[time-nuts] More Comments on GPS's ADEV

[Tom Clark, K3IO]

TvB replied to the GPS ADEV question with
> I like the questions you ask. Here's an updated plot:
>
> http://www.leapsecond.com/pages/3gps/gps-adev.gif
>
> Note that the tau 1 second Allan deviation is essentially
> the standard deviation of the sawtooth: in the graph the
> VP is about 55 ns, CNS about 15 ns, CNS2 about 3 ns.
> (CNS is an M12+; the CNS2 adds sawtooth correction).
>   
You might want to also take a look at my PowerPoint tutorials on
http://gpstime.com. Several of the talks contain my canonical "collected
wisdom" ADEV with some discussion, which I have attached.

The "L1 GPS" curve is similar to the one TvB posted. The two GPS lines
has had the Sawtooth removed  (the basic data was taken with HP 53131/2
counters using TAC32).

FYI -- Note that between 10,000 and 100,000 seconds, the uncalibrated
diurnal ionosphere signature just about doubles the noise level. The GPS
broadcast signals contain an approximate ionosphere correction that is
based on sunspot count, and most receivers follow the GPS ICD200
"recipe". However, this "cookbook" correction makes no attempt to
calibrate for day/night variations, or the variation of the ionosphere
with latitude. This typically gives rise to a diurnal clock signal that
is 10-20 nsec in size.

You can see these signatures clearly if you compare GPS with an atomic
clock, or if 2 GPS clocks that are separated more than ~100 km. If the
two clocks are closer, then the uncalibrated ionosphere is highly
correlated. In the 2002 USNO paper, you can see the daily 10-20 nsec
signature and as a bonus, an example of a 50 nsec ionosphere excursion
during a major solar event. We also show that all the ionosphere biases
are "common mode" and cancel out over a ~20 km baseline.

FYI #2 -- The 3 "blue balls" on this plot illustrate the clock
requirement of VLBI (Very Long Baseline Interferometry) which uses
H-Masers at sites all over the world. Some of the tutorials on
http://gpstime.com are my lectures to VLBI's telescope operators so that
they will better understand why clocks are important to them.

Regards, Tom


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