Bruce said:
"Unless you have a CSO or a hydrogen maser ... can't do that."
So I would generally answer if one can not measure any ills effects, Then it
does not mater to THAT person AND it is good enough.
But, Sounds like you could use a little more Practical experience.
Cause it is so simple to measure, most anyone can do it, even if it does not
mater to them.
I do it all the time and have nothing better than a HP10811
The First indirect way is to look at the position dispersion when in
position tracking mode.
OR
For a more direct way, set the GPSDO tracking TC very fast say at 10 sec or
so which will then allow plotting of the GPS ADEV noise.
The goal is to see how the GPS noise changes with different Antennas and the
ultimate goal is to see where it drops below the internal Osc noise.
You can See Tom's site for more on that.
And if you want to do it without ANY external equipment, It can be done with
some of LH plots and graphs good enough,
by placing the Osc in disable and ploting the Phase noise which for short
time periods will be the GPS noise.
More info on request.
ws
**********************
Bruce Griffiths wrotre
Unless you have a CSO or a hydrogen maser absolute measures of ADEV and
phase noise arent feasible for the range of Tau of interest.
Even an indirect method such as measuring the location of the apparent
minimum in ADEV between the GPS SV constellation observables and the
OCXO when it is undisciplined depend heavily on the ADEV characteristics
of the OCXO being used.
Bruce
********************
WarrenS wrote:
yeah,
So many variables, ALL the more reasion to just see what the overall
effect is on the more common type of GPSDO receviers at a few sites.
So did you have a better plan?
ws
****************
Bruce Griffiths Added:
WarrenS wrote:
Bruce wrote:
"Which antenna performance metric do you have in mind?"
Could do GPSDO hold over performance, but that would not be much of
a test of the antenna.
How about the antenna's effect on the ADEV Osc noise and Phase noise.
What else does the Time Nut care about?
Since the better timing receivers use carrier phase ...
I don't remember you ever finding ANY Time Nut that is now using one.
So may be simpler for now to just stick to the more common type of
GPSDO in use.
The Motorola M12+T and iLotus M12M use carrier phase smoothing of the
code phase observables.
Its highly likely that a number of the better performance GPS timing
receivers also use carrier phase smoothing.
Thus whether one is aware of it or not the antenna carrier phase
properties are likely to be of some importance.
In the absence of complete information on how your particular GPS timing
receiver uses carrier phase and code phase observables, the best you can
do is compare the performance of a range of antennas using a given
timing receiver.
Such results will only apply to a particular site and receiver.
Specifying the pertinent characteristics (eg isolated on a flat plain,
surrounded by a set of hills, mountains which obscure the sky below 10
degrees, surrounded by trees that obscure everything below 40 degeees
elevation, etc) of your antenna location and the particular GPS receiver
used will be helpful to others in selecting an antenna that suits their
budget, receiver, antenna location constraints, etc.
Bruce
ws
*************
Bruce said:
Which antenna performance metric do you have in mind?
There are several, some of which are considered in the paper:
http://www.novatel.com/Documents/Papers/effectofantenna.pdf
Since the topography surrounding the antenna, its height and location on
the Earth all affect measured performance any comparative measurements
should use the same receiver and antenna location.
Some estimates for the effect of multipath on code phase receivers can
be found:
http://bbcr.uwaterloo.ca/~wzhuang/papers/iee95_gps.pdf
http://bbcr.uwaterloo.ca/%7Ewzhuang/papers/iee95_gps.pdf
Since the better timing receivers use carrier phase smoothing of the
code phase timing, both the carrier phase and code phase performance of
the antenna are important.
A phased array antenna like the one in the following papers may provide
better performance than alternative antennae:
http://www.navsys.com/Papers/0001002.pdf
http://www.congrex.nl/07c12/papers/day1_s1_paper05_Konovaltsev.pdf
Some measurements with geodetic antennae:
http://www.fig.net/pub/fig2008/papers/ts05g/ts05g_03_eventzur_shaked_2816.pdf
Comparison of code phase and carrier phase time transfer:
http://tycho.usno.navy.mil/ptti/ptti2004/paper41.pdf
Bruce
******************
WarrenS wrote:
Brian wrote:
"There were also comments about surveying and timing antennas."
Those may of been from me, unsuccessfully trying to make a point of
the difference between what is 'Best' and what is 'GOOD enough'.
"about every national timing laboratory uses choke ring antennas.
... for timing stability reasons."
Then again they also have multiple CS and Just their Antenna budget is
likely more than the annual income of most time nuts.
Can you do a test to show IF there is ANY improvement for the AVERAGE
time nut when compared to a well setup (Tbolt) GPSDO using a TacoSalad
antenna?
Would be interesting to see a plot of cost vs. performance for the
various antenna types,
Scaled to show the performance improvement that the average Time nut
would see.
The TacoSalad antenna, originally cost me a total of $7.95, And took
under 30 seconds to build.
That cost should be discounted because those parts had been considered
just throw away junk up until now.
ws
**********************
----- Original Message ----- From: "Brian Kirby"
<kilodelta4foxmike at gmail.com>
To: "Discussion of precise time and frequency measurement"
<time-nuts at febo.com>
Sent: Monday, March 15, 2010 2:09 PM
Subject: Re: [time-nuts] Choke Ring Pictures
Dr. Clark passed on a tip that I used. Put the funnel in a microwave
oven and run it and see if the funnel warms up. If it warms up, you
do not use it. I do not know what type of plastic the funnel was
made out of; it was white, semi-transparent.
There were also comments about surveying and timing antennas. If you
investigate about every national timing laboratory uses choke ring
antennas. Some enclose the antenna unit and they temperature control
it. They do this for timing stability reasons.
The commercial timing antenna is bullet shaped and is operated
without a ground plane. They are patch antennas. When there is not
ground plane, the antenna picks up best from the overhead and less
towards the horizon. These antennas usually have a lot more gain
(30-50 db vs most normal antennas in the 15-25 db range).
Also in surveying, we cut off the horizon at 15 degrees in software.
A free Army Corp of Engineering manual on GPS Surveying is at
http://140.194.76.129/publications/eng-manuals/em1110-1-1003/toc.htm
The main difference in surveying and timing is in surveying they use
the carrier phase method, were in timing most use a solution derived
from the processing of the coarse acquisition code, in were the
receiver is in a fixed over-determined position . Some timing labs
are using carrier phase method, when they need more resolution.
Brian - KD4FM
****************
warrens wrote:
...
Preliminary results for the Taco Dish GPS antenna as an indoor
antenna are looking good.
Certainly worth considering if your GPS antenna is stuck indoors,
'Out of the rain in the living room'.
I find it best to rise it up near the ceiling such as on an upper
shelf with nothing above it.
It would be hard to tell the difference between the GPSDO
performance obtained from this or the Best outdoor antenna if using
a Tbolt set to the standard default settings.
Picture attached
ws
**************
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