Warren,
This will be fun. Are these standard TBolts or ones with external
oscillator (Rb)?
I won't address the issue of noise measurement in this email.
The first question is how long did you collect data among the sites?
The standard GPS Common View that the timing labs do is based
on 13 minute "tracks" (per satellite). In other words, it's not the raw
one second measurements they compare; it's the summary of the
entire 780 second track. The raw TBolt LO time (phase) data has
a lot of GPS signal and receiver noise and won't correlate at short
times. That's why GPSDO need such long time constants. Also with
1 SV in your test instead of 8 SV the noise will be all the greater.
Anyway, reduce the data in 10 to 15 minute chunks and see if that
helps at all. There's more to common view; not sure how much to
dump on you for starters. How did you try to correlate it? Send me
some of the raw data if you can.
For some light reading start with:
<http://tf.nist.gov/time/commonviewgps.htm>
<http://www.usno.navy.mil/USNO/time/gps/about-the-cggtts-data>
<http://www.nrc-cnrc.gc.ca/eng/services/inms/time-services/positioning-data.html>
<http://www.usno.navy.mil/USNO/time/gps/gps-timing-data-and-information>
You don't have to use the CGGTTS format for this initial trial but
the information about the file format will be a useful guide to how
common view works. The advantage of the format is then you can
compare against USNO and anyone else that publishes CGGTTS
files. But I'm getting ahead of myself.
For some deeper reading, please enjoy these:
"A review of time and frequency transfer methods"
http://tf.nist.gov/general/pdf/2311.pdf
"Time and Frequency Measurements Using the Global Positioning System"
http://tf.nist.gov/general/pdf/1424.pdf
"A Comparison of GPS Common-View Time Transfer to All-in-View"
http://tycho.usno.navy.mil/ptti/ptti2005/paper40.pdf
"Effects of the Rooftop Environment on GPS Time Transfer"
http://tf.nist.gov/general/pdf/2199.pdf
Google for "Novel GPS Survey Antenna" for details on that
pinwheel antenna mentioned in the above paper.
/tvb
----- Original Message -----
From: "WarrenS" <warrensjmail-...@yahoo.com>
To: "Discussion of precise time and frequency measurement" <time-nuts@febo.com>
Sent: Tuesday, October 18, 2011 11:08 AM
Subject: [time-nuts] Common View Tbolt-Tic
I'm doing some experiments in hopes of using the TBolt-Tic in a common view
configuration to lower the short term
noise.
That is, instead of comparing a local externally connected Tbolt Osc to the
GPS, I want to compare it remotely to one
of say Tom's supper H-masers or Cs references.
The single view Tbolt-Tic works great using averages of a half day plus to get
down to 1e-13, but being nuts I want to
do it Faster and better.
Test setup:
I have three COMPLETELY independent Tbolts running, each with its own
LadyHeather monitor.
Two of the Tbolts are a couple feet away from each other, the third one is a
few hundred miles away.
All three are in single satellite mode set to watch the same near overhead
satellite.
All three of these Tbolt oscillators are capable of short term noise a decade
or two better than the short term GPS
noise.
I also measured the TBolt's engine phase noise to be a decade lower than the
short term GPS noise.
All three are in disable mode, so that their plotted LH phase noise is for the
most part ALL due to the received
satellite signal noise.
My hope was that there would be a high correlation between the Phase noise of
their three Phase plots..
What I see is almost no correlation, they are all just doing their own thing
with random phase noise of a couple ns.
Just to check I tried the same test with a satellite that had an elevation of
around 45 deg with the same results.
One test I should run is to use the same antenna for two of the Tbolts,
But that defeats the whole purpose, which is to be able to compare by way of
LadyHeather's remote function, a local
Osc to an external Rb or Cs remote reference Osc.
Any suggestions?
Something is wrong somewhere.
Maybe it is that the Tbolt's engine noise is not what I measured it to be, but
I have double checked that a couple of
different ways.
Anyone have personal common view experience comparing Tbolts OR any other
thing using GPS.
ws
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