[time-nuts] NIST GPS Data Archive

[Skip Withrow]

In my quest to get some decent real-time comparison to GPS it was suggested
to look into the NIST TMAS system.  That is when I discovered the NIST GPS
Data Archive.  However, my first attempts at correlating my reception
results with the data archive were miserable failures.

I saw that Mike Lombardi from NIST was on several of the TMAS papers, so
shot off a quick email to him.  He was very kind and actually gave me a
call.  He cleared up several questions and the results are looking much
better.  Thanks!

I was originally using a dual-band L1/L2 receiver in fixed position mode
with WAAS corrections turned on.  Turns out the NIST archive is done with a
single-band L1 receiver operating in fixed position mode using only
receiver computed ionosphere corrections.

I also noted in the archive that the hourly values did not match the
10-minute values.  This is because the six 10-minute values are averaged to
give the hourly value.  AND the 10-minute values are the average of 600
1-second readings.

So it was time to put Lady Heather to work.  Using a Trimble Thunderbolt I
have plotted the 10-minute averages for the last seven days.  So my plot is
10-minute averages and the NIST plot is of the hourly averages.  A
comparison picture is attached.  The Thunderbolt is being driven from the
MHM-A1 maser which I thought was running about 3 nanoseconds per day slow,
but it appears to be much closer than that.  In the picture that is
attached both NIST and LH are plotted at 2ns/div vertical, the horizontal
scales are close but not exact.

Next task would obviously be to get some automation involved to collect the
NIST data, Thunderbolt data, and the dual-band data and start crunching
some numbers.

Regards,
Skip Withrow
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