[time-nuts] Some results of PRS10 and Trimble Resolution

[Tom Van Baak]

> 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.
http://www.gpstime.com/
http://tycho.usno.navy.mil/ptti/ptti2002/paper9.pdf

Note NIST's use of regular GPS receivers (Oncore?)
for frequency _stability_ (not time _accuracy_):
http://www.tf.nist.gov/service/fms.htm

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:
http://www.leapsecond.com/time-nuts.htm

> 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.

/tvb