[time-nuts] Sub mm measurements with gps timing antennas?

Tom Van Baak tvb at LeapSecond.com
Wed Apr 25 17:05:00 UTC 2012


Hi Attila,

Are you sure the customer said sub-mm and not sub-meter? I know
post-processing is really helpful, but the LEA-6 is a single frequency
receiver so all the advantage of L2 is lost for this customer. The
bullet antenna's don't even have an arrow for North ;-)

One thought -- seeing how this is a research project. It might be
possible to cross-correlate the post-processed data against the
Az-El of each SV along with ambient temperature over days or
weeks and thus actually measure the phase center profile as well
as tempco of the system. This would be no small effort, depending
on the math and programming skills of the researcher(s), but the
advantage for them is that is costs time instead of money. Then
armed with this "calibration" data (possibly unique to each unit),
it would be possible to reduce these effects, improving precision.
I have no idea how much. Still, an interesting project.

A simple test that could be done locally (refrigerator, sauna, etc.)
would be to measure the tempco of the entire system (antenna,
cables, LEA-6T) before they deploy it to a mountain. It may also
be the case that the system has both a temperature coefficient
and a temperature change coefficient so it's not a simple 2-point
test. You can probably ignore humidity and barometric pressure.

Another test would be to rotate the antenna at 1 RPH (revolution
per hour) and then look for modulation in the post-processed
solution. This would give a hint of the quality of the antenna. As
a baseline, try the same test using a precision gps antenna. I
have spare pin-wheel, choke-ring, and ground-plane antennas
that I could loan, but surely these are available where you are,
and probably cheaper than postage from here.

It seems that everyone else that does sub-ns precision timing or
mm positioning uses a large combination of tricks: dual-frequency
antenna and receiver, geodetic-quality antenna, passive or
active temperature control, phase-stabilized cables, GPS and
Glonass, external frequency reference, and post-processing.
Your customer is only using one from this long, expensive list.
So there may be a lesson there.

Can you share any data they have collected already? I would be
interested to know how far one could push a LEA-6T.

Thanks,
/tvb

----- Original Message ----- 
From: "Attila Kinali" <attila at kinali.ch>
To: <time-nuts at febo.com>
Sent: Wednesday, April 25, 2012 7:56 AM
Subject: [time-nuts] Sub mm measurements with gps timing antennas?


Moin,

We've a customer who does sub mm measurements using GPS in alpine
enviroment. This is done using LEA-6T modules, logging of raw phase
data and offline post processing using long averaging windows.

Now, the customer had some problems reaching the precision requirements
and i'm wondering whether one of the causes might be the use of a
Trimble Bullet antenna [1] (3V type) and not a geodetic antenna. Can it be
that the phase center of the Trimble Bullet antenna isn't as well defined
as it should be for this application? Would the addional gain of the 5V
version help (35dB instead of 30dB)? Or should we evaluate a different
antenna all together.

A major restriction in this application is that there is a very harsh
enviroment, temperature wise. We have measured -40°C to +30°C jumps in
just 2h. Most of the time the devices are below freezing temperature,
but can go up to 50°C when in direct sunlight.

The next big restriction is, that this is a research project. So
there isn't as much money available as there should be to "do it right".

So, if someone could give a few tips how to improve things, this would
be much appreciated.

Attila Kinali


[1] http://www.trimble.com/timing/bullet-gps-antenna.aspx?dtID=overview&

-- 
The trouble with you, Shev, is you don't say anything until you've saved
up a whole truckload of damned heavy brick arguments and then you dump
them all out and never look at the bleeding body mangled beneath the heap
-- Tirin, The Dispossessed, U. Le Guin






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