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

[Tom Miller]

You should be able to get the power for a OCXO way down with some very good 
insulation. Think Dewar container for the XTAL and heater. They just need a 
lot of power to get warmed up and you could do that prior to installation.

As to using a choke ring antenna, just use a steep peaked radome.


Tom



----- Original Message ----- 
From: "Attila Kinali" <attila at kinali.ch>
To: "Tom Van Baak" <tvb at leapsecond.com>; "Discussion of precise time and 
frequency measurement" <time-nuts at febo.com>
Sent: Wednesday, April 25, 2012 2:42 PM
Subject: Re: [time-nuts] Sub mm measurements with gps timing antennas?


Beside the harsh environment, this also means that the devices cannot be
reached for most of the year (there is usually a 3 months window when
the devices can be accessed)..and some years they cannot access them at all
(adverse weather conditions preventing ascend). For installation, a 
helicopter
has to fly everything up into the mountains, which means that pre-assembled
stuff cannot be transported, because it's too bulky (prevents whole
system calibration in a climate chamber). After installation the system
runs on solar power with a backup battery. But this doesn't guarrantee
power at all. The solar panel could be below a meter or two of snow.
Hence the whole system has to cope with periodic power los and has to
be as low power as possible (ie no OCXO, no Rb, no temperature 
stabilization).

The snow also prevents the use of choke rings, because they would accumulate
a lot of snow and ice, which would then cover the whole antenna.

But at least, there is hardly any electronic interference, a good sky
view (no trees), unless the system is mounted near a wall. And the
antenna cable is quite short (it was 15cm in the previous version
of the device and should be <50cm in the next)

> 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.

I think the ETH has climate chambers that could run such tests.
But i'm not sure how you'd test the antenna in such a chamber.


> Another test would be to rotate the antenna at 1 RPH (revolution
> per hour) and then look for modulation in the post-processed
> solution.

Hmm.. that's actually a quite nice test. Cool idea, thanks!

> 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.

Yes. The problem is, antennas that perform well in a city environment,
where temperature swings are quite limited, fail in high alpine environment.

But that's what my question originally aimed at. How much better can
we get using a better antenna? Is it worth doing? Or is it just a
waste of time and money?

BTW: what's a pin-wheel antenna? Google tells me contradictory things.

> 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.

The problem with most of those techniques is, that they are not available
for the price the customer can afford. A dual frequency receiver costs
a lot more than an of the shelf LEA6-T. Also these modules are usually
build with larger power budgets in mind, e.g. the Trimble BD920 uses
1.3W typical, while the 0.3W max(!) of the LEA6-T already hurt us a lot.
Using an external frequency reference is not possible with the LEA6-T.
It would be possible to do that when using one of the GPS chipsets from
u-blox, but therefor we would need to take at least a full reel (iirc 2000
pieces), which isnt exactly cost efficient. Beside, we would still need
to use a TCXO, because there is not enough power available for an OCXO
or even an MCXO.


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

We are "only" a supplier of the electronics and don't have access
to the data. And i don't think they have finished analysing the data
they collected already...But i can ask. What data do you have in mind?


Attila Kinali

[1] http://www.permasense.ch
-- 
Why does it take years to find the answers to
the questions one should have asked long ago?

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