[time-nuts] Common View Tbolt-Tic

WarrenS warrensjmail-one at yahoo.com
Wed Oct 19 11:25:29 EDT 2011


Thanks Guys, Gives me lots to consider and go over.
Not going to be quite as easy as I hoped. Then if it was easy it would not 
be Nut-fun.
Lots to learn, which I do best with experiments.

Sounds like using times much longer than 1 Hr is the way others do it,  But 
then they have different goals.
My thought is that a well setup TBolt using a good external oscillator will 
pretty much beat anything if let run long enough,
So what I'm first interested in is to see if there is a significant 
improvement possible over short time periods.

Preliminary test results using two Tbolts on the same Osc, with different 
antennas that are close together is:
Comparing the short term Tbolt Phase plots does not help much, they are 
pretty random noise short term.
Comparing them after passing thru a slow low pass helps.

On the other hand,
The two TBolt's PPT freq plots are tracking each other nicely.
This agrees with early test that showed the Tbolt's PPT data has much less 
short term noise than the Phase data, (but the PPT data is not so good long 
term).
I'm seeing tracking between these two Tbolts of better than 1e-12 using 
filter setting of 10 to 100 sec.
This is giving at least a 10 to one improvement compared to using just one 
Tbolt in single view mode.
Now that I have data showing what the Tbolt is capable of, need to redo the 
test where the antennas are separated by hundreds of miles, to see what the 
GPS is capable of over that distance.

Still TBD is how to best take advantage of the Tbolt's unique 
characteristics.
I'll respond to the other questions and show some LH graphs, once I get some 
better data.

Thanks,
any other comments/experiences and translations of what "the papers are 
saying" that apply to this effort are welcomed
ws

*********************
Warren,

This will be fun. Are these standard TBolts or ones with external oscillator 
(Rb)?
ws) All the above

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?
ws) I'm looking at short term results from seconds to an hr or so

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

*****************
Hi Warren,

> What is not too clear is how much of that is due to the Tbolt engine and
> how much is the "GPS Reference".

Do you have two working Tbolts with their orginal oscillator removed?

> From what I've seen in my test, a large amount of that noise floor is due
> to the GPS.

I think a dual Tbolt configuration would eliminate GPS instabilities and
rely more on the house standard. As proposed in previous email.

--

   Björn

*********************
Hi Warren,
...

The 2Tbolt-system is really a degenerated Common View time transfer
system, with both receivers beening colocated and using the same GPS
antenna signal.

Moving the receivers to different time-nut labs, we have a traditional
time-transfer system. Where we perhaps only lack L2 measurements, compared
to the serious time tranfer systems used by national labs.

     http://tf.nist.gov/time/commonviewgps.htm

--

    Björn
****************

I propose an experiment with two Tbolts running from the same antenna and
sharing the same oscillator, ie at least one of the Tbolts modified to
accept external 10MHz. This would take out most/all GPS system errors,
leaving receiver measurement noise.

Does anyone have two working Tbolts modified for external oscillator
available for testing?  I have one Tbolt and a loaner unit from a friend.
I am not very keen on modifying the two I have in my lab right now.

kind regards,

     Björn

************* 





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