree with this. See also the links to
GPS papers I posted earlier today which will give
you a feel for what level of accuracy or stability
you get from various GPS time transfer techniques.
Note accuracy and stability are two different goals.
Related to that, GPS-based frequency reference
products are plentiful, cheap, and in widespread
use while GPS-based time transfer products are
few, very expensive, and have a small use base.
I think most of us time-nuts use GPS as a source
of precise time interval (GPS as a ~ 1e-13 stable
frequency reference) rather than a source of
absolute time (~ ns accurate UTC). This is why
uncalibrated GPS receivers work for all of us.
See Rick's papers on M12 calibration at USNO.
Note NIST's use of regular GPS receivers (Oncore?)
for frequency _stability_ (not time _accuracy_):
So 10 ns _stability_ with GPS over a day is quite
doable on the cheap. < 10 ns _accuracy_ with GPS
is quite another matter and requires a huge amount
of work. On this list, DougH is probably the only
one who's pulled it off:
> Our units typically average the GPS 1PPS over
> 30 minutes, so having less than 3.33ns error on
> the 1PPS capture may not improve things much
> because while the error stays always at +-3.33ns
> it get's averaged over the measurement intervall.
3.3ns / 30 m = 2e-12 and sets your lower bound.
But it would depend on your choice of LO and PLL
if this is a limiting factor or not.
Here's something to try: deliberately degrade your
1PPS TIC resolution in software and see what effect
it makes on the stability of your RF or 1PPS output.
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