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

[Attila Kinali]

Moin,

On Wed, 25 Apr 2012 10:05:00 -0700
"Tom Van Baak" <tvb at LeapSecond.com> wrote:

> 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 ;-)

Yes, it is sub-mm. They are already doing sub-cm with the current
setup, but it isn't precise enough yet. IIRC during the first tests
last year they got <4mm precision. And yes, using a LEA6-T with a Trimble
Bullet antenna. I don't know what they actually do with the raw phase
data, beside that they average over several hours and use a 100m baseline
reference consisting of two stations mounted at a fixed position.
 
> 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.

Hmm.. That would be an idea, but i don't know whether it is feasible.

Maybe i should here expand a little bit what the project actually is.
The effort is part of the Permasense[1] Project, which does measurements
of different parameters of permafrost soil/rock in high alpine regions.
The idea of this sub-project is to measure the exact movement of rocks
and other "solid" and "unmovable" things with regard to weather conditions.

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?